I remember reading somewhere that modern seatbelts are a tradeoff in design. For the buckle to be big enough to not deform in an accident it would have to be much heavier and latch stronger and less people would use them. They had to design them light, simple and as minimally intrusive as possible so the most people would wear them. The baby/toddler carriers/seats are a good example, it takes a lot longer to buckle them in than it does an adult and if it took the adult that long most would just skip the seatbelt.

Wait until you have a seat belt buckle/latch fail in an accident. My mother did in 1989, and GM paid all the medical bills no questions asked.

I doubt that would happen today.

Everything in engineering is a tradeoff in design.
Seatbelts work very very well, the chance of an injury due to a seatbelt jamming closed is tiny compared to the number of saves from wearing one.

If you are concerned you should also wear a cycle helmet when driving. More people suffer serious head injuries in car accidents than in cycling accidents.

As an engineer, I agree 100% that every design is a compromise. But I'm still not convinced that it is impossible to bias that compromise just a little bit more towards robustness.

And no one need worry that I'm not going to wear a seatbelt for fear of it jamming - I understand physics!

I was just asking myself, "How often do seat belt buckles actually jam?" Is there a statistic anywhere out there?

Seat belts are probably cut all the time by first responders, but I suspect that few are actually because the buckle jammed, as Martin also alluded to. I would think that the majority of cases where the belts are cut have nothing to do with the buckle. The structural damage to the vehicle or the final position of the belted occupants probably make it easier to cut the belt than try to reach the buckle.

I didn't watch that video, but was the buckle actually jammed? If a car interior is on fire, the last thing I want to do is stick my head inside a flaming oven to reach over and unbuckle the belt. Is that the reason why they're yelling for something to cut the belt?