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As 4:20 threatens to become as popular as happy hour, police are scrambling to figure out a reliable way to make sure that the dude who’s zoning out in the Taco Bell drive-thru isn’t too stoned to drive. The problem is, roadside breathalyzer tests administered to drunks don’t work for pot smokers, forcing cops to take suspected stoners into the station for a blood test. Now the Swedish company SensAbues is offering something of a fix. A study published in the Journal of Breath Research last week found that its proprietary breath-testing device can detect recent use of a wide range of drugs, including prescription meds, cocaine, and marijuana.

“The device is based on a filter, you can actually just blow through it,” developer Olof Beck told a Swedish interviewer. “It’s very, very easy to collect the sample.”

But here’s why Beck probably isn’t going to be landing contracts with the Colorado or Washington state troopers anytime soon. First off, his device only detects the presence of marijuana, not whether a driver has consumed enough to be impaired. And even more problematic from a cop’s point of view: The filter samples have to be analyzed in a lab—which means no instant field results.

Those technical obstacles may not be insurmountable. If science can figure out how to grow weed in vehicles, then surely it can develop some kind of mobile testing center for Beck’s breathalyzer filters.

But then, there’s an even more basic question that American society has yet to answer: How high is too high to drive?