Geometric Design of Highway

There are several design factors on which the geometric design of a highway depends, the most important are as follows:

1.  Design Speed
2. Topography
3. Traffic Factors
4. Design hourly volume and capacity
5. Environmental and other factors

Highway Cross Section Elements

The important surface characteristics of the pavement are the friction, unevenness, light reflecting characteristics and drainage of surface water.

1. Friction
   
   The friction between the tyres and the pavement is one of the main factors in the determination of operating speed and stopping sight distance of vehicles. Whenever a vehicle negotiates a horizontal curve lateral friction comes in to play and counteract the centrifugal force. The coefficient of friction or skid resistance offered by the pavement surface under various driving and surface condition is important with reference to the safety.
   A skid occurs when the slide without revolving or when the wheels partially revolve i.e., when the path travelled along the road surface is more than the circumferential movements of the wheel due to their rotation.
   A Slip occurs when a wheel revolves more than the corresponding longitudinal movement along the road.
   
   

   Factor affecting Friction:

   • Type of pavement surface (Cement concrete, Bituminous, WBM etc)
   • Its relative roughness
   • Condition of the pavement (Wet, Dry, Smooth, Rough etc)
   • Type and condition of tyre (New with good treads or smoothened or worn out)
   • Speed of vehicle
   • Brake efficiency
   • Load and tyre pressure
   • Temperature of tyre and pavement
   • Type of skid, if any
   
   Indian Road Congress (IRS) has recommended a value of 0.35 to 0.40 for the longitudinal coefficient of friction depending on the speed and a value of 0.15 in case of lateral friction.


2. Pavement Unevenness
   
   Even pavements are desirable to have higher operating speed, less tractive resistance, less fuel consumption and less fatigue and accidents.
   The pavement surface condition is measured by Bump Integrator in terms of unevenness index, which is the cumulative measure of vertical undulations of the pavement recorded per unit horizontal length of the road.
   A value less than 150 cm/KM is desirable for high-speed highways and a value of 250 cm/KM is satisfactory up to a speed of 100 KM/H and a value more than 350 cm/KM is considered very uncomfortable even at speed of 50 KM/H.


3. Light Reflecting Characteristics
   
   Night Visibility very much depends upon the light reflecting characteristics of the pavement surface. The glare caused by the reflection of the head light on Wet pavements is considerably more than on the dry pavement.
   Light coloured or white pavement surface gives good visibility at night particularly during rains but they produce glare and eye strain during bright sunlight.
   Blacktop pavement surface provides very poor visibility at night, especially when the surface is wet.


4. Cross Slope or Chamber
   
   Cross Slope is the slope provided to the road surface in the transverse direction to drain off the rainwater from the road surface.
   
   Camber is provided because of the following reasons:
   • To prevent the entry of surface water into the subgrade soil.
   • To prevent the entry of water into the bituminous pavement layers.
   • To drain off the rainwater as soon as possible and to allow the pavement to get dry soon after the rain.
   
   Camber of a pavement depends on:
   
   • Type of pavement surface
   • Amount of Rainfall
   
   
   
   
   

Width of pavement or Carriageway

The width of the pavement depends on the number of traffic lanes and width of the traffic lane. The maximum width of a vehicle as per IRC specification is 2.44 m.

Width of Carriageway

Class of Road	Width of Carriageway (m)
Single Lane	3.75
Two lanes, without raise kerbs	7.0
Two lanes, with raised kerbs	7.5
Intermediate carriageways (except on important roads)	5.5
Multi-Lane carriageway	3.5 per lane

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