Road Characteristics:
Pavement Surface Characteristics :
The important surface characteristics are:
1) Friction :
The friction between the vehicle tyres and pavement surface is one of the important factors determining the operating speed of vehicle and distance requirements in accelerating and stopping the vehicles. The coefficient of friction or skid resistance offered by the pavement surface is important for safety.
Skid and Slip
Skid :
Occurs when the wheels of the vehicle slide without revolving or when the wheels partially revolve.
i.e. when the path travelled along the road surface is more than the circumferential movement of the wheels due to their rotation, as brakes are applied.
When brakes are applied the wheels are locked fully or partially, and the vehicle moves forward, the longitudinal skidding takes place which may vary from 0 to 100%.
Slip :
Occurs when a wheel revolves more than the corresponding longitudinal distance along the roads. Slipping usually occurs in the driving wheel of a vehicle when the vehicle rapidly accelerates from stationary position on pavement surface which is wet and slippery.
Factors affecting friction or skid resistance are :
- Type of pavement surface, namely bituminous, cement concrete, WBM or earth surface
- Roughness of the surface
- Conditions of pavement namely dry, wet, smooth, rough, etc.
- Condition of tyres
- Speed of vehicles
- Brake efficiency
- Load and tyre pressure
- Temperature of road surface and tyre, etc.
2) Unevenness of Pavement :
If pavement surface is even and with less undulations, vehicles can travel at a higher speed. Pavement unevenness also affects the vehicle operation cost, comfort and safety. Fuel consumption and wear and tear of tyres and other moving parts increases with increase in pavement unevenness.
3) Light reflecting characteristics :
Night visibility is affected by the light reflecting characteristics of the pavement surface.
Light coloured or white pavement surface gave good visibility at night during rains, and they produce glare and eye strain during bright sunlight.
Bituminous surface (black top pavement) on the other hand provides very poor visibility at nights, especially when the surface is wet.
Braking test :
Braking test is used to determine the skid resistance of pavement surface. If the brakes are applied till the vehicle comes to stop, it may be assumed that wheels are fully locked and the brake efficiency is 100%.
During braking test at least two of the following three measurements are needed in order to determine the skid resistance of the pavement :
- Braking distance (L), meters
- Initial peed (u), m/s
- Actual duration of brake application (t sec)