it's not worrying at all to me. take a look at colorado - after legalization, accidents dropped as well as crime. plus it's way, way easier to drive while high than it is to drive while you're drunk.
That's not necessarily the major point here. Normal marijuana smokers retain lots of traces in their blood for weeks or more and theoretically can be at DUI levels behind the wheel at any time. Considering it's more of a norm in these states, there could be higher %s of people consuming mj behind the wheel making easy targets for police busting.