It's in Newton's 2nd law
The force you experience during a crash is defined by Newton's 2nd law.
Hence the force exerted on a person in a crash is a function of three things, mass, initial speed, time to come to a stop. Once a crash is initiated, there is not much you can do about u and m. However, the longer the time period over the deceleration, the less force you will experience. This is where the seat belt comes in. It stretches during a collision, increasing the time over which the force is exerted on the passenger, thereby reducing the force they experience.