Last month’s vote to legalize weed in Colorado and Washington will present a myriad of logistical and legal challenges as citizens and lawmakers figure out how to live with the new paradigm. One such challenge was addressed by President Obama this morning. A report from the Verge raises another: what do you do about driving while high?

Most would agree that drivers whose abilities are impaired–no matter the substance or circumstance–should be kept off of the road. The questions facing Colorado and Washington lawmakers, however, are more complicated: How high is the limit? How do you develop a reliable roadside test for cannabis inebriation? How do bodily THC levels relate to impairment?

As of now, police rely on observational impairment tests to make an arrest, then suspects are taken elsewhere for blood testing. This, predictably, is not the most efficient method in the world, so public- and private-sector researchers are looking for a breathalyzer-style roadside test. Their options range from the pedestrian (performing cheek swabs and running tests on the saliva) to the Orwellian (scanners that test for sweat-borne THC while taking a suspect’s fingerprint).

Whatever the method that’s settled on, authorities will then have to contend with the uneasy correlation between THC levels and impairment. Unlike alcohol, which has a relatively predictable effect on people, cannabis’s impairment level depends heavily on tolerance and the strain of weed being smoked–two people with the same amount of THC in their blood could have vastly different levels of high going on. Take it from Office of National Drug Control Policy Director Gil Kerlikowske, who said “I’ll be dead — and so will lots of other people — from old age, before we know the impairment levels” of the plant.

Whatever happens, officers will probably have to rely on those tricky observational tests for the time being. “I believe something will be invented,” said Sergeant Mark Crandall of the Washington State Patrol Drug Evaluation and Classification Program. “But even once it’s invented, it would probably take years to bring it to market and then even more time to get it accepted by the court systems.

(Photo: Théo/Flickr)