A recent study in Germany on the effects of cannabis and driving should really put the fearmongers at ease for now. This study using a driving simulator found that 1) driving impairment does not really correlate with blood THC levels, except above a threshold of >15 ng/mL in serum (roughly 7.5 ng/mL in whole blood), and 2) any impairment that does occur apparently dissipates and driving ability returns to baseline after a mere three hours following toking.
Of course, smoking very large quantities, or very high potency strains, and/or eating cannabis edibles, will undoubtedly lead to longer periods of greater impairment than observed in the study. But as a general rule, the results of the study make sense.
Thus, zero-tolerance policies for driving under the influence of cannabis, as well as setting an arbitrary per se blood THC limit analogous to alcohol, appear to be unjustified. That is not to say that a reasonable prima facie blood THC limit (i.e. above which creates a rebuttable presumption of guilt) is bad--Colorado currently sets theirs at 5 ng/mL--but setting it too low or rigidly per se would punish far too many non-impaired drivers. And there is no evidence that zero-tolerance or per se limits for THC actually save lives.
Of course, this is not to suggest that driving stoned is a good idea or without risk. But driving under the influence of cannabis alone is less risky than driving under the influence of alcohol, and the often hysterical fears of prohibitionists are largely unwarranted, while legalization advocates are unsurprisingly vindicated once again.
Remember: "when in doubt, wait it out."
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