Two years ago a California initiative to legalize marijuana on a county-by-county basis for people over 21 was voted down, 53% to 46%. California has traditionally held some of the most liberal marijuana laws, first legalizing medical marijuana in 1996. 2010′s Proposition 19, which would have made marijuana similar in status to alcohol, failed in part because it didn’t address a fundamental issue—a THC driving limit.

Unlike alcohol, which offers a clear gauge of a person’s current drunkenness with the use of a breathalyzer test, marijuana is impossible to test in real time. THC, marijuana’s psychoactive agent, stays in the system for weeks and scientists don’t currently have a test to determine how stoned a person is at any given moment.

States are working on solution, however.

Reuters reports “Dr. Marilyn Huestis of the National Institute on Drug Abuse, a government research lab, says that soon there will be a saliva test to detect recent marijuana use.” The saliva test currently being developed still won’t detect current levels of stoned-ness—only whether the person has smoked recently or not. “I’ll be dead – and so will lots of other people – from old age, before we know the impairment levels,” said White House drug czar Gil Kerlikowske.

The issue raises all kinds of philosophical questions: namely, if we can’t scientifically measure impairment, how do we even know it’s real? Does 2 hits on a joint impair you but not me? If there is no universal standard on impairment as there is with alcohol (e.g. blood alcohol level), how are we supposed to regulate it? If we can’t regulate it, how do we legalize it? What if when I see blue, you see green, man?

Without answers to these questions, making a simple rule about how long you have to wait after smoking pot before driving a car might be a helpful solution. As much as a fleet of State Troopers armed with a new saliva test with which to harass you sounds like a potential nightmare, the “marijuana breathalyzer” might actually be a good thing.

Just as most wine lovers aren’t gunning to drink a bottle and get behind the wheel, most marijuana enthusiasts aren’t looking to take three gravity bong hits and head out on the highway. I think most of us could probably agree on some reasonable amount of time you should wait before driving a car after smoking pot. Maybe an hour? 90 minutes? Having a standard rule like this through which cops could test stoned driving, similar to how they enforce drunk driving rules, might actually give future marijuana legalization initiatives a better chance of passing.

And legalizing pot would do all kinds of great things, like create new tax revenue streams, cut government spending and keep people out of jail—as long as you don’t smoke and drive.

According to Reuters, “voters in Colorado and Washington state will decide this fall whether to legalize the drug for recreational use, bringing a new urgency to the issue.”