I won't start the car until all passengers in my car have their belts on correctly. I've had cheeky children passengers who undo them, or try the under the arm trick - I just pull over out of the traffic and wait until they comply.
There are many people who don't know these things, common sense or not. If they didn't learn them at school or at home, at least they can learn from social media. "You don't know what you don't know."