6 BASIC CROSS SECTIONAL ELEMENTS OF HIGHWAY PAVEMENT

The characteristics of cross-sectional elements are important in highway geometric design because they affect safety and comfort. There are 7 basic cross sectional elements of a highway pavement:

  1. curvature
  2. width of carriage way
  3. Control
  4. road margin
  5. width of formation
  6. right of way (ROW)

1.curvature

Camber or cant is the cross slope provided in the middle of the road surface to raise it in the transverse direction to drain the rain water from the road surface. The objectives of providing camel are:

  • Surface protection especially for gravel and bituminous roads
  • Sub-grade protection by proper drainage
  • Quick drying of pavement which in turn increases safety

Too steep a slope is undesirable as it will corrode the surface. Camber is measured in 1 in n or n% (eg 1 or 2 in 50) and the price depends on the type of pavement surface. The values ​​suggested by the IRC for the different categories of pavement are given in Table 1. Common types of camels are parabolic, straight, or a combination thereof (see figure below).

different types of camber
different types of camber
Table 1: IRC value for camel
Surface overweight Light
Type Rain Rain
Concrete/Bituminous 2% 1.7%
Gravel / WBM 3% 2.5%
Of clay 4% 3.0%

2.width of carriage way

The width of the carriageway or the width of the sidewalk depends on the width of the traffic lane and the number of lanes. The width of the traffic lane depends on the width and clearance of the vehicle. Side clearance improves operation speed and safety.



The maximum permissible width of a vehicle is 2.44 And the desirable side clearance is 0.68 m for single lane traffic. For this, the minimum lane width for a single lane road should be 3.75 meters.

However, the side clearance requirement is around 0.53 m on either side and 1.06 m at the centre. Therefore, a two-lane road requires a minimum of 3.5 meters for each lane.

The desired carriageway width recommended by IRC is given in Table 2.

Table 2: IRC Specification for Carriage Way Width:
Single Lane 3.75
two lane, no curb 7.0
two lane, raised edge 7.5
intermediate train 5.5
Multiple-lane 3.5
lane width
lane width

3.Sanctions

The curb denotes the boundary between the carriageway and the shoulder or islands or sidewalks. The picture below shows the different types of curbs.

different types of curbs
different types of curbs

low or mountable curbs

These types of curbs are provided in such a way that they encourage traffic to remain in the traffic lane and also allow the driver to enter the shoulder area with little difficulty. The height of this curb is approximately 10 cm above the edge of the sidewalk with a slope that allows the vehicle to climb smoothly. It is usually provided in median and channelization plans and also helps in longitudinal drainage.

semi-barrier type curb

When pedestrian traffic is high, these restrictions are provided. Their height is 15 cm above the edge of the sidewalk. This type of curb prevents encroachment of parking vehicles, but in case of acute emergency it is possible to cross this curb with some difficulty.

barrier type curb

They are designed to discourage vehicles from leaving the pavement. They are provided if there is a considerable amount of pedestrians. They are placed at a height of 20 cm from the sidewall edge with a steep batter.

submergence restriction

These are used in rural roads. Curbs are provided at the sidewall edges between the sidewalls and shoulders. They provide lateral confinement and stability to the pavement.

4.road margins

The portion of the road beyond the carriageway and onto the road may generally be called the road margin. The various elements that make up the road margin are given below.

shoulders

Shoulders are provided at the side of the road and are intended for housing stalled vehicles, acting as an emergency lane for vehicles and providing lateral support for base and surface courses. The shoulder should be strong enough to bear the weight of a fully loaded truck even in wet conditions. Shoulder width should be sufficient to allow working space around the stopped vehicle. It is desirable for the shoulders to have a width of 4.6 m. A minimum width of 2.5 meters is recommended for 2-lane rural highways in India.

parking lane

Parking lanes have been arranged in urban streets for side parking. Parallel parking is preferred as it is safer for vehicles on the road. In case of parallel parking, the width of the parking lane should be at least 3.0 metres.

bus-bay

Bus bays are provided by removing the curbs for the bus stops. They are provided so that they do not obstruct the movement of vehicles on the way of the carriage. They should be at least 75 meters from the intersection so that the traffic near the intersection is not affected by the bus-bay.

service road

Service roads or frontage roads provide access to controlled highways such as freeways and expressways. They run parallel to the highway and will usually be separated by a divider and access to the highway will be provided only at selected points. These roads are designed to avoid congestion on expressways and also the speed of traffic in those lanes does not decrease.

cycle track

Cycle tracks are provided in urban areas when the volume of cycle traffic is high. A minimum width of 2 m is required, which can be increased by 1 m for each additional track.

trail

A footpath is a special right of way for pedestrians, especially in urban areas. They are provided to protect pedestrians when both pedestrian traffic and vehicular traffic are high. The minimum width is 1.5 meters and can be increased depending on the traffic. The pavement should be either as smooth as the pavement or smoother than it should be to encourage pedestrians to use the pavement.

guard rail

Usually when the road is on an embankment they are provided on the side of the shoulder. They serve to prevent vehicles from exiting the embankment, especially when the height of the fill exceeds 3 m. There are various designs of guard rails. Guard stones painted alternately in black and white are commonly used. They also give better visibility of curves at night under vehicle headlights.

5.width of formation

The width of the building or the width of the road is the sum of the width of the sidewalk or carriageway including the divider and the shoulders. It does not include additional land in formation/cutting. The values ​​suggested by the IRC are given in Table 3.

Table 3: Width of construction of different categories of roads
Street Width of road m. In
classification plain and hill and
rolling terrain steep terrain
NH/SHO 12 6.25-8.8
mdr 9 4.75
ODR 7.5-9.0 4.75
we are. 7.5 4.0

6.right of way (ROW)

Right of way (ROW) or width of land is the width of the acquired land along the alignment of the road. It should be large enough to accommodate all cross-sectional elements of the highway and can reasonably provide for future development. To prevent ribbon development along highways, control lines and building lines may be provided. The Line of Control is a line that represents the closest extent of future uncontrolled construction activity in relation to the road. The building line represents a line on either side of the road, between which no construction activity is allowed between and the road. The right of width of the road is controlled by:

  • width of formation: It depends on the class of the highway and the width of the road and the margin of the road.
  • embankment height or depth Cutting: It is controlled by topography and vertical alignment.
  • slope of the embankment: It depends on the height of the slope, type of soil etc.
  • Drainage systems and their sizes Which depends on rainfall, topography etc.
  • vision distance ideaVisibility is restricted on the inner side of the curve due to presence of some obstructions like building structure etc. on the curve etc.
  • Reserved land for future widening: Some land is to be acquired in advance in anticipation of future development like widening of road.

The importance of reserved land has been emphasized by the following. Additional width of land is available for construction of roadside facilities. Land acquisition later is not possible, as land can be occupied for various other purposes (building, business etc.).

The general ROW requirements for built up and open areas specified by the IRC are given in Table 4.

Table 4: general right of way to open areas
Street Width of road m. In
classification

plain and

rolling terrain

hill and

steep terrain

open area

NH/SHO 45 24
mdr 25 18
ODR 15 15
we are. 12 9

built up area

NH/SHO 30 20
mdr 20 15
ODR 15 12
we are. 10 9

A typical cross section of a ROW is given in the figure shown below.

right of way (row)
right of way (row)

Author

Dr. Tom V Matthews (IIT Bombay)

Er. Mukesh Kumar

Photo of author
Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.