Please explain to me how not wearing a seatbelt at any time makes the "roads more dangerous". The intent of the law is that the "roads" not be "more dangerous" to others, which is the only reason for the "Rules of the Road" comprehended by the states police power.
One suspects that there's been a rash of people who've crashed through windshields at high rates of speed, thus endangering lookers-on and other motorists. This, of course, has made the "roads much more dangerous after sunset." :)