When a car is moving with a high velocity, its momentum is also high and the people in the vehicle are also in the same momentum. We know that momentum is inversely proportional to time. So if the momentum is bought to 0 at a short time then force acting on a body is very large. So when a car meets with an accident, the person falling front with the presence of stretchable seat belt as these elastic seat belt reduce the momentum in more time to reduce the force.