It's settled. Pot is legal in Washington and Colorado. But how much is too much to drive? That's what officials are trying to figure out, and it isn't as easy as measuring a driver's blood alcohol content. Marijuana stays in the system long after its side effects wear off. And there is no widely available test to determine whether someone is driving while impaired. Lawmakers in Colorado, which also allows medical marijuana, have failed to set a driving limit for THC – the psychoactive chemical in marijuana. They say they'll try again next year. Washington's marijuana law sets a driving-while-intoxicated limit for THC. But that standard already is being criticized as arbitrary.