Cannabis is recreationally legal in 4 states and the District of Columbia, and medically legal in 23 states in the US. One of the main concerns of marijuana opposers is stoned drivers, and while as a safety issue this has yet to cause any major problems, law enforcement is looking for effective ways to test drivers on site for marijuana. The issue at hand is that the main chemical in marijuana, THC, can stay in a persons blood stream almost a month after having smoked, leaving a sober driver to fail a THC test weeks after the effects have dissipated. A company in Canada is currently working on the prototypes for an upcoming THC breathalyzer, but has yet to release the details on exactly how it will work. This also leaves more details to be filled within regulation as different states have different legal limits for THC in your system.
Then there's the whole mess of differing levels of impairment between states. In Washington and Montana 5 nanograms/milliliter (ng/mL) of THC counts as "too stoned to drive." In Pennsylvania, that limit is 1 ng/mL. And as Nicholas Lovrich, a political scientist at Washington State University, told Reuters, these limits are based more on politics than on science. So until both the science and policy surrounding cannabis advance beyond their present states, don't expect these devices to be entered into evidence in your DUI case anytime soon
Original article: http://www.engadget.com/2015/07/13/breathalyzer-for-pot/