About 20 years ago, while driving on US Route 1, I was sitting next to another car at a traffic light.
In my peripheral vision, I saw what looked like a fire.
It turned out to be a couple of white-trash types who were igniting…something…in a glass pipe, with a propane torch.
As soon as I figured out what I was looking at, I floored my gas pedal, and put as much distance as possible between myself and those druggies.
However, I have to point out to the OP that a dropped cigarette butt–or even a dropped MJ joint–is also a possible cause of a vehicular fire. If one of those items is dropped and is not quickly retrieved/extinguished, it is very possible for the situation to result in a conflagration that encompasses the entire vehicle.
In any event, when a driver or passenger is addicted to nicotine, or marijuana, or to something of a much “harder” nature that is “smoked”, that act of smoking in the car has a real potential to result in a vehicular fire.