When a driver has accidentally travelled onto the road shoulder the risk of crashing will be less if the vehicles can either stop on the shoulder or safely travel back into the traffic lane.

The vehicle will be better able to do both of these things if the shoulder is wide enough and if the vehicle tyres are able to grip the surface of the shoulder.

A sealed road surface provides the best grip for tyres. When a vehicle leaves the road, and especially when this happens at high speed, stopping and/or steering the vehicle back onto the road will be easier if the vehicle tyres are able to grip the shoulder surface.

An adequate shoulder width makes it easier for a driver to steer the vehicle back onto the road at a shallower angle, reducing the chances that the driver will ‘overcorrect’ and travel into oncoming traffic. Shoulders that are too wide create a hazard if they are used as an additional lane.