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Friction between the wheel and the pavement surface is a crucial factor in the
design of horizontal curves and thus the safe operating speed.
Further, it also affect the acceleration and deceleration ability of vehicles.
Lack of adequate friction can cause skidding or slipping of vehicles.
- Skidding happens when the path traveled along the road surface is more
than the circumferential movement of the wheels due to friction
- Slip occurs when the wheel revolves more than the corresponding
longitudinal movement along the road.
Various factors that affect friction are:
- Type of the pavement (like bituminous, concrete, or gravel),
- Condition of the pavement (dry or wet, hot or cold, etc),
- Condition of the tyre (new or old), and
- Speed and load of the vehicle.
The frictional force that develops between the wheel and the pavement is the
load acting multiplied by a factor called the coefficient of friction and
denoted as  .
The choice of the value of is a very complicated issue since it depends on
many variables.
IRC suggests the coefficient of longitudinal friction as 0.35-0.4
depending on the speed and coefficient of lateral friction as 0.15.
The former is useful in sight distance calculation and the latter in horizontal
curve design.
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