When the car catches fire, Heather cuts the jammed seat belt with a pocket knife after many unsuccessful attempts to remove it. This is not at all necessary. You can slowly pull the seat belt from top to slacken it, crouch and slid down the seat and remove the belt from the body.
Richard has ample time in saving himself from death. While his car was being pushed towards the precipice, he had enough time to remove his seat belt, open the door, jump out and run for his life.