National Standards TCVN 4054:2005 for Highway - Specifications for design

NATIONAL STANDARD

TCVN 4054:2005

HIGHWAY - SPECIFICATIONS FOR DESIGN

Foreword

The TCVN 4054:2005 replaces the TCVN 4054:1998.

The TCVN 4054 : 2005 is compiled by Board of Technical Standard TCVN/TC98/SC2 Road Traffic Structure, approved by the Directorate for Standards, Metrology, and Quality, and promulgated by the Ministry of Science and Technology.

HIGHWAY - SPECIFICATIONS FOR DESIGN

1 Scope

1.1 This Regulation prescribes requirements in design, construction, renovation, and upgrade of highway. Specialized roads such as: expressway, urban roads, industrial roads, forestry roads, and other road types must be designed in accordance with field-specific standards. It is permissible to adopt appropriate road classes in this document upon designing rural road design.

Where highway is related to other structures such as railway, irrigation structures or crossing residential areas, cities, cultural, historical heritage sites, etc., other applicable regulations of the Government pertaining to these structures shall apply in addition to this document.

1.2 In special circumstances, it is possible to adopt technical criteria under other standards as long as they pass economic - technical analysis.

Road segments to which technical criteria other than those mentioned above apply must be designed in a concentrate, consistent manner throughout the length of the route and the total length of these road segments must not exceed 20% the length of the design route.

2 Reference documents

Documents below are necessary for the application of this document. Where a reference document is cited with its publishing year, the specified version will prevail. In respect of reference documents not cited with publishing year, the latest version will prevail, including all its amendments

TCVN 5729 : 1997 Freeway and Expressway - Design requirements.

22 TCN 333 Procedures for earth, gravel compression in laboratory.

22 TCN 16 Procedures for measuring road roughness with 3 m long gauge and feeler.

22 TCN 171 Procedures for structural geological survey and design of road base stability solutions in areas prone to erosion, landslide.

22 TCN 211 Procedures for design of flexible pavement.

22 TCN 221 Design standards for traffic structures in areas prone to earthquake.

22 TCN 223 Procedures for design of rigid pavement.

22 TCN 237 Regulation on road signaling.

22 TCN 242 Procedures for environmental impact assessment in developing projects, feasibility study and design.

22 TCN 251 Procedures for testing general elasticity modulus of flexible pavement using Benkelman beam.

22 TCN 334 Graded aggregate bases and subbases pavement - Specification for construction and acceptance.

22 TCN 262 Procedures for survey and design of road bed filling on weak ground.

22 TCN 272 Design standards for bridges.

22 TCN 277 Standard for road surface examination and evaluation using International Roughness Index (IRI).

22 TCN 278 Procedures for determining road traction using sand surface method.

22 TCN 332 Procedures for determining CBR of soil and gravel in laboratory.

3 General provisions

3.1 Design requirements

3.1.1 General requirements in design require multi-facetted research in addition to compliance with this document in order to develop safe, effective road and aim for sustainable, long-term development.

3.1.2 All road elements must be adequately incorporated: topographic map, cross section, longitudinal section, utilization of terrain to create uniform road with adequate visibility and mechanical stability in order to:

- serve appropriate traffic flow and facilitate reasonable traffic quality;

- ensure maximum safety and convenience for vehicles and road users;

- yield high economic effectiveness via evaluation criteria, expenditure on construction and maintenance, expenditure pertaining to transport costs, transport time, and traffic accident forecast;

- minimize negative environmental impact, create reasonable ecological balance to allow roads to contribute towards scenery of local area.

3.1.3 In principle, high-class highway (class I, II, and III) must not travel through residential areas. The design must take into account:

- connection of roads to cities, especially major cities;

- solutions for isolating from local traffic, especially high-class roads to maintain mobility.

Highways must be able to maintain:

- mobility, displayed at high speed and shortened travel time and safety in driving;

- accessibility, vehicles arrive at destination conveniently.

The aforementioned functionalities are not compatible with one another. Thus roads of higher classes, greater traffic flows, and long travel time must be restricted of accessibility to promote mobility; whereas roads of lower classes (class IV, V, VI) must have adequate accessibility.

Roads of high class must:

- Isolate local traffic from traffic on high-class roads.

- Avoid residential areas while paying attention to connection to other cities, especially major cities that require traffic through city center.

3.1.4 Investment projects in stages must be taken into account in long-term general solutions. Investment stages must be appropriate to traffic flow at the stages in question and must be parts of the general investment solutions which mean most, if not all structures built in investment stages will be utilized.  Implementation of investment stages must take into account land reservation for future complete structures.

3.2 Design vehicle

3.2.1 Design vehicle means a popular type of vehicle for the calculation of road factors. Selection of type of design vehicle shall be decided by persons entitled to investment. Dimensions of design vehicles are specified in Schedule 1.

Schedule 1 - Dimensions of design vehicles

All dimensions in meter

3.3 Design vehicle flow

3.3.1 Design vehicle flow means the number of cars converted from other vehicles crossing through a cross section in a period of time for the future year. Future year means the 20^th year from the year in which the class I and class II roads are used; the 15^th year from the year in which class III and class IV roads are used; 10^th year from the year in which class V, class VI roads and upgraded, renovated roads are used.

3.3.2 Rate of conversion from other vehicle types to cars conforms to Schedule 2.

Schedule 2 - Rate of conversion from other vehicle types to cars

3.3.3 Types of design vehicle flow

3.3.3.1 Average daily design vehicle flow for future year (abbreviated as “Ntbnđ”) is presented as xcqđ/nđ (converted cars/24 hours).

This flow serves as reference for the purpose of choosing design class of roads and calculating other factors.

3.3.3.2 Peak-hour design vehicle flow for future year (abbreviated as “Ngcđ”) is presented as xcqđ/h (converted cars/hour).

This flow rate serves determination of number of lanes, prediction of traffic flow, traffic arrangement, etc.

N_gcđ can be determined by:

- extrapolating from N_tbnđ using time variables where statistics are available;

- using the 30^th peak hour flow rate of reporting year where adequate reports on hourly vehicle flow in 1 year are produced;

- using N_gcđ = (0,10 ÷ 0,12) N_tbnđ if no special studies are conducted.

3.4 Design class of road

3.4.1 Design classification refers to framework of technical regulations of roads in order to:

- serve intended traffic function in traffic network;

- satisfy design pass-through flow rate (this criterion is open to expansion due to cases and roads of importance that are not crowded or are temporarily not crowded);

- set separate requirements pertaining to standards in each design class depending on the terrain in order to yield reasonable investment and economically effective.

3.4.2 Technical classification of roads in road network is based on functionality and design flow of roads in road network and specified under Schedule 3.

Schedule 3 - Technical classification of highway by functionality and design flow

*) This value serves reference purposes only. Road class should be selected based on road function and functionality.

3.4.3 Each route must have a minimum length of road that is at the same class. This minimum length is 5 km for roads of class IV or lower and 10 km for other roads.

3.5 Design speed, (Vtk)

3.5.1 Design speed means speed used for calculation of necessary technical parameters of roads in difficult situations. This speed is different from permissible speed limit imposed by road authority. Permissible speed limit depends on actual road situations (climate, weather, road conditions, traffic conditions, etc.).

3.5.2 Design speed of road classes depends on terrain and is specified under Schedule 4.

Schedule 4 - Design speed of road classes

4 Cross-section

4.1 General requirements for arrangement of highway cross-section

4.1.1 Arrangement of elements such as roadway, shoulder, median strips, frontage roads, and auxiliary lanes (climbing lanes, acceleration/deceleration lanes) on road cross section must be appropriate for traffic arrangement and allow all vehicles (motor vehicles, mopeds, non-motorized vehicles) to move safely, conveniently and utilize roads effectively.

Depending on design class and design speed of roads, arrangement of the aforementioned elements must adhere to traffic arrangement under Schedule 5.

Schedule 5 - Traffic arrangement on road cross section

4.1.2 Minimum width of elements on cross-section depends on design class of road under Schedule 6 for plain and hill terrain, Schedule 7 for mountainous regions.

Schedule 6 - Minimum width of elements on cross section for plain and hill

Schedule 7 - Minimum width of elements on road cross section in mountainous regions

4.1.3 Designing road cross section requires close study of land use planning of regions through which the road travels, consider investment staging for cross-section area (for class I and class II roads), considers land reservation for upgrade, future expansion, identifies road margin in accordance with applicable regulations of the Government.

4.2 Roadway

4.2.1 Roadway consists of an integer number of lanes. This number is preferably an even number unless either direction of travel facilitates significantly larger traffic than the other or special traffic arrangement is in effect.

4.2.2 Number of lanes on cross section is dependent on road class as defined under Schedules 6 and 7 and must be compliant with the formula:

where:

n_lx means the required number of lanes rounded in accordance with 4.2.1;

N_cđgio means design flow during peak hour compliant with 3.3.3;

N_lth means actual traffic capacity, in the absence of study and calculation can be:

- 1800 xcqđ/h/lane where median strips separating directions of travel and lane separators for separating motor vehicles from non-motorized vehicles are available;

- 1500 xcqđ/h/lane where median strips separating directions of travel but not lane separators for separating motor vehicles from non-motorized vehicles are available;

- 1000 xcqđ/h/lane where median strips separating directions of travel and lane separators for separating motor vehicles from non-motorized vehicles are not available.

Z refers to traffic capacity coefficient:

0,55 for V_tk ≥ 80 km/h;

0,55 for plan and 0,77 for mountainous region for V_tk = 60 km/h;

0,85 for V_tk ≤ 40 km/h.

The number of lanes must be calculated using the above formula if number of lanes in roadway is expected to exceed values defined under Schedule 6 and Schedule 7.

4.2.3 Width of one lane

Usually, width of a lane notwithstanding of class conforms to values under Schedule 6 and Schedule 7.

4.3 Shoulder

4.3.1 Depending on road class, a minimum width of shoulder must be reinforced in accordance with Schedule 6 and Schedule 7 (values in brackets). Structure of reinforced shoulder conforms to Article 8.8.

4.3.2 Roads with minimum design speed of 60 km/h must be outfitted with guiding strips. Guiding strips mean the solid (white or yellow) lines of 20 cm in width on reinforced shoulder and close to roadway. Where crossing traffic is allowed such as junction or lane merger, guiding strips shall be dotted lines (in accordance with road signaling regulations). Where lane separators are installed on class III roads to separate cyclist lanes on reinforced shoulder, the guiding strips shall be 2 solid white lines the width of each line is 10 cm and each line is 10 cm away from the other (adding up to a total width of 30 cm).

4.3.3 Where auxiliary lanes such as climbing lanes, acceleration/deceleration lanes are built, these auxiliary lanes will be in position intended for reinforced shoulder. The remaining width of shoulder must be at least 0,5 m either naturally or by expanding.

4.3.4 Roads for non-motorized vehicles

In respect of class I and class II roads, lanes for non-motorized vehicles must be separated from lanes for motorized vehicles (Schedule 5) and accommodate local vehicles on frontage roads; in respect of class III roads, lanes for non-motorized vehicles must be located on reinforced shoulder (which is separated from lanes for motorized vehicles via lane separators, see 4.5).

Width of cyclist lanes (b) in one direction is expressed in meters and determined using the formula below:

b = 1 x n + 0,5

In which: n refers to the number of cyclist lane in a direction.

Traffic capacity of each cyclist lane is 800 bicycles/h/direction. Where cyclist lanes are located on reinforced shoulder, the reinforced shoulder can be expanded to accommodate width b (width of reinforced shoulder in this case equals b plus width of lane separators). Width of cyclist lane must be further examined for travel capacity of other non-motorized vehicles.

4.3.5 Surface of lanes for non-motorized vehicles must have equal evenness to adjacent lanes for motor vehicles.

4.4 Median strips

4.4.1 Median strips shall only be built if the roads consist of at least four lanes (see Schedule 5) and comprise separation area and reinforced safety strips to the side.  Minimum dimensions of median strips are specified under Schedule 8, Figure 1.

Schedule 8 - Structure of median strips

legend:

a) elevated;              b/ same elevation, coated;                          c/ depressed to collect water

Figure 1 - Design of median strip

4.4.2 Where road base is separated into two parts, width of a road base serving one direction consists of roadway and two shoulders where the right shoulder complies with Schedule 6 or Schedule 7 depending on the terrain and the left shoulder has preset width and 0,5 m of reinforced shoulder width. Guiding strips of 0,2 m in width are required at the edge of reinforced shoulder.

4.4.3 Where median strip is less than 3 m in width, separation segment is coated and surrounded by curb.

Where median strip is 3 m to 4,5 m in width:

- where median strip is outfitted with curb, soil in the separation segment must not stain road surface (soil level is lower than the curb), the curb must have a minimum height of 18 cm and containing compressed clay to prevent water from seeping into the road surface below.

- grass or bush vegetation of up to 0,8 m in height is recommended.

- where median strip is more than 4,5 m in width (to reserve for future lane expansion, or division of individual road bases), a concave design is recommended together with structures for collecting water and preventing water from seeping into the road base. Shoulder structure complies with Article 4.4.2.

4.4.4 Median strips must be interrupted to facilitate turning facilities. Turning facilities must be located:

- at least 1 km away from one another (where width of median strip is below 4,5 m) and up to 4 km away from one another (where width of median strip is greater than 4,5 m);

- before major tunnels and bridges.

Length and shape of turning facilities must be sufficient to allow tri-axle trucks to turn around. Turning facilities must conform to vehicle trajectory and prevent vehicle from colliding with the curb.

4.5 Lane separators

4.5.1 Lane separators shall only be built for cases detailed under Schedule 5 to separate lanes for bicycles and non-motorized vehicles on reinforced shoulders (or expanded reinforced shoulder) from lanes for motorized vehicles.

4.5.2 Placement and design of lane separators can utilize any of the following solutions:

- two solid lines in accordance with 22 TCN 237 (for class III roads);

- soft crash barriers (corrugated plates). Height measured from road surface to the top of the plates shall be 0,8 m.

In the aforementioned cases, the crash barriers can be located on reinforced shoulder as long as road margins are at least 0,25 m away from the outermost lanes for motor vehicles.

Width of lane separators consists of: width of barriers (or line markings) plus width of road margin.  

4.5.3 Lane separators must be interrupted at most every 150 m for drainage purposes. Turning facilities for non-motorized vehicles must overlap turning facilities for motorized vehicles in accordance with Article 4.4.4.

4.6 Frontage road

4.6.1 Frontage roads are roads on either or both sides of class I and class II roads that:

- prevent all vehicles (motorized, non-motorized, foot travel) from freely entering, exiting class I, class II roads;

- satisfy travel demand of all vehicles above in either or both directions (between designated entries and exits to class I and class II roads).  

4.6.2 Class I and class II roads, frontage roads shall be situated in locations with significant traffic such as: roads travelling through residential areas, industrial parks, tourist attractions, forestry and agricultural farms, etc. Where frontage roads are not feasible (due to investment staging, difficulties, etc.), Article 4.6.6 must be adhered to.

Determination of local traffic demand above must be examined and forecasted in accordance with economic, cultural, societal planning of each segment where frontage roads are to be built.

4.6.3 Frontage roads must be physically separate from class I and class II major arterial roads. The length of each frontage road segment (between designated entries and exits of class I and class II road) must be at least 5 km. Frontage roads can be on both sides of arterial roads; each side may accommodate one or two directions of travel (to best suit local traffic); Frontage roads on both sides of major arterial roads can be connected via underpasses or overpasses (without intersecting major arterial roads) between designated entries and exits.

4.6.4 Frontage roads can be located in road margin of class I and class II major arterial roads. In this case, road margins shall conform to applicable regulations from the edge of the outermost structure of frontage roads.

4.6.5 Frontage roads shall be designed in accordance with standards for class V and class VI roads (in plains or hills) as long as width of road base can be reduced to a minimum of 6 m (in case of two-way frontage roads) and a minimum of 4,5 m (in case of one-way frontage roads). Cross section of frontage roads shall be determined by design counseling entities, depending on practical situation.

4.6.6 Where frontage roads are not built, class I and class II roads must be isolated from lanes for bicycles and non-motorized vehicles on reinforced shoulder, protected by crash barriers that are at least 0,8 m high from road surface.

4.7 Auxiliary climbing lanes

4.7.1 Accounts shall be taken to build additional auxiliary climbing lanes if all requirements below are met:

- flow of climbing vehicles exceeds 200 vehicles/h;

- flow of climbing trucks exceeds 20 vehicles/h;

- longitudinal gradient ≥ 4 % and length of slope ≥ 800 m.

Where construction of climbing lanes is expected, it is necessary to compare economic - technical indicators between building climbing lanes or lowering longitudinal gradient.

Climbing lanes are usually considered for two-lane roads without median strips and with limited overtaking conditions.

4.7.2 Structure and arrangement of climbing lanes

- width of climbing lane is 3,5 m or 3 m in case of difficult conditions;

- climbing lanes should be separate lanes or built on reinforced shoulder; where width of reinforced shoulder is insufficient, it is permissible to expand to maintain a minimum of 3,5 m in width of climbing lanes and 0,5 m in width of ground shoulder (these climbing lanes are shared by bicycles, non-motorized vehicles and trucks).

- transition to climbing lanes must be located 35 m before the climb and expand at a ratio of 1:10; transition to primary lanes must taper at a ratio of 1:20 (with the length of the transition is 70 m).

4.8 Acceleration/deceleration lane

Entries and exits from class I and class II roads must be outfitted with acceleration/deceleration lanes. Structure of acceleration/deceleration lanes conforms to Article 11.3.5.

4.9 Lateral gradient

Lateral gradient of elements on cross section of straight roads is specified under Schedule 9. Lateral gradient on curved roads must conform to regulations pertaining to superelevation (Article 5.6).

Schedule 9 - Lateral gradient of elements on cross section

4.10 Vertical clearance

4.10.1 Vertical clearance means spatial limits allowing vehicle movement. Obstacles, structures associated with roads such as signs, lamp posts, etc. are not allowed in the vertical clearance.

4.10.2 Minimum vertical clearance of road classes is compliant with Figure 2. In case of renovated roads, vertical clearance is not required to be increased as long as such vertical clearance is not lower than 4,30 m in case of difficulties. If this is the case, overhead structure indicating the vertical clearance must be installed at least 20 m leading up to the area with limited vertical clearance.  

Vertical clearance of junction between motor vehicle roads and railway conforms to 22 TCN 272 (depending on track gauge and type of locomotive).

All dimensions in meter

legend:

a) Roads with V_tk ≥ 80 km/h and median strips;

b) Roads of all classes without median strips;

Figure 2 - Vertical clearance of roads

4.10.3 Where lanes for non-motorized vehicles (or footpath) are isolated from those for motor vehicles, minimum vertical clearance of lanes for non-motorized vehicles and footpath are a rectangle of 2,5 m high, 1,5 m wide. This vertical clearance may be adjacent to vertical clearance of lanes for motor vehicles or separated by separators in the same manner as vertical clearance in tunnels (Figure 3).  

4.10.4 Vertical clearance in tunnels conforms to applicable tunnel design standards and is illustrated in Figure 3.

Shoulder shall be utilized to accommodate crash barriers.

All dimensions in meter

Note: The left side indicates the case where footpath and cyclist lanes are connected to roadway; the right side indicates the case where footpath and cyclist lanes are isolated from roadway.

Figure 3 - Vertical clearance in tunnels

4.10.5 Width of roadway on bridges:

- bridges with length ≥ 100 m, road width conforms to vertical clearance standards of bridge design;

- bridges with length < 100 m, road width equals roadway width plus necessary width to allow traffic of pedestrians and non-motorized vehicles that is not wider than road base width;

- bridges with length < 25 m, road width equals bridge gauge.

5 Topographic map and longitudinal section

5.1 Visibility

5.1.1 It is necessary to maintain visibility to improve driving safety and psychological reliability to drive at design speed.

Minimum values of stopping sight distance, oncoming vehicle sight distance, and overtaking sight distance are specified under Schedule 10.

Schedule 10 - Minimum sight distance on road

Sight distances are measured from eye elevation of drivers at 1 m above the roadway to oncoming vehicles of 1,2 m in height and road obstacles of 0,1 m in height.

5.1.2 Sight distance must be inspected during design. Obstacles in areas with restricted visibility must be removed (felling trees, cutting slopes, etc.). Removed obstacles must be 0,3 m lower than line of sight. In case of extreme difficulty, it is permissible to employ mirrors, warning signs, speed limit signs, or no overtaking signs.

5.2 Road elements on topographic map

5.2.1 Roads on topographic map consisting of straight segments are connected by circular curves. Where design speed is ≥ 60 km/h, straight line will be connected to circular curves via transition curves.

5.2.2 Where curves of opposite direction meet, sufficient length must be given to accommodate transition curves or superelevation.

5.3 Curves on topographic map (horizontal curves)

5.3.1 Horizontal curves of minimum radius shall only be used in extremely difficult situations. Curves of regular minimum radius are recommended for regular application and curves should conform to natural terrain to provide best driving quality.

Regulations on curve radius are specified under Schedule 11.

Schedule 11 - Minimum horizontal curve radius

5.4 Expansion of roadway in curves

5.4.1 Inner lanes of curves must be expanded. Where radius of horizontal curves ≤ 250 m, roadway shall be expanded to an extent specified under Schedule 12.

5.4.2 Where roadway consists of more than 2 lanes, each lane must be expanded by 1/2 of the values under Schedule 12 and divisible by 0,1 m.

In respect of vehicle types with special models, it is necessary to re-examine values under Schedule 12.

5.4.3 Expansion must be implemented on both sides of the curves. In case of difficulties, it is permissible to implement expansion on either side of the curves.

Schedule 12 - Expansion of roadway to both sides of horizontal curves

Dimensions are in millimeter

5.4.4 The expansion is located on reinforced shoulder. Guiding strips (and other structures such as auxiliary lanes for non-motorized vehicles, etc.) must be located on the right side of the expansion. For the purpose of expanding road base, the remaining earth shoulder must be of at least 0,5 m in width.

5.4.5 The expansion overlaps superelevation or transition curve. Where these two elements are not available, the expansion shall be:

- evenly distributed on the straight line and the curve;

- located on the transition and expanding evenly (linearly). Expand by 1 m over a minimum length of 10 m.

5.5 Superelevation and superelevation transition

5.5.1 Superelevation means the outer edge of the roadway banked towards the inner edge of the roadway.

Superelevation gradient conforms to radius of horizontal curve and design speed under Schedule 13. Superelevation gradient must not be greater than 8% and must not be lower than 2%.

5.5.2 Shoulder of the reinforced segment must be of the same gradient and direction as the superelevation; shoulder of the non-reinforced segment on the outer edge of the curve must be sloped towards the outer edge of the curve.

5.5.3 Separate roadway should be located on separate superelevation.

5.5.4 The length of superelevation transition (where the curve contains superelevation) must not be lower than the values specified under Schedule 14.

Schedule 13 - Superelevation gradient corresponding to horizontal curve radius and design speed

5.5.5 Superelevation transition

Superelevation is implemented by rotating roadway on the outer edge of the curve around the center of the road until roadway is of the same gradient and until superelevation gradient is achieved. Where superelevation median strips are built, they shall rotate around inner or outer edge of the road.

5.5.6 Superelevation transition and expansion transition shall overlap transition curve. Where transition curves are not built, these transitions shall be evenly distributed on the curves and the straight line.

5.6 Transition curve

5.6.1 Where V_tk ≥ 60 km/h, transition curves are required to connect straight lines to circular curves and vice versa.

5.6.2 Superelevation gradient (isc) and length of superelevation transition (L) are dependent on radius of horizontal curve (R) and design speed (V_tk) and not lower than values under Schedule 14.

Schedule 14 - Superelevation gradient (isc) and length of superelevation transition (L)

5.6.3 Transition curves can be an Euler spiral, a cubic parabola, or a curve with multiple arcs.

5.7 Longitudinal gradient

5.7.1 Depending on design class of roads, maximum longitudinal gradient is specified under Schedule 15. In case of difficulty, the maximum longitudinal gradient can be increased by 1% to a maximum of 11%.

Gradient of roads located at a height of 2000 m above sea level must not exceed 8 %.

5.7.2 Longitudinal gradient of roads travelling through residential areas must not exceed 4%.

5.7.3 Longitudinal gradient in tunnels must not be greater than 4 % and must not be lower than 0,3 %.

5.7.4 Longitudinal gradient of cut roads must not be lower than 0,5 % (or 0,3% in case of difficulty and up to 50 m in length).

Schedule 15 - Maximum longitudinal gradient of road classes

5.7.5 Length of the approach to vertical curves must not exceed values under Schedule 16; where length of the approach to vertical curves, segments of 2,5 % gradient and sufficient length for vertical curve are required.

Schedule 16 - Maximum length of vertical curve

All dimensions in meter

5.7.6 Minimum length of change in grade line slope must be sufficient to accommodate vertical curve and must not be lower than values under Schedule 17.

Schedule 17 - Minimum length of change in grade line slope

5.7.7 In case of horizontal curves of small radius, vertical curves in Schedule 16 must be lowered in accordance with Schedule 18.

Schedule 18 - Reduction to longitudinal gradient of horizontal curves with small radius

5.8 Vertical curve

5.8.1 Change in grade line slope in longitudinal section (more than 1 % where design speed ≥ 60 km/h, and more than 2 % where design speed < 60 km/h) must be connected via vertical curve (summit-type or valley-type)

- These curves can be either circular curves or parabolas.

5.8.2 Radius of vertical curve must conform to terrain, aid driving process and aesthetic of roads and must not be lower than values under Schedule 19.

Schedule 19 - Minimum radius of summit-type and valley-type vertical curve

5.9 Hairpin curves

5.9.1 The use of hairpin curves must be very restricted except for use on mountain terrain.

5.9.2 Technical specifications at turning facilities on hairpin curves are specified under Schedule 20.

Schedule 20 - Technical specifications for turning facilities on hairpin curves

6 Cooperation of route elements

6.1 Cooperation of road elements:

- in creating adequate visibility, providing adequate information to allow drivers to respond to situations;

- in creating reliability and convenience to allow drivers to perform well, effectively and without fatigue;

- in avoiding obstructed locations, hallucinating or distracting locations;

- in establishing structure conforming to natural scenery, improving natural beauty of the vicinity.

6.2 Provisions under 6.1 are mandatory in respect of roads with design speed of exceeding 800 km/h,, recommended for roads with design speed of exceeding 60 km/h and reference for roads of other road classes.

6.3 The design process must be conscious effort of designers and must not hike up construction costs. Where construction costs increase, investment effectiveness must be taken into account.

6.4 Elements on topographic map

6.4.1 When depicted on topographic map, it is better to implement routes with multiple large-radius curves than straight routes with shorter curves, routes that utilize the terrain (forest edge, hill, river bank) than routes that cut through geographic features or requiring special structures (walls, road bridges, etc.).

6.4.2 Small angle of change requires large horizontal curve radius. See Schedule 21.

Schedule 21 - Minimum horizontal curve radius dependent on angle of change

6.4.3 Design process should avoid sudden changes:

- radius of a curve must not be greater or lower than that of an another adjacent curve by twofold;  

- horizontal curves of minimum radius must not be located at the end of straight routes;

- length of the curve should be equal to or greater than the transition curve leading up to the curve.

6.4.4 Where two directions of travel on a road are separate, it is recommended to design two routes with independent road bases and expanded median strips to incorporate the terrain or two separate road bases to save materials and achieve better-looking and more stable structures.

6.4.5 In respect of high-class roads, it is recommended to connect to and from horizontal curves via continuous Euler spiral.

6.5 Incorporate topographic map and longitudinal section

6.5.1 Avoid placing multiple vertical curves on long straight lines (or horizontal curves with large radius) to avoid routes with obscured sections.

In order to avoid convoluted routes, it is recommended to avoid placing multiple curves along straight routes.

6.5.2 It is recommended to design equal number of horizontal curves as number of vertical curves with overlapping peaks. Where deviation is required, deviation between peaks of horizontal curves and vertical curves must not be greater than 1/4 the length of horizontal curves.

6.5.3 It is recommended to design horizontal curves curving towards the outside of vertical curves.

6.5.4 Vertical curves of small radius must not situated inside of horizontal curves. Radius of valley-type curve (Rlom) must be greater than that of horizontal curve (Rnam).  

6.6 Combination with natural scenery

6.6.1 Geographic and natural elements of the area must closely studied to reasonably incorporate without violating natural rules, avoid structures that require cutting deep, filling high and avoid using specialized structures.

6.6.2 Regulations on cut slopes (Schedule 24 and Schedule 25) must be based on mechanical principles of soil and rocks.  The slopes can be:

- altered to conform to horizontal slopes common in the area;

- rounded at the top and expanded on both ends;

- slopes that are less than 1 m should be of a 1:4 ~ 1:6 gradient due to insignificant amount of materials involved and rounded at the top and bottom;

- slopes of a greater height should be outfitted with steps. Steps help stabilize the slopes, prevent erosion and should have shrubs planted around.

7 Road base

7.1 Basic requirements and design principles

7.1.1 Road base must be stable, capable of retaining geometric dimensions, sufficiently strong to withstand vehicular load and natural factors throughout its useful life.

In order to develop appropriate design solutions, it is necessary to survey geographic, topographic, hydrographic conditions (especially presence of water sources, factors that eroding, destroying road base) and closely study relevant data.

Construction of road base must keep destruction to existing natural balance to the minimum, must not negatively affect the environment, must not destroy regional scenery. Do note that once the environment is destroyed, the road base will be destroyed due to lack of stability.

7.1.2 Design principles

7.1.2.1 Effective area of road base (in the absence of special calculation, such area is determined within 80 cm downwards from the bottom layer of pavement) must always:

- not be too humid and not be affected by external humidity sources (rainwater, groundwater, surface water);

- be able to sustain minimum CBR of 8 for class I, class II roads or 6 for roads of other classes with the topmost 30 cm;

- be able to sustain minimum CBR of 5 for class I, class II roads and 4 for roads of other classes within the next 50 cm.

In which: CBR indicates load-bearing capacity during lab test under the condition that soil is at standard compression according to 22 TCN 332 and soaked to saturation for 4 days.

7.1.2.2 Road base on weak soil

Compliant with 22 TCN 262.

7.1.2.3 Road base in areas with complicated geological conditions shall be compliant with 22 TCN 171.

7.1.2.4 Road base in areas prone to earthquake shall be compliant with 22 TCN 211.

7.1.2.5 In order to avoid harmful effect on the environment and scenery, accounts must be taken to the following principles:

- minimize destruction to vegetation. If possible, organic soil removed from cut road base should be reused to refill where soil was previously removed and fill slopes;

- minimize destruction of natural balance; avoid digging too deep, filling too high, and aim to balance amount of materials removed and fill. In respect of dangerous terrain, accounts should be taken to compare road base solution to road bridge, tunnel solutions. Height of slopes around road base should not exceed 20 m;

- where slope gradient exceeds 50%, consideration should be taken to build 2 independent road bases;

- areas around cut road base and low-fill road base should be eased up to the road bases (1:3 ~ 1:6) and smoothed out to conform to the terrain and traffic safety;

- minimize negative impact on social and economic life of local inhabitants. Location and clearance of drainage structures must be sufficient to not obstruct flood or destroy other foundations and bases, avoid obstruction to internal local traffic, respect local drainage planning.  

7.2 Road base width

Width of road base and width of elements of road base specified Schedule 6 and Schedule 7 are minimum values. Where additional lanes are designed, where median strips with coatings are placed with structural pillars or without coatings, where lane separators are placed, or where width of cyclist lanes must be increased according to calculation (b), designers must re-determined design width of road base.  

7.3 Road base elevation

7.3.1 Design elevation of road base is determined at the center of the road. In case of two independent road bases, there will be two design elevations on two separate longitudinal sections.

7.3.2 Design elevation on the edge of the road in river banks, approaches of small bridges, sewers, and flooded fields must be at least 0,5 m higher than flood level at calculated frequency in Schedule 30. Flood level must also include height of wake and waves crashing into the slopes.

In case of difficulty, especially when roads travel through densely populated areas and flood level is not sustained for more than 20 days, calculation of flood frequency should take into account economic, technical norms and environmental impact. When necessary, it is permissible to propose a decrease to design elevation of road base. In this case, design counselors are responsible for proposing and examining long-term stability of the structure and competent individuals entitled to investment decision shall decide.

7.3.3 Elevation of the bottom layer of pavement must be higher than the calculated groundwater level (or common standing water level) according to Schedule 22.

Schedule - Minimum height from calculated groundwater level (or common standing water level) to the bottom layer of pavement

All dimensions are in cm

7.3.4 Road elevation where circular manholes are located must be at least 0,5 m higher than top of the manholes.

Where thickness of pavement is greater than 0,5 m, such difference in height must be sufficient to facilitate construction of pavement thickness.

7.4 Materials forming road base

7.4.1 Soil forming the road base is gathered from cut bases, earthen piles, borrow pits. Soil collection must minimize impact on the environment as depicted under 7.1.2.5. Design and shape of borrow pits must not affect scenery and allow utilization following roadwork.

Soil, regardless of sources, must be tested for segregation and placed into layers.

All layers must be in alternating order. Where layers of soil with good permeability is on top of layers with poor permeability, the surface of the bottom layer must be inclined by 2 % to 4 % for drainage purposes.

7.4.2 Soil mixed with salt and plaster (more than 5%), humus, peat soil, eluvia soil, and mush (more than 10 % in concentration of organic matter) must not be used as road base.

Heavy clay with free swell ratio of exceeding 4 % must not be used in affected area (see 7.1.2.1).

Dust soil and weathered rock must not be used to fill the body of road base that can be flooded.

Materials of loose particles with high angle of internal friction should be used on road segments following bridge abutments and next to walls.

Where materials used are waste rock, soil and gravel, the biggest particle size is 10 cm for areas within 80 cm of affected areas and 15 cm for areas below; particles of this largest size must not be used more than 2/3 the thickness of the compressed soil (depending on the compressing equipment).

7.4.3 Weathered rock and easily weathered rock (schist, etc.) must not be used to fill road base.

7.4.4 Where road base is fill with sand, road base must be fill towards both slopes on the sides and road base top to prevent surface erosion and facilitate vehicle movement and pavement construction machinery. Soil fill both sides of the slopes must have minimum elasticity of 7; soil used on the top of the road base should be hill subbases. Soil used on the top of the road base must not be of loose materials so as to prevent rainwater and surface water from infiltrating the sand.

Thickness of materials deposited on both slope side must be at least 1,0 m and on top of road base (bottom of pavement) must be at least 0,3 m.

7.5 Processing natural soil before filling

7.5.1 Where natural base is less than 20 % in inclination, it is necessary to remove the organic soil and deposit directly.

Where natural base is 20% to 50% in inclination, the base must be formed into steps before road base is fill.

Where natural base is exceeding 50% in inclination, supporting structures (walls, rockfill, road bridges, overhang bridges, etc.) are required.

7.5.2 The bottom of fill base must adopt drainage solutions and solutions for preventing runoff from nearby slopes from stagnating at the bottom of the slopes.

Where fill base travels through paddy fields and areas with regular standing water, sludge dredging and soil replacement design is required. If possible, loose materials wrapped in layered filters (such as geotextile) should be used in areas with regular standing water or lime mixed with cohesive soil should be added for the purpose of soil replacement.  

7.5.3 Processing of natural base that is weak soil prior to road base filling must be compliant with 22 TCN 262.

7.5.4 In respect of special areas such as areas with moving sand, karst areas, and areas with complicated geological conditions, it is necessary to conduct geology survey and test to calculate and find structural solutions to achieve stable road base. These solutions must be appropriate to class of roads, road structures, and local geological conditions.

7.6 Road base compression

7.6.1 Road base must be of appropriate compaction under Schedule 23. In addition, body of road base that is affected by flood or ground water must be of minimum compaction of 0,95 regardless of road class. Road base following abutments and walls should have compaction higher than the required values under Schedule 23 by 1 % to 2 %.

Schedule 23 - Required compaction of road base (compacted in accordance with 22 TCN 333)

7.6.2 Within the affected area, compacted soil must have load-bearing capacity compliant with CBR under 7.1.2.1. Where soil cannot be compacted to required compaction under Schedule 23 or cannot achieve the required CBR after compaction, soil improvement, fortification, or replacement design is required to simultaneously meet all requirements below (conduct test to identify appropriate lime ratio, improvement ratio).

7.7 Design of cut slopes

7.7.1 Gradient of cut slopes

Depending on geological conditions and height of slope under Schedule 24, choose gradient of cut slopes. Prior to that, examine gradient of natural slopes with similar geological conditions in areas adjacent to area where the roads in question are located.

Schedule 24 - Gradient of cut slope

7.7.2 Where height of slopes is greater than 12 m, stability analysis and audit must be conducted via appropriate measures corresponding to the most disadvantages condition (water-saturated soil, rock). In case of slopes made from loose and less cohesive materials, sliding method on flat surface will be adopted; in case of slopes made from cohesive soil, sliding method on round surface will be adopted and minimum stability coefficient must be 1,25.

In case of slopes made from rocks, analysis and comparison with gradient of stable slopes (slopes of road base, structures, or natural geological features) in adjacent areas must be made.

7.7.3 Where slopes go through multiple layers of rock and soil, varying gradient is required to create a break in slope surface or, where gradient changes, bench of 1 m to 3 m in width and 5% to 10% inclined insloped towards the drain of rectangular or triangular in shape is required.

The aforementioned design is also required where slopes go through single layer of rock or soil at great height. In this case, each bench must be 6 m to 12 m away from the next.

7.7.4 Where slopes are prone to collapse of failure, a bench of minimum width of 1 m is required between the side drains to the bottom of the slopes. This bench is not required if protective walls are built or where slopes are less than 12 m in height.

7.7.5 Slopes of cut road base must be reinforced against erosion, localized landslide (such as planting vegetation, bush, anchoring, etc.). When necessary, protective walls and walls at the bottom of the slopes must be built to improve overall stability.

7.7. 6 Excess soil removed from cut road base must be disposed properly and must not be disposed down the fill slopes, or paddy fields, gardens, rivers and streams below. The pile of disposed soil must be leveled, planted with protective vegetation, and outfitted with appropriate drainage solutions.

7.8 Design of fill slope

7.8.1 Depending on height of peak of slopes and materials involved, gradient of fill slopes is specified under Schedule 25.

Schedule 25 - Gradient of fill slope

7.8.2 In case of fill slopes reinforced by rock, rocks must be of a size larger than 25 cm and stacked without binding (but with wedging) within 1 m to 2 m of thickness corresponding to gradient under Schedule 25; depending on gradient, stacking without binding and without even gradient is allowed. Areas contained by rock stacking without binding can be reinforced by piling large-sized rocks into layers followed by smaller-sized rocks into layers and operating heavy vibratory drum rollers until surface rocks are stable. Test placement is required to determine thickness of each layer, quantity of rocks required, and quantity of vibratory drums required. Test results serve as the basis for examining and commissioning rock surface (including its compactness).

7.8.3 In case of soil (or sand) road base travels through flooded areas, slope gradient must range from 1:2 to 1:3 for road base sections that are lower than regular flood level and 1:1,75 to 1:2 for road base sections that are lower than design water level.

7.8.4 Where slopes of soil-filled road base, a bench of 1 m to 3 m in width is required every 8 m to 10 m in height. Such bench must be insloped and outfitted with drain in accordance with 7.7.3. High slopes should be reinforced by rocks or pre-cast concrete slabs.

7.8.5 In case slope height is greater than 12 m, stability audit in accordance with 7.7.2 must be performed. Stability audit of flooded road base must take into account hydrodynamic pressure caused by hydraulic gradient. Earthened slopes must not be higher than 16 m; rock slopes must not be higher than 20 m.

7.8.6 Where slopes are high and steep and stability audit results are not satisfactory, supporting solutions are required to increase stability (embankment, armored), rock stacking without binding, with plastering or cement.

7.8.7 Slopes of fill road base must be reinforced in a manner appropriate to onsite hydrography and climate conditions in order to prevent erosion caused by rain, flow, waves, and change to flooded water level.

7.8.9 Borrow pits must be planned in advance and approved by local authority in the following manner:

- utilize deserted areas, areas where soil is of adequate quality and extraction condition is met;

- the environment is not affected, land use is efficient;

- combine land utilization with agriculture, aquaculture practices (where water is located, etc.).

8 Pavement and reinforced shoulder

8.1 General provisions

All lanes for motor vehicles and non-motorized vehicles, acceleration/deceleration lanes, climbing lanes, reinforced shoulder, road margins, and points-of-service of roads of all classes must be paved.

Pavement shall be designed in on the basis of traffic volume and composition, road class, use purpose of structures, construction materials, natural conditions, and applicable regulations. Pavement must be of sufficient strength, proper waterproofing, and durable throughout design period in order to withstand destructive impact of vehicles, climate, weather while possess adequate surface properties (grip, evenness, drainage, low dust) to enable safe, smooth traffic and protect the environment.

8.2 Standard load

Standard load is compliant with 22 TCN 211 for flexible pavement and 22 TCN 223 for rigid pavement.

8.3 Pavement design

8.3.1 Pavement design of highway may consists of surface course (one to three layers), foundation course (upper foundation course, lower foundation course).  Pavement can be located on the foundation or the topmost layer of the road base (see 8.3.7). Depending on the type of pavement (flexible or rigid), design vehicle volume, and road class, pavement can comprise all courses and layers above or one or two layers with multitude of functions.

8.3.2 Choosing types and placing surface courses

Choose among surface course types under Schedule 26.

Schedule 26 - Types of surface course

8.3.3 In order to limit reflection crack, where asphalt roads employ upper foundation layer (or lower layer) in form of soil or rock reinforced by inorganic binders, minimum thickness of the asphalt surface above (asphalt concrete, impregnated bitumen, bituminous surface treatment) should conform to Schedule 27 depending on road class.

Schedule 27 - Minimum total length of asphalt surface required on foundation of soil and rock reinforced with inorganic binders

8.3.4 Soil, rock, sand reinforced with binders (organic or inorganic) shall be used as upper or lower foundation materials for high grade pavement A1. In respect of road surface of cement concrete without steel rebar, foundation of soil, sand, rock reinforced by inorganic binders (cement, lime) of a minimum thickness of 15 cm is required.

In respect of asphalt concrete surface, it is possible to use chipping aggregate or hollow asphalt concrete as the upper foundation; type II chipping aggregate according to 22 TCN 334, macadam chipping or natural aggregate as lower foundation.

8.3.5 Choosing foundation of other materials for other road surface:

It is possible to use reinforcing soil, rock, sand, chipping aggregate, macadam aggregate, natural aggregate as foundation for high grade A2 and low grade surfaces.

8.3.6 Foundation course should be at least 20 cm wider than surface course towards either side.

8.3.7 Lower foundation layer (lower pavement layer)

Lower foundation layer:

- Creates roadway with uniform and adequate load-bearing capability;

- prevents humidity and water from infiltrating the ground and from the ground to the pavement foundation;

- creates anvil effect to maintain compaction quality of foundation layers above.

- allows vehicles to operate during construction of pavement without damaging the ground below (even in poor weather conditions).

The bottom foundation layer is made of appropriate materials in order to achieve:

- high compactness k = 1,00 ~ 1,02 (standard compactness);

- elasticity modulus E ≥ 50 MPa (500 daN/cm²) or CBR ≥ 10 (depending on type of soil);

- minimum thickness of 30 cm.

Bottom layer of foundation is required in order to replace 30 cm of the topmost soil section in class I, class II, and class III roads with at least 4 lanes if the topmost soil section of the roads does not meet requirements above. Bottom layer of foundation is required where road base is filled by sand, swelling clay and where roads are located in areas with regular rainfall or affected by different sources of humidity.

Materials of bottom layer of foundation can be soil with good aggregate (without using sand of all kinds), natural aggregate, lime-fortified soil (cement) at a low ratio.

Bottom layer of foundation should be wider than foundation course by 15 cm towards either side.

8.3.8 In any case, road surface of all grades should utilize on site materials (including industrial wastes) as the lower foundation layer and the bottom layer of foundation as long as prior study and experiment and approval by competent authority are provided.

8.3.9 Thickness of all layers in pavement must take into account construction conditions and minimum thickness of each material.

Minimum thickness is determined to be 1,5 times the size of the largest aggregate particle in the layer. Effective compact thickness of regular asphalt concrete must be within 8 cm to 10 cm; effective compact thickness of other reinforced materials must not be greater than 15 cm and of non-reinforced materials must not be greater than 18 cm.

8.3.10 Apply cohesive layer between asphalt concrete layers and between asphalt concrete layer and other asphalt road surfaces if these surfaces are not built close to each other chronologically and if asphalt concrete is applied on top of the old asphalt concrete layer.

8.3.11 Apply cohesive and absorbent asphalt layer when situating asphalt road surfaces on top of foundation of reinforced soil, rock and foundation of chipping aggregate, natural aggregate, macadam aggregate.

8.3.12 Apply bituminous surface treatment on top of foundation of chipping aggregate or foundation of other loose materials to prevent water from seeping into road base and damage caused by construction machinery if construction of the foundation takes place before construction of other layers by a definite period of time.

8.4 Calculation of pavement

Structure, calculation, and design of pavement of highway must conform to flexible pavement design standards and rigid pavement design standards.

Other solutions that have adequate basis and parameters can be used for reference and calibration. Several pavement solutions must be developed depending on conditions and material costs for the purpose of economic and technical comparison. Where roads perform important functions and have little traffic in the first few years, investment on pavement structures should be divided into multiple stages (on the basis of long-term structural design).

Lateral slope of pavement is determined by materials of the surface course under Schedule 9 and must not be lower than 1,5%.

At superelevation and junctions where slope gradient cannot be lower than 1,5%, these sections must be kept to a minimum in length.

8.5 Traction

8.5.1 The topmost surface, when necessary, must be coated with a layer for traction with appropriate micro structure in order to maintain average depth of sand filling H_tb (mm) according to standards depending on design speed and danger level of roads according to Schedule 28.  

Schedule 28 - Traction requirement (according to 22 TCN 278)

8.5.2 Design of the pavement surface may employ traction evaluation using traction factors such as applying emergency brake on heavy-duty vehicles or other methods that employ pendulums.

8.5.3 Roads that do not meet traction standards must be outfitted with slip warning signs and speed limit signs.

8.6 Roughness

8.6.1 Roughness of roads determined via IRI (mm/km) must be compliant with Schedule 29.

Schedule 29 - Roughness requirement according to IRI (according to 22 TCN 277)

8.6.2 Roughness can also be evaluated using 3 m long ruler according to 22 TCN 16

In respect of high grade A1 surface (asphalt concrete, cement concrete), 70% of the slits must be less than 3 mm and the remaining 30% of the slits must be less than 5 mm. In respect of high grade A2 surface (see Schedule 26), all slits must be less than 2 mm; in respect of low grade B1 and B2 surface, all slits must be less than 10 mm.

8.7 Road surface on bridge

8.7.1 Road surface on bridges and road bridges must be designed separately and of the same grade as adjacent road segments.

8.7.2 Design solutions that allow vehicles to enter and exit bridges easily, safely are required, especially where roads and bridges converge.

8.8 Pavement design of reinforced shoulder

8.8.1 Where roadway for motorized vehicles and reinforced shoulder are not separated by lane separators or only separated by road markings (see 4.5.2) which means motorized vehicles may encroach or stop, park on reinforced shoulder on a regular basis, in respect of flexible pavement, reinforced shoulder must be calculated and designed in accordance with applicable road surface standards and the following requirements:

- withstand calculated vehicle load (standard vehicle/lane/24 hours) of 35% to 50% of the calculated vehicle load of adjacent lanes for motorized vehicles;

- the topmost surface of reinforced shoulder must be of the same type as the surface of adjacent lanes for motorized vehicles;

- reinforcing structures must be taken into consideration so that existing structures can be utilized for the purpose of road expansion and upgrade;

- maintain minimum elasticity modulus according to 22 TCN 211;

- audit tensile - bending strength and slip of the heaviest possible wheels that can park on reinforced shoulder (impact factor and repeated factor shall not be taken into account for the purpose of audit);

- where economic conditions permit, reinforced shoulder should be similar to pavement structure.

8.8.2 Where lane separators are installed between roadways for motorized vehicles and reinforced shoulder of class I and class II roads to prevent motorized vehicles from encroaching or parking on the shoulder (lane separators must be 30 cm to 80 cm higher than the road surface, see 4.5.2), the type of pavement and minimum elasticity modulus may comply with 22 TCN 211 minus one grade (for example, class I roads may be outfitted with grade A1 and grade A2 pavement and minimum required elasticity modulus corresponding to class II roads).

8.8.3 Where roadway for motorized vehicles are of solid surface (cement concrete) and not outfitted with lane separators to prevent motorized vehicles from encroaching or parking on the shoulder, the reinforced shoulder must also be of cement concrete with 18 cm in minimum thickness of shoulder slabs of cement concrete. Cement concrete slabs of shoulder must also be designed to join via longitudinal slits (with cement concrete slabs of adjacent lanes for motorized vehicles) and cross slits. The foundation is also of the same materials as that of the main roadway plus at least 10 cm towards the outside of reinforced shoulder.

8.8.4 Where class I, class II, class III roads where roadway for motorized vehicles are of cement concrete and outfitted with lane separators to prevent motorized vehicles from entering the shoulder and parking on the shoulder, the reinforced shoulder can be of rigid or flexible structure. In this case, where rigid shoulder is employed, the minimum thickness of the cement concrete slabs must be 12 cm for single-layer foundation of conventional materials (flexible or semi-rigid). Where flexible shoulder is employed, provisions under 8.8.2 must be adhered to.

8.9 Pavement of frontage road

Deepening on vehicle volume forecast, socio-economic situations on both sides (population distribution) and humidity and heat conditions, design of pavement for frontage roads must adhere to instructions under applicable design procedures for flexible pavement and rigid pavement (regardless of geometric standards of frontage roads mentioned under 4.6.5).

9 Design of water drainage structures

9.1 Planning for water drainage system

Overall planning of water drainage system must be implemented first and foremost consisting of drainage structures such as catch water drain, side drains, collecting gutters, bridges, culverts, groundwater gutters, borrow pits, evaporating tanks, etc. which must closely cooperate with one another. Location, dimensions, and structures of drainage structures must be reasonable, compliant with general drainage planning of the area, highly effective, and low cost.

Placement of road base drains must collect and prevent water from freely flowing towards the road base and incorporate with culverts to drain through roads; determine drain direction of channels and canals towards culverts or streams and connect drainage drains with culverts or streams. Vice versa, the placement of culverts must take into account rapid drainage demand from gutters and drains.

Placement of drainage structures on roads must take into account irrigation demands. Flood drainage must also be taken into account after road construction.

9.2 Surface and shoulder drainage

9.2.1 Straight lines and curves whose radius is not required to be superelevation (Schedule 11), road cross section shall be of a camber type with cross gradient compliant with 4.9.

Where curves with radius specified under Schedule 13 must be of superelevation, superelevation gradient must be compliant with Schedule 11 and superelevation should be located on road segments with minimum longitudinal gradient of 1% for road base and surface drainage.

9.2.2 Where class I and class II roads are outfitted with median strips, drainage drains are required on both sides of median strips at superelevation curves. Where lane separators are not coated and are depressed, gutters (either open or concealed) must be located at the lowest point of lane separators (gutters must be 20 cm - 30 cm wide, 20 cm - 30 cm deep). Where lane separators are coated and the kerbs are higher than the roadway, collecting wells and drainage pipes of 20 cm - 40 cm in diameter are required to carry water to drainage structures and out of roadway, minimum gradient of drainage pipes must be 0,3 %. Where vertical pipes meet horizontal pipes, transition wells (inspection wells) are required.

9.2.3 Where lane separators are not coated, protrude and roads are outfitted with curbs, straight lines or curves must be outfitted with devices collecting water seeping through soil below lane separators and directing water away from the road base. It is permissible to install absorbent materials below the bottom of pavement, in the middle of separators and drainage pipes of 6 cm to 8 cm in diameter which are wrapped in filter fabric.

9.2.6 Where rain volume on roadway of class I and class II roads with multiple lanes is significant, in respect of roads on fill slopes, crest of slopes must be reinforced against erosion or accompanied by walls of concrete or construction mason with 8 cm to 12 cm in height along the outer edge of the reinforced section to prevent water from flowing over the slopes; rainwater on roadway will move along the walls towards dedicated drainage slopes.  

9.3 Side drains

9.3.1 Side drains are built to drain rainwater from road surface, shoulder, slopes of cut road base, and both sides of fill, fill and cut and cut road base that are lower than 0,6 m.

9.3.2 Dimensions of side drains in normal conditions do not require hydraulic calculation. Where side drains drain road surface, side walk, plots along the roads and basins on both sides of the roads, dimensions of side drains must be calculated in accordance with hydraulic formula to a maximum depth of 0,8 m.

Cross-sectional area of the drain can be a parallelogram, triangle, rectangle, or semicircle. Drains of parallelepiped cross section is common with bottom width of 0,4 m, minimum depth from ground elevation of 0,3 m, gradient of drains of cut road base may equal that of cut slopes depending on geological characteristics, gradient of drains of fill road base can be 1:1,5 - 3. It is possible to employ drains of triangular cross section with 0,3 m in depth, 1:3 in gradient sloped towards roadway and 1:1,5 in gradient sloped towards the opposite direction for fill road base and 1:m depending on gradient m of cut road base; drains of triangular or rectangular cross section are allowed in rocky areas.

9.3.3 For the purpose of preventing sludge build-up at the bottom of the drain, longitudinal gradient of the bottom of the drain must not be lower than 0,5 % and can be 0,3 % in special circumstances.

9.3.4 For the purpose of surface drainage planning, water from drains of fill road base must not overflown into cut road base unless the length of cut road base is less than 100 m; water from catch water drains, channels, etc. must not flow to side drains while water from side drains must always be directed towards nearby depressions or water bodies or drainage structures. Sewer pipes of 0,75 m in diameter are required every 500 m in length of parallelepiped drains or 250 m in length of triangular drains to drain water from side drains. Dimensions of sewer pipes do not require hydraulic calculation.

9.3.5 Locations where water is drained from side drains must be separate from fill road base. Where borrow pits are next to fill road base, side drains of cut road base must direct water towards the borrow pits. Where borrow pits are not available, side drains of cut road base shall be parallel to the centerline of the roads until height of fill road base is greater than 0,5 m at which points the drains deviate from the road base until depth of the drains equals 0.  

9.3.6 In respect of agricultural areas, where side drains are incorporated with irrigation purposes, dimension of the drain can be increased and road base must be protected against erosion.

9.3.7 Where side drains travel through residential areas, the drains must be made of materials such as rock or concrete and tiled with cloth and outfitted with rainwater collection system.

9.3.8 Side drains in tunnels should be of a larger dimensions for improved drainage capability and built of rock or concrete.

9.3.9 Where gradient of the drains is greater than gradient at which erosion occurs in the drains, design appropriate reinforcement depending on water speed (rock tiling, rock construction, concrete construction). Where conditions allow, the inside of the drains should be reinforced by loose rocks without binding or rock construction notwithstanding drain gradient to improve drainage capability and lessen maintenance demand.

9.4 Catch water drain

9.4.1 Where mountain basins are directed towards major arterial roads or where height of cut slopes ≥ 12 m, catch water drains shall be required to catch water flowing towards roads, redirect water to drainage structures, streams, roadside depressions, and prevent water from flowing towards side drains.

9.4.2 Catch water drains must be planned appropriately regarding direction, longitudinal gradient, and drainage clearance. Catch water drains must be parallelepiped, of 0,5 m in minimum bottom width, 1:1.5 in gradient, depth determined by hydraulic calculation, and containing water at a level that is at least 20 cm away from the top and not deeper than 1,5 m.

9.4.3 Where catch water drain is of a significant length, they should be divided into short segments. Flow rate of water calculated for each segment equals that of water flowing through the last cross section of each segment which is flow rate of water flowing from the basins to the drains plus flow rate of water from drains and gutters above.

9.4.4 Gradient of catch water drains usually depends on geographical conditions to prevent erosion of the drains. Where terrain condition compels catch water drains to have a greater gradient, appropriate reinforcement solutions for the bottom of the drain are required either in form of ripraps or concrete slabs, or chute drain type. For the purpose of preventing sludge build-up, drain gradient must not be lower than 3 0/00 - 5 0/00.

9.4.5 Where terrain gradient is significant or basin area is significant or terrain prone to landslide, two or more catch water drains are allowed. Vice versa, where lateral gradient is insignificant and basin area leading up to the side drains is insignificant, catch water drains are not required as long as drainage capability of side drains must be examined.

9.4.6 Catch water drains must be at least 5 m from the peak of fill slopes; soil removed for the purpose of construction of catch water drains must be used to form interceptor drains that travel downhill; the interceptor drains must have lateral gradient of 2 % insloped and at least 1 m away from slopes of cut road base.

Where catch water drains are required to prevent water from flowing towards fill road base, catch water drains must be at least 5 m away from side drains if any, and at least 2 m from the bottom of the slopes of fill road base if side drains are not built; in this case, soil removed for the purpose of construction of catch water drains must be used to form interceptor drains the travel towards the road base and have lateral gradient of 2 % insloped.

Catch water drains should be too far from road base as their effectiveness will be diminished.

9.4.7 In respect of deep cut roads that employ stepped slopes, for the purpose of preventing erosion caused by rainwater, drains should be built along the steps to direct water to interceptor drains or stepped drains and to natural streams or to sewerage structures in stepped or sloped form.

9.4.8 The rate at which flow rate of catch water drain and side drain is calculated shall be 4%.

9.5 Drain gutters

9.5.1 Drain gutters are designed to carry water from localized depressions to the nearest drainage facilities or side drains, catch water drains to depressions or culverts or act as transition between rivers, streams and upstream, downstream of sewers.

9.5.2 Drain gutters should not be longer than 500 m. Soil removed for the construction of drain gutters must be used to built small embankments along the drain gutters. Where drain gutters are built along road base, the edge of the drain gutters must be at least 3 m to 4 m away from the bottom of the road base slopes and embankments of 0,5 m to 0,6 m in height are required between the drain gutters and road base.  

9.5.3 Drain gutters should be as straight as possible, where drain gutters change in direction, curve radius should be 10 to 20 times the width of the upper bottom of the drain gutters and at least 10 m.

9.5.4 Cross section of drain gutters shall be determined via hydraulic calculation; depth of drain gutters should be at least 0,5 m; width of bottom of drain gutters must not be lower than 0,4 m; edge of drain gutters must be at least 0,2 m higher than the water level therein.

9.5.5 The frequency at which flow rate of drain gutters is calculated shall equal that of relevant drainage structures.

9.6 Interceptor drains and stepped drains

9.6.1 Where drain gradient is significant, for the purpose of preventing drain erosion, interceptor drains or stepped drains shall be required.  Drainage structures shall be selected by comparing solutions based on specific conditions. Interceptor drains and stepped drains are used in gutter segments with significant gradient connecting upstream and downstream of sewers with natural streams, gutter segments draining from draining structures along slopes of fill or cut roads, gutter segments connecting catch water drains to natural streams or culverts.

9.6.2 Cross section of interceptor drains is usually of a rectangular shape with width and depth determined in accordance with hydraulic calculation and dependent on design flow rate, gradient of interceptor drains, permissible speed at which materials of interceptor drains are not eroded, and dimensions of structures connected to the interceptor drains.

9.6.3 Interceptor drains can be of concrete, reinforced concrete, or construction rocks. For the purpose of reducing water speed in interceptor drains, the bottom of interceptor drains is built with rough edges and baffle walls or tanks are usually built at the end of interceptor drains.

9.6.4 Stepped drains shall be outfitted with baffle tanks where gradient of gutters and drainage channels is very significant. Stepped drains are usually of a rectangular shape in cross section and made of concrete, reinforced concrete, or construction rocks. Width and height of stepped drains, depth and length of baffle tanks, height and thickness of baffle walls shall be determined via hydraulic calculation and dependent on dimensions of structures connected to interceptor drains.

9.6.5 Structure of interceptor drains and stepped drains shall conform to common design. Where no common design is appropriate, design of interceptor drains and stepped drains may consult the following provisions:

- height of interceptor drains and stepped drains must be at least 0,2 m higher than the minimum water level;

- to prevent erosion, anchors installed 0,3 m - 0,5 m deep below the bottom of interceptor drains are required every 2,5 m - 4,0 in length;

- gradient of interceptor drains should not exceed 1:1,5 or where the gradient exceeds 1:1,5, stepped drains shall be required;

- stepped drains shall be design so that riser ranges from 0,3 m - 0,6 m and gradient of each tread ranges 2 % - 3 %.

9.6.6 Frequency at which flow rate of interceptor drains and stepped drains is calculated shall equal that at which flow rate of structures related to interceptor drains and stepped drains is calculated.

9.7 Groundwater drainage structures

9.7.1 Where ground water level is high or groundwater from slopes may affect road base stability, appropriate solutions must be taken.

9.7.2 On a case-by-case basis, the following types of subsurface ditches are allowed:

- Subsurface ditches located deep below side drains, shoulders, and pavements to limit groundwater level underneath roadway;

- Subsurface ditches located in cut slopes to prevent slopes from being damp and prevent groundwater from seeping out of slopes;

- Subsurface ditches behind walls and behind walls of tunnels, bridge abutments.

9.7.3 Subsurface ditches can be of open type or enclosed type. Open subsurface ditches will only be used in case of high groundwater level whereas enclosed subsurface ditches will be used in case of deep groundwater level. Bottom width of subsurface ditches ranges from 0,3 m to 1 m depending on depth and construction conditions.

9.7.4 General design of concealed subsurface ditches: The topmost part of the ditches are covered by compact watertight materials (soil) to prevent rainwater from seeping into the ditch; followed by 2 inverted layers of grass and vegetation to prevent soil from entering filter materials below; followed by a layer of sand and gravel or pebble; and then a drainage pipe or channel.

9.7.5 Where subsurface ditches are built in slopes of fill roads to prevent groundwater from seeping outside, subsurface ditches must be of a type where one side is accompanied by watertight walls throughout its length and the other side is built in accordance with inverted filter model.

9.7.6 Rocks for filling the ditches must be of a non-weathered type and non-soluble; drainage pipes installed in ditches are usually of concrete with minimum drainage diameter of 15 cm - 20 cm or of ceramic, construction bricks or rocks with diameter of 30 cm - 50 cm with 0,3 m - 0,6 m in length of pipe segment; drainage pipes shall be placed close to one another with a gap of 1 cm - 0,5 cm to allow water to enter the pipes.

10 Bridges, culverts, tunnels, and stream crossings

10.1 Bridge of all kinds (river bridges, railway and footpath, overbridges, bridges, etc.), culverts, and tunnels of highways must be designed in accordance with specialized design standards.

10.2 Cross section of bridges and tunnels of highways must satisfy roadway requirements under 4.10.5.

Dimensions, shapes, and characteristics of cross section of bridges and tunnels must be compatible with those of roads connected to said bridges and tunnels; dimensions of roadway on bridges must be unchanged, other elements on bridge cross section can be narrowed down in difficult situations in a manner that does not alter cross section of roads leading up to and from bridges and tunnels. Cross section of small bridges must not be narrowed down from design standard of the route.

10.3 Median strips on bridges shall be designed as follows:

- where width of median strips is less than 3 m, surface of median strips shall be similar to roadway of bridges and the median strips shall be accompanied by barriers and safety devices;

- where width of median strips is greater than 3 m, the median strips can be replaced with a strip of 0,75 m in width and 0,25 m in height and accompanied by barriers and safety devices.

10.4 Topographic and longitudinal section elements of bridges and tunnels such as minimum curve radius, superelevation transition curve radius, transition curve radius, superelevation radius, expansion, maximum gradient, minimum radius of vertical curve, etc. must comply with design standards applicable to the road class. In respect of large bridges, medium-sized bridges and tunnels, for the purpose of improving traffic capability and driving comfort and safety, longitudinal gradient should not be greater than 4 % and expansion of roadway is required where radius of curve is small.

Where summit-type vertical curves are located on both sides of bridges in order to transition from bridge elevation to ground elevation on river banks, sections at matching elevation according to longitudinal section of the bridges are required on both sides of the bridges to accommodate vertical curves with the starting points of vertical curves must be at least 10 m away from the bridges.

10.5 Location of river-crossing bridges must be selected so that the bridges satisfy economic, technical, geological, hydrographic, traffic convenience and safety requirements and based on the following criteria:

10.5.1 Regarding economy, technical engineering, and environmental protection

- total construction and operation costs converted to the current year is at the minimum or NPV index is at the greatest;

- construction process is at the minimum in length;

- local construction materials are prioritized;

- ship clearance is convenient and safe;

- impact of bridge construction on surrounding environment is at the minimum;

- traffic convenience and safety is guaranteed;

10.5.2 Regarding hydrography, topography, and geomorphology

- river bed must be stable and even;

- width of rivers is at the minimum, shoals are small in size, water is of adequate depth, rivers do not have branches, old sections, sludges;

- flow state rarely changes;

- flow directions in flood season and dry season are nearly identical;

- centerline of large and medium-sized bridges must be perpendicular to main waterway. In difficult circumstances, the centerline can be diagonal from main waterway facilitating watercraft operation as long as safety is guaranteed; or perpendicular to river basins and not perpendicular to main water not facilitating watercraft operation. Bridge clearance must not be designed in a way that it narrows main waterway.

10.5.3 Regarding geology

Bridge locations should be areas with base rock near river beds where geological properties of river banks are suitable and stable and avoid areas prone to erosion, karst, plaster.

10.6 Frequency at which hydrograph calculation is conducted for roadside structures are specified under Schedule 30.

Schedule 30 - Frequency of hydrography calculation of structures on highway

In percentage

10.7 Where the length of culverts built below fill road base equals width of road base at the top of the culverts, the culverts shall be surrounded by walls to maintain stability of slopes of fill road base, prevent slip and erosion of road base. For minimum thickness of soil cover on circular culverts and box culverts in which steel rebar withstanding vehicle load must not be present, see 7.3.4.

Compactness of soil cover on culverts must be similar to that of road base; soil fill of culverts must be of the same type as soil fill of road base.

Culverts built in cut roads must be accompanied by collecting wells located upstream to gather water from side drains and streams. Where culverts are located at great depth and flow is significant, collecting wells shall be replaced by baffle tanks and interceptor drains shall be required to redirect flow from streams to culverts. Where deep cut road base travels through flow, it is permissible to build aqueducts to carry water across the roads.

Minimum clearance shall be 0,75 m for up to 15 m in length. In order to facilitate maintenance and repair, clearance of 1 m is recommended for culverts below 30 m in length. Culverts of 1,25 m in clearance will have permissible length of exceeding 30 m.

In general, culvert clearance should be selected in non-pressured state. Semi-pressured and pressured state only apply to high fill roads and where soil fill is hardly absorbent for water from upstream. Longitudinal gradient of culverts must not be greater than gradient downstream from the culverts. Culvert gradient should range from 2 % to 3 % to prevent sludge build-up.

10.8 In respect of stream crossings, where bridge construction is ineligible, pontoon bridges or ferries are allowed as substitute. Approaches to ferries must have common gradient from 8 % to 12 % depending on the terrain, be at least 9 m in width, and tiled with cement or riprap.

Parking lots and other services are required in vicinity of ferry stations and pontoon bridges.

10.9 Low-class highways where traffic is suspended during storm seasons, it is possible to build embankment roads or submersible roads:

- crossing large, even river basins that is mostly not deep;

- crossing slow-moving streams;

- crossing valley-type terrain at foot of mountains;

- embankment roads may incorporate culverts or embankment bridges to minimize standing water upstream from embankment roads and improve drainage capability of embankment roads in case of heavy flood;

- maximum depth at which traffic is still allowed on embankment roads is specified under Schedule 31.

Schedule 31 - Permissible flood depth on embankment roads (4% of flood frequency)*

Minimum roadway width of embankment roads and submersible roads is 7 m; surface of embankment roads and submersible roads shall be of cement concrete or riprap. Gradient of slopes of embankment roads shall be 1:2 for upstream slopes and 1:3 to 1:5 for downstream slopes. The slopes must be reinforced by concrete or rocks. Foot of slopes on downstream side must be reinforced by walls in form of riprap with minimum anchorage depth of 0,7 m. Foot of road slopes and river beds must be reinforced against erosion. Width of reinforced plot shall be 2 m upstream and 2,5 - 3 times the flow speed downstream. Reinforcement materials for erosion protection are usually riprap without binders or with cement plasters.

Both ends of embankment roads and submersible roads must be accompanied by warning signs and indicators of permissible flood depth for traffic. Marking posts must be installed along embankment roads to indicate roadway; water gauges must be installed on embankment roads to inform drivers about flood depth.

11 Junction

11.1 General requirements

11.1.1 Goals: As conflicts, accidents, and congestions usually take place at junctions, design of traffic junction must resolve conflicts (or minimize to a certain extent) in order to:

- allow reasonable traffic capacity of junctions in order to improve traffic quality;

- ensure traffic safety;

- yield economic effectiveness;

- provide aesthetic value and environmental hygiene.

The first two goals are the most important ones and thus must be guaranteed at all time.

11.1.2 Design of junctions must take into account the following factors:

a) Traffic factors:

- functions of roads in network;

- traffic volume: traffic entering junctions, traffic in all turns, current (existing junctions), forecast (20 years for capital construction, 5 years for short-term traffic arrangement); average traffic volume per 24 hours, traffic volume in rush hours;

- composition of traffic, characteristics of special vehicles;

- foot traffic volume;

- parking spaces in vicinity of junctions (if any).

b) Physical factors:

- mountainous regions where junctions are located and natural conditions;

- local planning, drainage conditions;

- angle of roads and possibility of improvement;

- requirements pertaining to environment and aesthetic.

c) Economic factors:

- construction and maintenance expenditure;

- compensation and tile clearance costs;

- technical economic analysis indicators.

d) Scenery factors;

e) Human factors:

- habits, discipline, skills of drivers;

- discipline, social ethics of road users and roadside inhabitants.

11.1.3 Junction classification

Method: depending on conflict solutions, junctions shall be classified as follows:

a) Grade-separated junctions where structures (tunnels or bridges) separate conflicting flows of traffic. Two primary types:

- complete grade-separated junctions: junctions where branching roads are available allowing vehicles to move in any direction;

- incomplete grade-separated junctions: junctions where vehicle can only move in one designated direction. Traffic flows primarily move through this junction to be separated from other traffic flows.

b) At-grade junctions:

- simple junctions: where conflicts are acceptable (flow of turning traffic is below 30 xcqđ/h and turning speed is below 25 km/h). This type may or may not be expanded;

- channelized junctions: where turning flows of vehicles have specific requirements (pertaining to turning flow and speed), such turning lanes will be separated and protected (via traffic islands or gores). Channelized junctions will create advantageous angle of conflicting traffic flows and make space for yielding vehicles that are prepared to interrupt other traffic flows;

- roundabout junctions: junctions that direct conflicting traffic flows around a circle.

c/ Traffic light-controlled junctions: junctions that separate conflicting traffic flows by time of movement. This type is not recommended for use on highway, especially if speed limit is greater than 60 km/h.

11.1.4 Choosing types of junctions. Types of junction shall be selected based on factors (see 11.1.2), technical and economic indicators, creativity of designers and by consulting data under Schedule 32.

Schedule 32 - Scope of use of junction types

11.2 Grade-separated junctions

11.2.1 Structure and clearance

Determination of direction of use (overpass or underpass) must:

- prioritized priority road;

- utilize terrain and advantages in construction;

- be correlated to other junctions;

- be made via technical and economic analysis.

Clearance must be guaranteed in accordance with 4.7:

11.2.2 Roadway on main road crossing grade-separated junctions

In respect of grade-separated junctions, roadway of main road crossing the junctions must not be narrower than those before and after the junctions. In addition:

- median strips of the roads below must be expanded to accommodate pillars of overpass and safety equipment if overpass piers are installed;

- each direction of travel must accommodate a transition lane of 3,75 m in width to the right of the road. Such lane must be of sufficient length to serve as acceleration and deceleration lanes (see 4.8);

- a space of 1,5 h in width is required (where: h is kerb height of footpath).

11.2.3 Left-turning lanes are categorized into 3 types:

- indirect left turn (270^o turn)

- semi-direct left turn (90_o turn over three quarter angles);

- direct left turn (90^o turn over a quarter angle).  

Indirect left turns are considered for use if flow of left-turning vehicles is less than 500 xcqd/h.

Semi-direct left turns are considered for use if flow of left-turning vehicles is greater than 500 xcqd/h.

Direct left turns are considered for use if flow of left-turning vehicles is greater than 1500 xcqd/h.

11.2.4 Cross section of left turns and right turns

Cross section of turns (left and right) is determined in accordance with 4.2. However, the following minimum parameters must be adhered to;

- where the turn is greater than 80 m in length, more than 2 lanes shall be required;

- where the turn is less than 80 in length, 1 lane is allowed as long as reinforced shoulder is built to allow trucks to overtake a parking vehicle.

11.2.5 Design speed in grade-separated junctions is specified under Schedule 33.

Schedule 33 - Design speed of turns

In km/h

11.2.6 Grade-separated junctions with turns must not be less than 4 km away from each other.

11.3 At-grade junctions

11.3.1 Route and angle of junctions

- roads in junctions should not be curved, if curves are inevitable, curve radius must not be lower than regular minimum radius depending on road class;

- the best angle is right angle. Where angle is lower than 60o, improvement must be made to angle of junction;

- junctions should be placed in even location. Where gradient exceeds 4%, adjustment to visibility must be made;

- longitudinal cross section of secondary roads must not violate or alter cross section of main roads. Where two roads are of the same class or similar priority, vertical design and adequate traffic, drainage shall be required.

11.3.2 Design vehicle and design speed

11.3.2.1 Design vehicle

Where car volume is greater than 60 %, cars will be used as design vehicles; where car volume is lower than 60 %, trucks will be used as design vehicles. Where trailer truck volume is greater than 20 %, trailer trucks will be used as design vehicles.

11.3.2.2 Design speed of turns

For forward-moving lanes, design speed of the road shall apply.

For right-turning lanes, design speed shall be 60 % lower than design speed of the main road cutting through the junctions. For left-turning lanes, design speed falls under 2 categories:

- minimum design must not be greater than 15 km/h;

- advanced design must not exceed 40 % the design speed of roads outside of the junctions.

11.3.3 Superelevation and lateral force factor

Maximum elevation in junction shall be 6 %. Superelevation in residential areas must not be greater than 4%. Lateral force factor allowed in junctions shall be 0,25.

11.3.4 Visibility in junctions

A field of view in junctions (see Figure 4) must be guaranteed for:

- non-priority vehicles when they are at a distance away from points of conflict. This distance equals:

- non-priority vehicles must be able to see priority vehicle (on the right) when priority vehicles is at a distance away from points of conflict. This distance equals  .

where:

V_A means design speed of non-priority vehicles, in km/h;

V_B means design speed of priority vehicles, in km/h.

Note: Crossed lines: area where obstacles are not required to be cleared.

Figure 4 - Guaranteed field of view in junction prioritizing right side

11.3.5 Acceleration/deceleration lane

Acceleration/deceleration lanes shall be located where vehicles transition onto roads of different class. Acceleration lanes are required for when vehicles move for roads with low design speed to roads with higher design speed. Vice versa, deceleration lanes shall be required.

11.3.5.1 Deceleration lanes shall be of parallel or tapered type (see Figure 5a and 5b) whereas acceleration lanes must be of parallel type (see Figure 5c).

11.3.5.2 Acceleration/deceleration lanes shall have width of 3,5 m. Length of transition segment must be at least 35 m in length (expand by 1 m over 10 m in length). Length of the speed change segment is determined in accordance with in positive acceleration of 1 m/s² and negative acceleration of 2 m/s². Length of deceleration segment must not be lower than 30 m, length of acceleration segment must not be lower than 120 m.

11.3.5.3 Acceleration/deceleration lanes should be located where longitudinal gradient is lower than 2 %. If longitudinal gradient greater than 2 % is inevitable, adjustment must be made depending on gradient or parameters must be multiplied by a factor of 1,2.

11.3.5.4 Entries and exits of acceleration/deceleration lanes must allow adequate visibility towards whichever lane the vehicles are moving towards.

legend:

Figure 5 - Arrangement solutions of acceleration/deceleration lanes

11.3.6 Traffic islands in at-grade junctions

a) Traffic island is a structure that:

- eliminates excess area between lanes;

- distinguishes turning lanes;

- localizes points of conflict and creates advantageous for conflicting traffic flows;

- allows vehicles to wait to turn and merge;

- allows pedestrians to rest and wait when crossing the street;

- accommodates traffic control facilities.

b) Placement principles and design:

- the fewer island is preferred over the more islands;

- larger islands are preferred over smaller islands;

- islands must be located so that: priority traffic flow is facilitated, traffic flow that must be slowed down is slowed down, traffic flows that must be prevented from entering junctions, vehicles clearing the junctions must be able to do so without confusion.

c) Setback:

In order to prevent collision, traffic islands must recede from the outer most roadway to create setback.

Setback at the entry of traffic flow must be 1 m ~ 1,5 m. Setback at the exit of traffic flow must be 0,5 m. Traffic circumference can be connected via even curves and rounded with radius of 0,5 m.

Surface of setback shall be similar to that of roadway and be marked with zebra marking.

11.4 At-grade junction with railway (level crossing)

11.4.1 Level crossings must be located outside of stations, shunting tracks, railway tunnel openings, station signaling posts.  The best incident angle is a right angle and the minimum incident angle is 45^o.

11.4.2 Level crossings must not be located:

- where highway of Vtk ≥ 80 km/h meets railway;

- where highway of Vtk < 80 km/h meets high-speed railway (120 km/h) especially if visibility is compromised.

11.4.3 Level crossings (that are not outfitted with barriers or guards), visibility must be ensured to allow drivers to see trains. Area specified under Figure 6 and Schedule 34 must not contain vision-obscuring obstacles.

Schedule 34. Area along railway without obstacles from level crossing

 (*) In case of limited geographic conditions, highway can be 5 m away from the outermost rail and marked by “stopping line” (section 3.5 Appendix 8) and sign No. 122 (stop) (STOP sign under Appendix 3) of 22 TCN 237. Line of sight on highway must be at least 5 m in length whereas line of sight on railway must be compliant with Schedule 34.

Figure 6 - Area without obstacles for visibility between highway and railway

11.4.4 Roadway of highway at level crossing must have minimum width of 6 m and length of sight stopping distance S1 according to Schedule 10 from the outermost rail plus 5 m.

11.4.5 In respect of level crossings, highway must have no gradient (0 %) or contain vertical curves based on superelevation of railway within at least 16 m (excluding transitions of vertical curves); this distance can be reduced to 10 m in case of difficult conditions.  

11.4.6 Highways at junctions should use reinforced concrete slabs of minimum side length of 2,0 m from the outermost rail; this distance can be reduced to 1 m in case of difficult conditions.

11.5 Grade-separated junctions:

11.5.1 Design of highway in vicinity of power lines and telecommunication lines must adhere to requirements of relevant sector and the followings:

- minimum vertical clearance from road surface to overhead telegraph lines and telecommunication line is 5,5 m;

- minimum horizontal distance from edge of the road to posts of the aforementioned lines must be 4/3 the height of the posts and 5 m;

11.5.2 Vertical and horizontal distances from highway to power lines shall conform to applicable laws.

11.5.3 Where highway cuts through water pipes, steam pipes, oil pipes, heating pipes, or underground power lines, applicable regulations of respective sector must be adhered to.  

12 Traffic safety devices

12.1 Signs are compliant with 22 TCN - 237

12.2 Road markings are compliant with 22 TCN 237.

12.3 Road margin posts

12.3.1 Posts act as guidance; where fill slopes are at least 2 m in height in curves with small radius and where bridge approaches are located, posts must be placed on the ground and compliant with spacing under Schedule 35. Where guard rails are installed, posts are not required.  

Schedule 35 - Post spacing dependent on horizontal curve radius

All dimensions in meter

Posts are circular, squared, or triangular in cross section with minimum dimension of 15 cm. Posts shall be 0,6 m in height and at least 35 cm in buried depth.

Paint color shall be compliant with road signaling regulations as long as they must be reflective or coated with a reflective strip of 4 cm wide and 18 cm long that is 30 cm to 35 cm away from the top and facing the oncoming traffic.

12.3.2 Where fill road base is higher than 4 m, roads, bridges, road bridges, overpasses, piers and abutments of overpasses, footpaths in tunnels, etc. must be outfitted with guardrails. 

Balconies can be of concrete or corrugated steel bars. Guardrails must be of a minimum thickness of 4 mm, minimum cross section height of 300 mm to 350 mm and corrugated to improve strength.

Bars and pillars of guardrails are designed and inspected in accordance with load-barding capability under Schedule 35.

Guardrails must extent past the protected areas in both directions by at least 10 m.

12.3.3 Where bars and posts of guardrails are of similar materials, mechanical tests compliant with Schedule 36 must be conducted.

Schedule 36 - Mechanical requirements for guardrails

12.4 Lighting

Highway shall not receive artificial lighting throughout its length except in major junctions, major bridges, tunnels, and residential areas where artificial lighting is considered. Illuminance must not shift by more than 1 candela/m2 per 100 m in length between illuminated spots to non-illuminated spots to prevent glare.

13 Service structures

13.1 Trees

13.1.1 Trees are mandatory elements of road design projects. Trees serve to reinforce other structures, provide shade, scenery, etc. dampen noise, dust, and prevent glare.

13.1.2 Grass: median strips and separators and traffic islands, if not coated, must accommodate grass.

Surface of cut and fill roads must accommodate grass planted via seeds, etc. to prevent erosion and improve aesthetics.

Grass cultivar selection requires consulting agriculturists and should be mixed to stay green throughout the year. Grass must not be taller than 5 cm. Grass taller than 5 cm must be trimmed.

13.1.3 Bush vegetation

Bush vegetation improves the aesthetics, prevents glare of opposing traffic, prevents dust and noise.

Bush vegetation is planted in median strips and steps around fill and cut road base. Bush vegetation must not be planted on small traffic islands.

Maintenance, trimming, replacement, and pruning are required to prevent vegetations from exceeding 0,8 m in height.

13.1.4 Trees

Trees must be planted outside of earthened shoulders. Trees can be planted on both sides of the road or in roadside clusters.

Tree selection requires consulting agriculturists and trees suitable with soil conditions, roots not harmful to road conditions, branch snapping is prevented should be prioritized.

13.2 Bus stop

13.2.1 Bus stop is divided into 3 types:

- Simple bus stop. The bus stops on roadway to the right. The bus decelerates and accelerates on the outermost lane;

- Bus lay-by. The bus stops partly on roadway and partly on shoulder. The bus decelerates and accelerates on the outermost lane;

- Segregated bus stop. The bus stops outside of roadway, on elevated area separated via rocks, guardrails, dividers. The bus decelerates and accelerates partly on the outermost lane and partly on separated bus lane.

13.2.2 Scope of use of bus stop:

a) Where bus frequency is lower than values under Schedule 37, adopt simple bus stop, otherwise bus lay-by will prevail.

Schedule 37 - Bus stop limit

In addition to Schedule 37, bus lay-by is required:

- where road shoulder is greater than 3 m in width;

- where road shoulder is 2 m to 3 m in width and motorbike flow rate is greater than 50 vehicle/h per direction;

- where conditions for other bus stop types that are 15 m away from designated crossing locations are not sufficient.

b) In respect of roads with V_tk ≥ 80 km/h, segregated bus stops are required.

13.2.3 Stopping space design

- simple stop where bus stops on roadway and passengers wait on reinforced shoulder;

- lay-by stop which has minimum width of 3 m from the edge of roadway. Passenger waiting area is 1,5 m in width and 15 m in length and illustrated under Figure 7;

- segregated stopping spaces have entrances and exits, including acceleration and deceleration lanes.

13.2.4 Location of bus stop

- bus stop must be on the rightmost side in the direction of traffic;

- each bust stop must be 300 m to 500 m away from one another. Bus stop must not be located on curves whose radius is below the regular minimum radius of horizontal curve;

- bus stop must be located on both sides of the road, end points of opposing bus stops must be at least 10 m away from the other;

- bus stop can be located before or after junctions. Distance from stopping spaces to junctions must take into account acceleration lanes, observation time (if before junction), braking distance (if after junction) and affect of stopping spaces to traffic capability of junctions. Where stopping spaces are after junctions, bus stop must be at least 50 m away from the center of the junctions.

Where stopping spaces are before junctions, bus stop must be at least 40 m away from center of the junctions in respect of roads with V_tk ≤ 60 km/h or 60 m away from center of the junctions in respect of roads with V_tk ≥ 80 km/h.

Where junctions contain zebra crossings, bus stopping spaces must be at least 10 m away from the zebra crossings.

13.3 Rest areas and other service locations

13.1.1 Rest areas and service locations should be located on highways with V_tt 60 km/h. Rest areas serve to reduce fatigue, improve traffic safety, and utilize tourism potentials.

13.3.2 Rest areas and service locations must be isolated from the road. Entrances and exits to such areas must incorporate acceleration and deceleration functions. Guidance signs must be erected on main roads in accordance with instructions under 22 TCN 237.

13.3.3 Rest areas

Small rest areas: have approximate area of 3000 m2 with stopping spaces, may include permanent fixtures (less than 10 rest spots, tables, chairs, roofs, drinking fountains, historical and geographical information panel).

Large rest areas: have area of above 5000 m2; have stopping spaces for cars, trucks, and buses. May include services under management of local government such as: medical stations, petrol stations, repair stations, drinking and goods vendors, public telephone, or posts stations.

13.3.4 Large rest areas shall be 60 km to 100 km away from one another. Small rest areas shall be 15 km to 30 km away from one another.

Hotels can be located on roads above 100 km in length.

Relevant local authority must be consulted for the selection of location and service capacity.

13.3.5 Surface of parking spaces must coated with sufficient strength. Minimum dimensions of parking spaces:

- car: 2,5 m x 5 m;

- truck: 4 m x 20 m;

- bus: 5 m x 15 m.

13.3.6 Trees must be prioritized in rest areas in order to:

- isolate rest areas and roads, create relaxing scenery;

- isolate areas in rest areas and parking spaces. Tall trees are recommended in parking spaces.

13.4 Tollbooths

13.4.1 Tollbooths shall be located:

- before major bridges and tunnels;

- at complete grade-separated junctions;

- at appropriate locations;

- distance between tollbooths must not be lower than 70 km.

13.4.2 Lanes in tollbooths

13.4.2.1 Number of lanes in tollbooths depends on:

- traffic flow in peak hours of design year;

- the length of line of waiting vehicles does not exceed 500 m;

- tolling period This period depends on method of tolling: manual, semi-automatic, or automatic;

- separate lanes if: multiple tolling methods are used (cash, tickets, magnetic card, etc.) or multiple vehicle compositions are tolled such as motorized vehicles, trucks, trailer trucks, etc.;

- in respect of tollbooths close to cities, several middle lanes may incorporate alternating traffic direction depending on time of peak traffic volume (significant vehicle in one direction in the morning, significant vehicle in the other direction in the evening);

- bypass routes are built to accommodate oversized vehicles.

13.4.2.2 Lane width in tollbooths:

- motorized vehicle lane shall have 3,8 m in width and be outfitted with vehicle counting devices;

- lanes are separated by islands of 30 m in length and 2 m in width. The islands accommodate tollbooth personnel, barriers, tolling, vehicle count, guiding instruments, etc.;

- motorbikes should have at least 2 separate lanes: (2 x 1m )+ 0,5 m = 2,5 m;

- roads in tollbooths (throughout the length of the queue) must be of cement concrete.

13.4.3 Other provisions

13.4.3.1 Clearance of tollbooth gates must be at least 5 m in height. Width of lanes entering and leaving tollbooths (including traffic islands and lanes reserved for further expansion) must be sufficient. Length of queue must be sufficient and can be up to 800 m.

13.4.3.2 Tollbooths must not be located at the end of slopes whose gradient is greater than 3 %.

13.4.3.3 Tollbooths must be illuminated; working spaces must be equipped with communication system (radio, telephone, etc.) ventilation and noise dampening system.

13.4.4 Tollbooths must contain:

- director office;

- security office;

- vaults for money and goods storage;

- dressing rooms and employee wardrobes;

- canteens;

- gender-segregated toilets;

- backup generators.

14 Environmental protection

14.1 During design process, analysis and evaluation of environmental impact brought by construction and utilization of the route must be conducted in order to minimize environmental impact and comply with applicable regulations;

14.2 Study of environmental impact is conducted in 2 steps:

- In fundamental design: conduct preliminary evaluation of environmental impact to study, choose alignment solutions in accordance with 22 TCN 242;

- In technical design and construction drawing design: conduct detail study of environmental impact, analysis of economic benefits and losses to propose and decide appropriate solutions in construction and utilization.