How high is too high to drive? That’s the question lawmakers in Colorado and Washington are grappling with now that marijuana has been decriminalized across both states. The issue has sparked intense political debate over the past few weeks, and at this point, there doesn’t seem to be a clear solution in sight. While there is broad agreement that impaired drivers should be kept off the road, some worry that unreliable technology and murky science could lead to unwarranted convictions.

Drugged driving remains illegal across all 50 states, and the recently passed pot laws in Colorado and Washington will do nothing to change that. Thus far, though, there’s no uniform system for determining whether a driver is in fact under the influence of marijuana. Some states police drugged driving by enforcing hard limits based on the chemical composition of a driver's blood. Under these standards, known as per se limits, drivers who test above a certain level for THC (the active ingredient in cannabis) are automatically subject to DUI convictions, much like those whose blood alcohol content (BAC) levels are in excess of .08 percent.

Washington’s Initiative 502, which went into effect earlier this month, sets the legal driving limit at five nanograms of active THC per milliliter of blood. Nevada and Ohio have each implemented a per se limit of two nanograms, while several other states have zero-tolerance policies.

In Colorado, lawmakers have yet to agree on a statewide THC limit, despite the fact that Amendment 64 has already been added to the state constitution. Per se provisions have come up for a vote on three occasions over the past two years, but were struck down in each instance, underscoring both legal and scientific ambiguities at play.