Many or all of the products featured here are from our partners who compensate us. This may influence which products we write about and where and how the product appears on a page. However, this does not influence our evaluations. Our opinions are our own.

Hazy. Clumsy. Unreliable. No, we’re not talking about a marijuana-impaired driver, but rather certain testing criteria used to weed them out.

In several states, motorists are subject to “per se” marijuana laws, which set a specific limit of THC that drivers can have in their blood. THC is the main psychoactive element in cannabis. However, these laws tell us next to nothing about who’s fit to get behind the wheel, according to a May 2016 study from the AAA Foundation for Traffic Safety.

Researchers examined toxicology reports and sobriety exams from more than 600 drivers who had been arrested on suspicion of marijuana impairment. Results showed that increasing the amount of THC does not necessarily heighten its effects. Furthermore, chronic marijuana users may test positive for THC long after use despite no longer being impaired, while other people may show legal amounts of THC even when their driving indicates they’re a danger.

These findings call into question our understanding of marijuana’s effect on the brain and body, and how we decide which drivers will face prosecution for using it.

THC doesn’t tell the whole story

Six states currently have per se marijuana laws. In these states, motorists who’ve been pulled over can be charged with impaired driving if they exceed the legal limit for THC.

State THC Limit (measured in nanograms per milliliter of blood) Source: Governors Highway Safety Association Pennsylvania 1 ng/ml Nevada 2 ng/ml Ohio 2 ng/ml Colorado 5 ng/ml Montana 5 ng/ml Washington 5 ng/ml

Unlike per se limits for alcohol, however, THC thresholds aren’t backed by science. There’s no evidence that these limits correlate with impairment, says Dr. Barry Logan, one of the AAA study’s authors. Instead, he says, they amount to arbitrary guesses, and misleading ones at that.

One issue is that the body eliminates much of the THC soon after ingestion, often before authorities can draw blood for a test. Police don’t yet have a method to determine a driver’s exact blood THC content in the field and must transport suspects to a health facility to test them. Logan estimates that a person could have a blood THC level close to 50 nanograms per milliliter when pulled over, yet that level could fall under legal limits by the time blood is taken. Field tests for weed, similar to the Breathalyzer for alcohol, may help eliminate this problem one day, he says, but they’re still a long way from being reliable or affordable.

This is a concern when you consider how many reckless drivers might go free despite hitting the roads high as a kite. According to the AAA report, roughly 70% of arrestees who were stopped due to driving that suggested impairment registered THC levels under 5 ng/ml once tested.

Conversely, chronic users could show trace amounts of THC even if it’s been up to 10 days since their last toke, Logan says. “Someone who smoked yesterday isn’t impaired,” he says. “We don’t benefit by getting that person off the road.”

Higher THC levels don’t always mean higher drivers

Conventional wisdom suggests that the more pot you smoke, the worse you drive. While the AAA study showed that motor skills do dip after using marijuana, there’s little evidence that increasing THC concentration amplifies its effects.

To measure this, AAA researchers looked at Drug Recognition Expert exams. These gauge impairment using physical tests — such as balancing on one leg, balancing with eyes closed and walking a straight line — plus diagnostic factors like pupil size and blood pressure. They examined results for one group of “high-concentration” drivers (THC above 5 ng/ml) and another group of “low-concentration” drivers (THC below 5 ng/ml).

The high-concentration group differed significantly from the low-concentration group in only four of a possible 15 indicators and only one involving motor function, the finger-to-nose test.

In essence, two people may show similar physical symptoms despite having ingested very different amounts of cannabis. Yet under per se laws, one may skate while the other would face prosecution.

Behavior more revealing than bloodwork

As more states legalize marijuana and confront the issue of policing weed-impaired drivers, Logan hopes they turn to better alternatives than per se laws. While there isn’t a perfect method to determine how high is too high, he says the fairest way to prosecute is by using reckless driving behavior as primary evidence, with blood THC levels as supporting evidence.

“The presence of THC can strengthen the observation that a driver is impaired, but the exact amount isn’t all that important,” he says. For that reason, he also suggests ditching blood tests in favor of using saliva samples, which can be collected on-site and more quickly reveal the presence of THC, although not the specific amount.

Drivers who exceed their state’s THC limit could face costly citations and court fees, license suspension and a sharp spike in car insurance rates, among other penalties.

» MORE: How much does a DUI cost?

Alex Glenn is a staff writer for NerdWallet, a personal finance website. Email: [email protected].