Actually, all available evidence points to the fact that legalising drugs like heroin actually decreases their usage. Would you suddenly take up heroin if it was legal? Do you know anyone who would?
But then why is alcohol and tobacco use so prevalent, since those substances are legal? Did the number of drinkers decrease in the U.S. after Prohibition was lifted?
I also wonder if we would see the same sort of safety issues if, say, alcohol was banned but marijuana and cocaine were legalized (driving under the influence, violence--not with pot, I suppose)?