Imagine driving your car to the grandparents' house. It's a small family car that seats five but you've got six in the back seat anyways - plus two car seats. On one end in a pink car seat is your youngest daughter and on the opposite end is a 5th grade boy squeezing against the car door while sharing a one butt seat and belt with his 5 year old brother. In the middle, strapped into a brown car seat is your four year old boy with Autism.
He's either been in the car for too long, is annoyed by the sound of the car on the freeway, thirsty, hungry, has to go the bathroom. . . whatever the reason, he is randomly and repeatedly shouting out the same silly word that he heard one of his brothers say a few miles back while intermittently kicking both front seats, smacking his brother and sister, and trying to unfasten his car seat. Eventually, the car seat comes undone and your son is trying to climb up to the front of your car while laughing hysterically.
You can't threaten a punishment, there isn't a chance in hell that you're going reason with him to stop by reminding him of the danger of taking off your seat belt in the car. You just pray to God not to let you get into a car accident and remind yourself that THIS is why you never drive places and this is why you do not go out in public.