Watch out – your days of driving around at 20mph post-spliff in an old Fiesta might be coming to an end. Weed has long been the drug of choice for motorists, mainly because it's impossible for the police to prove on the spot that drivers have been blazing even if all the signs are there. Currently, the fastest way to establish if someone's been blazing is a blood test – not the most road-friendly check around. 

Scientists at Washington State University are working on a portable breath test that'll return an instant result. Stoned motorists have become a concern in the state since residents voted to legalise recreational marijuana in 2012. In fact, blood samples taken from drivers in 2013 showed that a quarter of them tested positive for THC, the main active ingredient in marijuana.

Chemistry professor Herbert Hill is working with a student called Jessica Tufariello to build a handheld device that will tell an officer if there's any THC in the suspect's bloodstream. They're hoping to test out the breathalyser on people at the beginning of next year.

Speaking to the News Tribune, Hill said: “We believe at least initially that it would lower the false positives that an officer would have. They would have a higher level of confidence making an arrest." A spokesperson for the Washington State Patrol said that the department would “welcome anything that will help us get impaired drivers off the road".

A 2002 study found that a glass of wine affected driving skills more than a spliff, although those who combined both substances were the worst drivers of the lot. 

If the technology proves reliable, it's likely that the weed breathalyser will spread to this country – we imagine that any government would leap at the chance to rake in some extra cash through DUI fines. If police can crack down on stoned drivers on the spot, expect scenes of small, suspiciously-smelling cars crammed with teenagers driving at a snail's pace to become a thing of the past. 

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