I am not sure I believe in a modern car catching fire on start-up - more likely, I should have thought, when components start to get hot. And still less do I believe in the fire spreading so quickly that the time taken to remove a seat belt is significant. When a car I was driving caught fire, there was easily time to stop on the hard shoulder and get people and luggage out. In fact, although the car was a write-off, there was time for a police car to stop and put out what was still quite a small fire with their extinguisher.
While other fires will be different, immediate combustion is probably limited to car bombs - in which case not having done up a seat belt will not save occupants.
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