Legal tests used to determine whether drivers are impaired after using marijuana in six states have no scientific basis, according to a study by the US’s biggest motoring association.
The American Automobile Association (AAA) is calling for laws to be scrapped after a study commissioned by its safety foundation found it was not possible to set a blood test threshold for THC, the chemical in marijuana that makes people high, that can reliably determine whether a driver is impaired by the drug.
Despite the research, the laws in five of the six states automatically presume a driver guilty if that person tests higher than the limit, and not guilty if it’s lower.
This means drivers who are unsafe may be going free while others may be wrongly convicted, the foundation said.
The foundation recommends replacing the laws with ones that rely on specially trained police officers determining if a driver is impaired, backed up by a test for the presence of THC rather than a specific threshold. The officers are supposed to screen for dozens of indicators of drug use, from pupil dilation and tongue colour to behaviour.
The foundation’s recommendation to scrap the laws in Colorado, Montana, Nevada, Ohio, Pennsylvania and Washington comes as several more states consider adopting similar laws.
At least three states, and possibly as many as eleven, are due to vote in the Autumn on measures to legalise marijuana for either recreational or medicinal use, or both.
“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,” said Marshall Doney, AAA’s president and CEO. “In the case of marijuana, this approach is flawed and not supported by scientific research.”
Determining whether someone is impaired by marijuana, as opposed to having simply used the drug at some time, is far more complex than the simple and reliable tests that have been developed for alcohol.
There is no science that shows drivers become impaired at a specific level of THC in the blood. A lot depends upon the individual. Drivers with relatively high levels of THC in their systems might not be impaired, especially if they are regular users, while others with relatively low levels may be unsafe behind the wheel.
Some drivers may be impaired when they are stopped by police, but by the time their blood is tested they have fallen below the legal threshold because active THC disappears quickly. The average time to collect blood from a suspected driver is often more than two hours because taking a blood sample typically requires a warrant and transport to a police station or hospital, the foundation said.
In addition, frequent users can exhibit persistent levels of the drug long after use, while THC levels can decline more quickly among occasional users. Nine states, including some that have legalised marijuana for medicinal use, have zero-tolerance laws for driving and marijuana that make not only the presence of THC in a driver’s blood illegal, but also traces, which can linger for weeks after use.
Mark A R Kleiman, a New York University professor specialising in issues involving drugs and criminal policy, said: “A law against driving with THC in your bloodstream is not a law you can know you are obeying except by never smoking marijuana or never driving.”
He said rather than switching to a new kind of law as AAA recommends, states should consider simply making it a traffic violation.
Studies show that using marijuana and driving roughly doubles the risk of a crash, Kleiman said. By comparison, talking on a hands-free cellphone while driving — legal in all states — quadruples crash risk, he said. A blood alcohol content of .12, which is about the median amount in drunken driving cases, increases crash risk by about 15 times, he said.
Driving with “a noisy child in the back of the car” is about as dangerous as using marijuana and driving, he added.
The exception is when a driver has both been using marijuana and drinking alcohol because the two substances together greatly heighten impairment, he said.
The foundation also released a second study that found the share of drivers in fatal crashes who had recently used marijuana doubled in Washington after the state legalised it for recreational use in December 2012. From 2013 to 2014, the share of drivers who had recently used marijuana rose from 8% to 17%.
While it stopped short of blaming the crashes on that increase, AAA traffic safety director Jake Nelson said traffic fatalities went up 6% in Washington during that same period while the fatalities declined nationally.