Is friction acting for average speed if vehicle passing on a banked road?

Question

Roads are usually banked for the average speed if vehicle passing over them. However, if the speed of a vehicle of a vehicle is somewhat less or more than this the self adjusting static friction will operate between the tyre and the road, and the vehicle will not skid.

The above line are from my physic book. Can anyone explain what " if the speed of a vehicle of a vehicle is somewhat less or more than this the self adjusting static friction will operate" it does mean? Shouldn't the friction component always be acting along with the normal force component?

Answer 1

Normally, if a vehicle turns in a curved path on a road, something has to provide an inward, radial centripetal force to cause the vehicle to change direction (accelerate). On a flat road, that something is the friction between the wheels and the road.

The roads that the book is talking about are banked so that, when a vehicle is travelling at average speed, the horizontal component of the normal reaction force from the road will exactly provide the centripetal acceleration to cause the vehicle to turn along the curved path, so that no lateral friction is required between the wheels and the road to provide this force.

Therefore, if a vehicle is travelling faster or slower than this average speed on the banked road, some lateral friction on the wheels will be necessary to balance the radial centripetal forces. This will 'self-adjust' to provide the centripetal force that is required, unless the vehicle is going so fast that the limiting static friction is reached - at that point the vehicle will skid off the road.