Why is it difficult to maintain constant speed of a vehicle?​

Roads consist of curves, gutters, etc. Thus not allowing the vehicle to maintain a constant speed. Also it can be explained as due to a change in the frictional surface of the road. hope it help you.

The road surface is always rough, so the frictional force between the tires and the road surface make it difficult to maintain constant speed because acceleration always changes.

r=rho l/a where a is the area, l is the length and rho is resistivity so the greater the area the lesser the resistance thats why the earth pin is thicker bcos current follow the shortest path, so less resistive .