Measuring ‘drunk’ is fairly easy; the more alcohol a person drinks, the more alcohol appears in that person’s blood and the worse that person becomes, somewhere on a scale from tipsy to wasted. On the other hand, measuring “high” is much fuzzier — much to the dismay of some states’ law enforcement officials.
According to a study by the AAA Foundation for Traffic Safety, blood tests that attempt to quantify marijuana use are essentially useless for assessing how bad a driver is. In other words, the study found that people with low blood levels of THC – or delta-9-tetrahydrocannabinol, the main psychoactive component of marijuana – can still act as if they are really stoned. On the other hand, some people may have off-the-chart THC readings but still behave normally.
The finding is critical because several states have already set legal limits on the amount of THC a person is allowed to have in their blood while driving. AAA concluded that such limits are “arbitrary and unsupported by science, which could lead unsafe drivers to go unpunished and others wrongfully convicted of drunk driving.”
For the study, AAA researchers combed through arrest records for drunk driving, as well as results from toxicology tests and Drug Recognition Expert (DRE) exams. This exam includes roadside sobriety tests such as walking and turning, standing on one foot, and nose touching.
The researchers compared the DRE exam results of 602 drivers who had only THC in their blood at the time of arrest with those of 349 volunteers who passed the test drug-free and sober. First, the researchers confirmed that the pot-smoking drivers did worse on the exam than sober people. For example, 55.5 percent of drug-free people passed the walk-and-turn test perfectly, while only 6 percent of pot smokers passed.
But when the researchers looked for a link between DRE exam scores and blood THC levels, the link went up in smoke. The THC levels of the 602 cannabis-smoking drivers ranged from 1 to 47 nanograms per milliliter of blood. While higher THC levels seemed to correlate with more errors on the nose touch test, error rates on the other tests had no such association with THC levels.
Similarly, when the researchers looked for an association between error rates and THC levels above or below 5 ng/ml – the legal limit in Colorado, Washington and Montana – they also found no clear difference.
The authors noted that of the drivers who failed the fasting tests, 80 percent had THC levels of 1 ng/ml or more. But of those who passed the tests, 30 percent also had THC levels of 1 ng/ml or higher. “Based on this analysis, a quantitative threshold for per se laws for THC after cannabis use cannot be scientifically supported,” the study authors concluded.
The conclusion is in line with that of other researchers who also found no correlation between THC levels in the blood and disorders. The disconnect may be related to the fact that THC is rapidly metabolized, and its presence in the blood may depend on both a person’s dose and usage pattern. Infrequent smokers tend to see rapid drops in blood THC levels, while regular users can sustain higher THC levels for longer.
Still, it is necessary to dissuade people from smoking weed while driving, argues AAA, because it can interfere with driving. In a separate study published by the foundation, researchers reported that the percentage of drivers involved in a fatal car accident who had recently smoked marijuana doubled from 8 percent in 2013 to 17 percent in 2014 in Washington state. The state legalized marijuana for medical use in 2012. However, most of those drivers had multiple drugs in their system and there was an overall increase in accidents.
Nevertheless, the foundation called the rise “alarming.” It recommends that until scientifically valid restraint measures are put in place, law enforcement should use a combination of behavioral and psychological tests to assess whether drivers using marijuana are safe to drive.