A common argument. Used by people advocating legalization of drugs, against prohibition, etc.
It may sound silly regarding traffic, but it's the argument used by people who advocate legalization of drugs. It was the argument used by folks against prohibition. It's used by people advocating wider gun ownership, offering immigration immunity, any number of things.
Actually, murders continue even though there are laws against it. Burglary, rape, embezzlement, all manner of crimes. If we legalized all of them, the prisons could empty and bigawd, the country would be crime-free in one fell swoop.
If only stupidity could be criminalized, a felony, at least they couldn't vote.