David Zalubowski/AP

As more states legalize recreational marijuana, they're also looking to police people who drive while stoned. Research to create a breathalyzer for marijuana is advancing—but evidence shows that driving under the influence of pot doesn't actually cause accidents.

Colorado breathalyzer company Lifeloc Technologies, which received a $250,000 state grant last year to develop the equivalent of a marijuana breathalyzer, is moving forward with the new technology, the Durango Herald reports.

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It's a growth industry: a Vancouver company called Cannabix Techologies is also developing a handheld pot breathalyzer, and according to a press release, a model is in the prototype stage. Meanwhile, researchers at the University of Akron and Washington State University are working on their own marijuana breathalyzer projects, one of which is called the "Cannibuster."

The science behind a pot test isn't as simple as an alcohol breathalyzer, however, because evidence of marijuana can linger in a user's body for more than a week. In order to get around this, pot breathalyzers should only detect Delta-9 THC, the psychoactive ingredient in the drug. Right now, the only sure method of detecting this is a blood test, which can take a long time to get results.

All of the states that currently allow recreational marijuana—Colorado, Washington, Oregon, and Alaska, as well as Washington, D.C.—outlaw driving stoned. The current standard for marijuana impairment is five nanograms of THC, which some say is an arbitrary limit unsupported by science. The "marijuana critic" for the Denver Westword, an alt-weekly, once tested three times higher than that limit 15 hours after smoking. Colorado police issued 87 marijuana DUI citations in the first quarter of 2015, the Herald reports.

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But there's also no evidence that driving stoned leads to an increased rate of traffic accidents: according to a study by the National Highway Traffic Safety Administration released in February, about 12.6 percent of drivers have evidence of marijuana in their systems. What is dangerous is driving stoned and drunk—consuming both substances together is "more likely to be a traffic safety risk factor than when consumed alone," NHTSA says.

Colorado Governor John Hickenlooper signed a bill last week increasing penalties for drunk driving.

Casey Tolan is a National News Reporter for Fusion based in New York City.