Working in Mayfair in London I get to see a lot of wealthy people in the back seats of their Bentleys. And they are never, ever wearing a seat belt. That's illegal in the UK, but I imagine that these wealthy scofflaws understand that they are unlikely to get into trouble that actually hurts them (the fine for not wearing the belt is £500).
But you can't be a law of physics scofflaw, if you are unrestrained in a car then inertia's going to get you. Just ask Princess Diana. You do not want to be flying around inside a metal box.
Which brings me to taxis. Why do people not wear seat belts in the back of a taxi? When entering a taxi recall Scotty saying "A canna' change the laws of physics, Captain". Put the seat belt on.
And having flown a lot and seen a stewardess go flying when we hit unexpected turbulence: wear the seat belt on a plane all the time. In fact, I think there ought to be three signs in a plane: seat belt (always illuminated), no smoking (always illuminated) and a new "it's safe to get up" sign that goes on and off and is a replacement for the current seat belt sign.
And if you need any more convincing, just ask Jimmy Saville.