With lawmakers and road safety lobbyists raising concerns about a potential uptick in impaired driving incidents due to marijuana legalization, dozens of firms have been racing to develop a reliable, roadside test for cannabis impairment.

But while there have certainly been plenty of attempts, none of the devices seem able to do, um, precisely what they're supposed to do: namely, tell whether the person being tested is sufficiently impaired by the drug for it to effect their driving ability.

As Oakland-based Hound Labs puts it, "although measuring THC in blood, urine, or saliva is relatively easy, this type of testing doesn't distinguish recent use from chronic use [...] Frequent marijuana users often will have elevated levels of THC in their systems. Even if they haven't smoked marijuana in several hours and are therefore unlikely to still be impaired."

As NPR has reported, police in Washington can seek a warrant to test drivers suspected of cannabis impairment for THC, but the results from such tests often take weeks to come back. In the meantime, police have to rely on other signs, like the smell of marijuana in the vehicle, or the driver having red eyes, that could result in some drivers been targeted by false suspicions they were driving under the influence.

"Seventeen states have per se drug laws whereby it is illegal to drive with any detectable amount of a prohibited substance in ones' blood," according to AAA, adding that five states have established permissible limits, most notably for marijuana.

"Surprisingly, these laws are not based on any data that ties a driver's level of impairment to the presence or concentration of drugs in their body."

"There is understandably a strong desire by both lawmakers and the public to create legal limits for marijuana impairment in the same manner we do alcohol," Marshall Doney, AAA's president and CEO, told The Toronto Star. "In the case of marijuana, this approach is flawed and not supported by scientific research."

Although, according to the AAA, "It's not possible to set a blood-test threshold for THC, the chemical in marijuana that makes people high, that can reliably determine impairment," that hasn't stopped 18 states from placing limits on marijuana intoxication while driving.

h/t NPR, The Cannabist, The Star.