The way our drug policy will get more rational is for sub-Federal entities to make their own laws and dare the Feds to come enforce them. This is exactly what people mean when they talk about the states being laboratories of democracy. And this is a great test to see if all the "states' rights" small-government people are blowing hot air, or mean what they say.
That said, there are completely legitimate questions to ask as we're starting to see exactly this happening, and one of them is: to what degree does legalization affect driving safety? It must be said that this article addressing the problem in California gets an F for statistics because it equates "1 in 7 drivers at checkpoints tested positive for drugs" to "1 in 7 drivers is under the influence" - obviously those are two different things, plus the population that gets stopped at checkpoints (about 50% of the total) is enriched for alcohol and drug use.
But that said, a great way to say legalization to hesitant voters is sell it as improved enforcement on drug use in dangerous circumstances, of which driving is #1. Tax it (as everyone except the politicians seem to be suggesting) and use some of that revenue for improved enforcement, which isn't being done systematically right now like it is for alcohol. This isn't just a gimmick either. I don't mind having this protection for alcohol and I'd like to have for all other mind-altering substances as well.