Heck, a lack of proper sleep will impair you more than smoking a joint. Not that I condone toking and driving. It's not harmless as some people suggest, but then neither is Vitamin A or water. Too much of either and you're going to have a bad day.
However, some police are even beginning to question just how much marijuana impairs a person's ability to drive, although unquestionably there are some strains of marijuana that one definitely shouldn't plan on driving while or after toking.
And I will also contend that in most accidents where pot is the leading cause, it isn't so much a case of impairment but a case of toking while driving. It's especially difficult to pass and receive a joint while trying to drive. The distraction level is huge, unless you have a nice pair of hemostats.
I would say "no problem"...............unless they can determine how much is safe, then zero should be acceptable!