When a car is going down a hill and it isn't on "parking mode", unbalanced forces are acted upon the vehicle because the hill causes the vehicle to move. The unbalance force when a car crashes into a building is the wall of that building. That car was moving at a certain speed at a specific direction and the wall caused a halt to the car's movement also causing a possible change to the car's direction. A two-car collision has two unbalanced forces: both cars. The reasoning behind that answer is that both cars are coming from two different directions and both cars abrupt to each other's direction and motion.
