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CNN affiliate KIRO tested drivers before and after they smoked marijuana to see how it affects their abilities. Something that had never occurred to me is how states with varying levels of legal marijuana use have to establish limits on how much would make you legally ineligible to drive a vehicle. They have, and if these test subjects are any indication, one would probably need to consume several times the legal limit before police will observe you driving erratically. That said, you should never operate a vehicle, or any heavy machinery, while impaired, legally or not. -via reddit