It is assumed that surfaces are frictionless and the weight of the car remains constant throughout its journey.
When the car travels at a uniform speed on the horizontal section of the road, the weight (say W ) of the car acts on the road. Since the car is in vertical equilibrium, a force from the ground/road acts on the car against the weight of the car, this force from the road is called the normal force (N) .
The diagram below shows the car at horizontal section of the road:

When the car is at the top of the hill its weight W acts on the road. Since the car is in vertical equilibrium, a force from the ground/road acts on the car against the weight of the car, this force from the road is called the normal force (N).
The diagram below shows the car at the top of the roller coaster road:

When the car is at the bottom of the hill its weight W acts on the road. Since the car is in vertical equilibrium, a force from the ground/road acts on the car against the weight of the car, this force from the road is called the normal force (N).
The diagram below shows the car at the bottom of the roller coaster road:


It is important to note that when the car is at the top or the bottom, it becomes parallel to the horizontal surface momentarily, therefore the entire weight W acts on the road.