I was that guy who did his seatbelt up and sat on it so it didn't beep. Until my wife and eye were on are way to our honeymoon wen a car in front of us lost control and flipped over the barrier. Ending up lay on the drivers door. We stopped partly because I have experience of these situations and because my wife is a nurological specialist sister.
When I got on top of the car I found that the dive wasn't moving and his wife was stood on him screaming. his wife wasn't wearing her seat belt and her window was open. The top half of her body had slid along the road as the car rolled becoming trapped under it. It won't go into detail but it wasn't a pretty sight. More people stopped, 2 of them wanted to help but couldnt look that the blood so they held the car steady whilst I pulled the casualty out. It was risky as we couldn't fully assess her but my wife said better to get her out ASAP so she could be treated rather than wait for fire crew to remove windscreen. An as the male wasn't moving we needed to get in to treat him.
Once out my wife (and by this point a passing gp) started to treat her using the combined first aid kits we had.
The male was fine. In a bit of shock but other than that ok. Why because he had his seatbelt on and could get it off coz his wife was stood on him. Wen the paramedics and police arrived we gave our details and left.
We were only 5 mins from the airport and wear so worried about missing our flight that we decided to change our bloody clothes in the airport toilet before the check in. Which went well.
Since that day I always wear my seatbelt. I plead for everyone else to do the same.
Side note. Whilst the female was being treated and I was pulled her husband out covered in her blood l, she asked my wife if we had any water. My wife got my bottle of water out of our car (as it was closest) to which the female asked if anyone had drank out of it. She was told yes and refused to drink in. I didn't find this out till I was on the plane.