Most people know that when a car that is moving at some amount of speed crashes into a stationary object or another moving object, damage will be done to at least one piece of the accident. When cars strike each other or a stationary object, there are really two accidents or collisions that occur in the great scheme of making contact with an object or entity.

The first accident in the scheme of the two accidents is the obvious one that causes a lot of damage to the car. Here, the car will crumple and any structural damage to the car will occur. This is the damage that everyone thinks of when they think of a car accident. This is the less dangerous part of a car accident as well because it doesn't involve the passengers of the car actually making contact with anything.
 
The second accident is less recognized because the two collisions come so close together that everyone thinks of a car accident as just one big accident. This collision occurs inside of the vehicle and involves mostly the passengers of the car. Here the forward momentum with which the passengers were travelling causes them to continue moving forward after the rest of the car has stopped. If unrestrained, they will keep continuing forward until another force acts upon them to stop the momentum. In car accidents, the other force is the steering wheel, the dashboard, a ceiling, the windshield, or something else in the car. 
 
The reason this second collision occurs is because of a simple rule in physics. It is said that an object at rest will stay at rest until another force acts on it. An object in motion will stay in motion until there is another force exerted on that object. In order for a person to move in a car, the person adopts the speed and momentum of the vehicle. The vehicle hits the other car or an object and stops. But the force of the other car or the object doesn't exert force directly on the passengers. So the passengers are allowed to continue moving forward until they are stopped by a seatbelt, an air bag, or a hard part of the car.
 
Another side effect of the two accidents is that it makes it possible for a person to be completely thrown from a vehicle in the event of an accident if there is no windshield or seatbelt to stop the forward motion. Being thrown from the car is extremely dangerous as it exposes the person to all of the activity and friction outside of the car.