Fatal road crashes involving marijuana double after state legalizes drug: Foundation research also shows that legal limits for marijuana and driving are meaningless -- ScienceDaily
I always suspected that this would be a likely problem, but the evidence to show that it was really was seems to have been slow coming forward. And the thing is, because of the way THC works and hangs around for a long time at detectable levels, it's a tricky one to respond to. (Short of saying any THC in the test will result in a punishment, I guess.)
Update: here's a story about a recent case in Australia illustrating the difficulties of making "drug driving" laws for THC. I see that the Greens recommend following British laws where they also test for impairment - but I've always been doubtful about the reliability of roadside impairment testing.