Measuring ‘drunk’ is pretty easy; the more alcohol someone drinks, the more alcohol shows up in that person’s blood and the more impaired that person becomes, falling somewhere on a scale of tipsy to wasted. Measuring ‘high,’ on the other hand, is far hazier—much to the dismay of some states' law enforcement.

Blood tests that try to quantify marijuana use are in fact useless at assessing how impaired a driver is, according to a study by the AAA Foundation for Traffic Safety. In other words, the study found that people with low blood amounts of THC—or delta-9-tetrahydrocannabinol, the main psychoactive component of pot—may still act as if they’re really stoned. On the other hand, some people may have THC measurements off the charts yet still act normally.

The finding is critical because several states have already set legal limits for the amount of THC a person can have in their blood while driving. AAA concluded that such limits are “arbitrary and unsupported by science, which could result in unsafe motorists going free and others being wrongfully convicted for impaired driving.”

For the study, AAA researchers combed through arrest records for impaired 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 that only had THC present in their blood at the time of arrest to those of 349 volunteers that took the test drug-free and sober. First, the researchers confirmed that the pot-smoking drivers did worse on the exam than sober folks. For instance, 55.5 percent of drug-free people passed the walk-and-turn test perfectly, while only six percent of the pot smokers managed that.

But, when the researchers looked for a link between DRE exam scores and the level of blood THC, the connection went up in smoke. In the 602 pot-smoking drivers, THC levels ranged from 1 to 47 nanograms per millilitre of blood. While higher THC levels did seem to correlate with more errors on the nose-touch test, error rates on the other tests had no such association with THC levels.

Likewise, when the researchers looked for a link between error rates and having 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 that failed the sobriety tests, 80 percent had THC levels of 1 ng/mL or greater. Yet, of those that passed the tests, 30 percent also had THC levels of 1 ng/mL or greater. “Based on this analysis, a quantitative threshold for per se laws for THC following cannabis use cannot be scientifically supported,” the study authors concluded.

The conclusion echoes that of other researchers that also noted no correlation between blood THC levels and impairment. The disconnection may be linked to the fact that THC is quickly metabolized, and its presence in blood can depend on both the dose and a person's usage patterns. Infrequent smokers tend to see quick drops in blood THC levels, while regular users may sustain higher THC levels for longer.

Still, there is a need to deter people from smoking pot while driving, AAA argues, as it can impair driving. In a separate study published by the foundation, researchers reported that the percentage of drivers involved in a fatal car crash who had recently smoked marijuana had 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 the number of crashes.

Nevertheless, the foundation called the increase “alarming.” It recommends that until scientifically valid measures of impairments are put into place, law enforcement should use a combination of behavior and psychological tests to assess whether drivers who use marijuana are safe to drive.