There seem to be loads around here. I think they justify it by assuming that because we live in a city, traffic doesn't move that fast. Some wear it as some sort of badge of honour though, as though being a risk-taker with your children is something to be proud of and something that we neurotic types should consider.
It doesn't normally affect me directly, but it did a couple of weeks ago when a Father turned up to pick up my DC and was about to drive off without DC in a childseat.