I don't think anyone has been able to conclusively say that marijuana legalization actually increases the percentage of smokers in any state where it has been implemented.

I really don't care about the increase. I care about disproving / proving DUI in court. Without a valid blood test / breath test and some standards, its going to be a rough go.

As to if high people are bad drivers, then yes they are. I don't even know how that's a debate. What constitutes high is the real debate.

My point is that all of this is already a problem. Legalization will, if nothing else, make it easier to conduct research and get funding to fix some of this.

Well, when pot was illegal they just arrested them on possession and didn't have to worry about the rest. Now, we need some standards or else some people are getting DUIs.

Do we have the science to know how safe/unsafe it is to drive under the influence of marijuana?

There have been a few studies out of CO that I can't find at the moment. The gist is that it's less safe than being sober, but a bit more safe than driving drunk. Unfortunately, the real finding is that combining the two is a lot less safe than either on their own and that a lot of drivers are choosing to cross-fade.