Actually marijuana is not yet sold recreationally in California since a 1 year moratorium on selling it to the public has been imposed. This length of time has been rationalized by saying it gives time to figure out how to tell if a driver is intoxicated by marijuana. Testing for traces of marijuana should appear to be straightforward, like testing for alcohol, but it is complicated by the fact that it could take days or even weeks for the inactive chemical tested for to clear your system even though you're no longer under the influence after a few hours (It is the metabolite, not the active ingredients, that stays in your system).
This moratorium is unjustified since other states have not imposed any of their own and no ill effects have occurred. Also, under the law, local jurisdictions can impose their own laws negating the cultivation or use of marijuana.