At relatively low consumption, intoxicants like alcohol and anti-anxiety drugs like Xanax impede more than a few hits of marijuana. That sad, if you just took hits from a bong or consumed pot brownies, you shouldn't be driving.
Tolerance builds in every drug. An alcoholic can consume a six pack and just barely be buzzed. A heroin addict can shoot up a half gram of dope in an hour only to stave off withdrawals while that same amount would kill a new user. For now, to avoid even more criticism, setting an acceptable level for THC in your body will have to suffice.
In almost all situations you broke a law that resulted in the cop pulling you over and you're exhibiting unusual behavior to boot. Just inspect your car's lights and license plate before driving out. And if you really have doubts about whether you can drive or not, don't. Luckily smoking pot doesn't give most people an inflated ego like alcohol.