Like we control it now? That argument still uses the assumption of increased use after legalization. I didn't turn into an alcoholic when I got 21 and I didn't start smoking either just because it became legal for me. The age limit was a joke. We could get anything at any time regardless of the law but it was harder to find people selling black market cigarettes and booze than it was to find illegal drugs.
If marijuana were legalized, there is no question in my mind that usage would increase. No question. And, my libertarian side says OK, but I still worry about the highways.