Question

does accleration increase down an inclined plane

Answer 1

Moving objects have momentum . Newton's First Law of Motion says that unless an outside force acts on an object, the object will continue to move at its present speed and direction.
Automobiles consist of several objects, including the vehicle itself, the passengers inside and any other loose objects in the vehicle.
Unless the objects inside the car are restrained they will continue moving at whatever speed the car is traveling even if the car is stopped by a crash.
Changing or stopping an object's momentum requires a force acting over a period of time. If momentum changes instantly, as in a car crash, the force is very, very great! If the momentum can be changed over a period of time, even a fraction of a second, much less force needs to be applied with less damage or injury.
In a head-on collision, if a passenger flies into the dashboard of a car, their momentum is instantly stopped, and serious injury is often the result.
If the passenger is restrained by a seat-belt, their momentum is reduced more gradually by the constant and smaller force of the belt acting over a longer period of time. Seat-belts can reduce the impact of a passenger to one-fifth of the impact suffered by the body of the car.