FWIW, I was told a long time ago that if we had the equivalent of the Breathalizer test for marihooch, it would already be legal. The problem is, that we have no way to control millions of stoned kids (and adults) from getting behind the wheel.
Not so; we still have roadside sobriety tests (walking a line, etc.).
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.