under a bus

For a long time now people have been using falling under a bus as a convenient way of representing mundane, random death. I don't want to get all philosophical but why a bus? In Melbourne we tend to see trams as the people mover in streets, though you hear a lot more of people falling under trains and dying, than you do under trams, for some reason. Of course the most common transport-related cause of death is car accidents, but for some reason these seem more gruesome and 'that's horrible' than falling under a bus, though most of us can expect on present indicators to die prolongedly in drawn-out and miserable care institutions. Is there something vaguely comforting about falling under a bus? Does one think of Big Ears driving the bus full of cotton-reel people, and it's only the Mayor of Toytown who's hit, and he just gets a red mark on his forehead?

Usually falling under a bus is summoned by people who generally indulge in thoughtless, risky self-harming behaviour eg smoking, as 'oh well, I could die from lung or throat or some other cancer, but I could fall under a bus, so...' (the 'so...' meaning, 'The randomness of the world means I refuse to partake in any consideration of what, all other things being equal, the likely outcome of my behaviour will be.') But I have also in work scenarios had it suggested to me that I keep various files and other research materials user-friendly in case I fall under a bus. Strangely, I did feel this was a pretty benign way of putting it, and I wouldn't have felt that way if instead it was suggested I might have been garrotted or fall off a tall building.

So what exactly is so great about falling under a bus?