The pyschoactive effects of the drug don't matter much. I think we can all agree that marijuana and even "hard" drugs don't impair you as much as alcohol.

Prohibition only has a modest impact on drug consumption, and most of this is just casual consumption...it does little to stop the would be drug abusers. Just as most people who use alcohol only do so casually, there are even a good number of heroin "users" who would not necassarily be classified as "addicts". This said, the people who are constatly on drugs probably wouldn't be too worried about driving under the influence anyways. The fact that the substance is illegal may even encourage the user to drive under its influence since they have already broken the law.

Go to a library or book store and find Drug War Crimes: The Consequences of Prohibition by Jeffrey Miron, it doesnt take long to read and explains the externalities caused by prohibition and how legalization would likely be better than decriminalization.