==But a cliff is an obvious threat.==

But there are also other people around and animals and high wind speeds and any number of other factors that impact safety. I bet you still follow stop lights when nobody is coming the other way.

==Because children are not responsible and hence have to be protected from them self.==

This is an argument for putting rails on cliffs, as children are not barred from cliffs. Seems like you have talked yourself into a pretzel.

==I said that back when they didn't exist people drove more careful. With all those protection in place in cars - many people drive more reckless having those protections in mind. ==

I get the point, people may drive more recklessly because they feel protected by the seatbelt. I would say that all regulations are a cost-benefit equation. In the case of seatbelts, the benefit of more people living is larger than the cost of lost freedom.