There's no established scientific standard that's equivalent to the well-known blood-alcohol content level. Pot's active ingredient, THC, can stay in your system for days, without much indication that you could be long past a state of highness.
"The presence of active THC is generally suggestive of recent marijuana use," the Auto Club of Southern California says in a statement. But that's about as much as you can say about it, scientifically speaking.
The AAA Foundation for Traffic Safety recently analyzed lab results of drivers arrested for driving under the influence of marijuana and concluded that "legal limits for marijuana and driving are problematic," according to the Auto Club of Southern California.