Highway Design: Cross Sectional Elements, Types and Drainage, Exams of Transportation Engineering

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Highway components, cross

sectional elements and types

Dr Rawid

Road cross section elements

• The features of the cross-section of the

pavement influences the life of the pavement

as well as the riding comfort and safety.

• Pavement surface characteristics affect both

life of road and riding comfort.

• Camber, kerbs, and geometry of various cross-

sectional elements are important aspects to

be considered in this regard.

Typical cross section (Fill)

Cross section in cut

Half cut

Cross section in half cut

Types of highway cross section

Camber (Cross slope)

• 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 slope is undesirable for it will erode the

surface. Camber is measured in 1 in n or n% and the

value depends on the type of pavement surface.

• The values suggested by GUIDELINES for various

categories of pavement is given in Table 1. The

common types of camber are parabolic, straight, or

combination of them

Width of carriageway

• Width of the carriage way or the width of the pavement depends on the width of the traffic lane and number of lanes.
• Width of a traffic lane depends on the width of the vehicle and the clearance. Side clearance improves operating speed and safety.
• The maximum permissible width of a vehicle is 2.44 and the desirable side clearance for single lane traffic is 0.68 m. This require minimum of lane width of 3.75 m for a single lane road. However, the side clearance required is about 0.53 m, on either side and 1.06 m in the center. Therefore, a two lane road require minimum of 3.5 meter for each lane.

Width of carriageway

Kerbs

• Kerbs indicate the boundary between the carriage way

and the shoulder or islands or footpaths.

• Low or mountable kerbs : This type of kerbs are

provided such that they encourage the traffic to remain

in the through traffic lanes and also allow the driver to

enter the shoulder area with little difficulty. The height

of this kerb is about 10 cm above the pavement edge

with a slope which allows the vehicle to climb easily.

This is usually provided at medians and channelization

schemes and also helps in longitudinal drainage.

Kerbs

• Semi-barrier type kerbs : When the pedestrian

traffic is high, these kerbs are provided. Their

height is 15 cm above the pavement edge.

This type of kerb prevents encroachment of

parking vehicles, but at emergency it is

possible to drive over this kerb with some

difficulty.

Different types of kerbs

Road margins , Shoulders

• The portion of the road beyond the carriageway and on the roadway can be generally called road margin. Various elements that form the road margins are :

Shoulders :

• Shoulders are provided along the road edge and is intended for accommodation of stopped vehicles, serve as an emergency lane for vehicles and provide 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.
• The shoulder width should be adequate for giving working space around a stopped vehicle. It is desirable to have a width of 2.0 m for the shoulders.