Roadside tests for marijuana impairment that are widely used by police departments across the U.S. are “inadequate,” according to a newly published commentary by a Rutgers University law professor who is urging policymakers to take a more scientifically robust approach to transportation safety while relying less on law enforcement’s ostensible expertise.

The paper, by William J. McNichol of Rutgers University’s Camden School of Law, asserts that current approaches to detecting cannabis impairment “either merely imitate tools used to identify alcohol based impairment without taking into account important differences between the effects of these molecules or they are pseudoscientific ‘police science.’”

“The substance-use science community must take the lead in formulating an informed marijuana impairment detection policy,” McNichol writes, “rather than leave this in the hands of law enforcement.”

As more jurisdictions legalize and regulate marijuana, the paper says, minimizing potential harm “depends on being able to promptly, accurately, and objectively identify dangerous, marijuana- based impairment on the highway and in the workplace,” McNichol argues. “Existing tools for making this on-the-spot, medico-legal determination are inadequate, owing in large part to their lack of scientific rigor.”

Unlike what he describes as a scientifically validated “close correlation” between blood alcohol concentration and fatal vehicle accidents, tests for the presence cannabinoids and their metabolites don’t provide a clear indication that someone is behaviorally impaired.

“The science is fairly clear that the presence per se of cannabinoids in bodily fluids ‘provides little, if any, information concerning behavioral impairment,’” the article says, citing past research.

“Even if there was a well-demonstrated correlation between impairment and a specific concentration of cannabinoids in bodily fluids,” it adds, “the analytical chemistry required to reliably quantify cannabinoids in body fluids is difficult to execute in the field. Police departments have been reported to have given up on it, even after expending significant resources on the attempt.”

Another approach to identifying impairment is the use of so-called drug recognition experts (DREs) or workplace impairment recognition experts (WIREs).

“Both DREs and WIREs claim that their expert opinions on the question of impairment are objective, reliable, and grounded in science,” McNichol writes, “even though the procedures they use were never designed to detect impairment and do not do so.”

The use of DREs, which originated at the Los Angeles Police Department, “is very popular among police officers and prosecutors,” he notes:

“It is a classic example of ‘police science’—techniques developed by police officers for use in their police work. That is to say, it is not science-based at all but is merely a police officer’s lay opinion encrusted with some of the trappings but little or none of the substance of science.”

For example, DRE protocols typically call for investigators to check to see whether a subject’s muscles feel “normal,” “rigid” or “flaccid,” the paper says, even though health care professionals “do not measure muscle tone this way…and would not recognize a police officer’s roadside squeezes as a valid method of measuring muscle tone.”

Other signs of impairment, according to DRE training, include a high heart rate and elevated blood pressure, McNichol points out, “even though about half of the U.S. adult population has high blood pressure when completely sober…to say nothing of the effect on one’s blood pressure of having a police officer measure it on the roadside or in a police station after an accident—settings that in 2009 the Kentucky Supreme Court recognized can fully account for elevated blood pressure.”

“The lack of scientific rigor in the DRE Protocol,” the paper says, “inevitably leads to inconsistent, almost random results.”

“In a 1998 controlled lab study,” it adds, “the DRE Protocol produced either false negative or false positive results 45.5% of the time—not much better than a coin toss.”

The solution to the problem, McNichol suggests, is simple—though not necessarily easy to achieve.

“We must actually do what, so far, we have only pretended to do,” he writes. “We must develop and adopt a scientifically valid, objective, and reliable method for detection of marijuana-based impairment that can be practically implemented in the field.”

A law professor, McNichol does not dig deep into alternative roadside tests for cannabis use, but he calls for the development of “new psychomotor tests” as well as further study into potential “biochemical markers of impairment—including the full range of natural and synthetic cannabinoids as well as their metabolites.”

“Most importantly, leadership of the legal and public policy discussions in this area must not be left in the hands of police officers and their associations, as is now the case,” he writes. “The substance-use science community must take the lead, not only in the basic research that provides the foundation for sound laws and policies but also in the legal and policy discussions that will establish those sound policies.”

The new editorial from McNichol, initially published online late last year, appears in the current issue of the Journal of Studies on Alcohol and Drugs. In the same issue, a pair from the Center for Medicinal Cannabis Research at the University of California at San Diego similarly calls for disinterested experts to guide development of new approaches.

“Developing more robust tools to identify cannabis-impaired drivers in an unbiased fashion is essential to keeping our roadways safe,” the UC San Diego authors said.

There’s another wrinkle to the cannabis testing debate: As recently acknowledged by a U.S. Department of Justice (DOJ) researcher, it’s unclear whether a person’s THC levels are even a reliable indicator of impairment.

On a podcast last year, Frances Scott, a physical scientist at the National Institute of Justice (NIJ) Office of Investigative and Forensic Sciences under DOJ, questioned the efficacy of setting “per se” THC limits for driving. Ultimately, she said, there may not be a way to assess impairment from THC levels, as law enforcement does for alcohol.

One complication is that “if you have chronic users versus infrequent users, they have very different concentrations correlated to different effects,” Scott said. “So the same effect level, if you will, will be correlated with a very different concentration of THC in the blood of a chronic user versus an infrequent user.”

Last October, a study preprint posted on The Lancet by an eight-author team representing Canada’s Centre for Addiction and Mental Health, Health Canada and Thomas Jefferson University in Philadelphia identified and assessed a dozen peer-reviewed studies measuring “the strength of the linear relationship between driving outcomes and blood THC” published through September 2023.

“The consensus is that there is no linear relationship of blood THC to driving,” the paper concluded. “This is surprising given that blood THC is used to detect cannabis-impaired driving.”

Most states where cannabis is legal measure THC intoxication by whether or not someone’s blood THC levels are below a certain cutoff. The study’s findings suggest that relying on blood levels alone may not accurately reflect whether someone’s driving is impaired.

“Of the 12 papers included in the present review,” authors wrote, “ten found no correlation between blood THC and any measure of driving, including [standard deviation of lateral position (SDLP)], speed, car following, reaction time, or overall driving performance. The two papers that did find a significant association were from the same study and found significant relationship with blood THC and SDLP, speed and following distance.”

The issue was also examined in a federally funded study last year that identified two different methods of more accurately testing for recent THC use that accounts for the fact that metabolites of the cannabinoid can stay present in a person’s system for weeks or months after consumption.

A 2023 congressional report for a Transportation, Housing and Urban Development, and Related Agencies (THUD) bill said that the House Appropriations Committee “continues to support the development of an objective standard to measure marijuana impairment and a related field sobriety test to ensure highway safety.”

A year earlier, Sen. John Hickenlooper (D) of Colorado sent a letter to the Department of Transportation (DOT) seeking an update on that status of a federal report into research barriers that are inhibiting the development of a standardized test for marijuana impairment on the roads. The department was required to complete the report under a large-scale infrastructure bill signed by President Joe Biden, but it missed its reporting deadline.

Meanwhile, National Transportation Safety Board (NTSB) last year warned that marijuana rescheduling could create a “blind spot” with respect to drug testing of federally regulated workers in safety-sensitive positions—despite assurances from then-U.S. Transportation Secretary Pete Buttigieg that the cannabis rescheduling proposal “would not alter” the federal drug testing requirements.

At a House committee hearing, Buttigieg had referenced concerns from ATA “about the broad public health and safety consequences of reclassification on the national highway system and its users,” which the trucking association voiced in a letter to the secretary.

As more states legalize marijuana, a federal report published last year showed that the number of positive drug tests among commercial drivers fell in 2023 compared to the year before, dropping from 57,597 in 2022 to 54,464 in the prior year. At the same time, however, the number of drivers who refused to be screened at all also increased by 39 percent.

Another question found that 65.4 percent of motor carriers believed current marijuana testing procedures should be replaced with methods that measure active impairment.

At the time, the report from the American Transportation Research Institute (ATRI) noted a 65,000-driver deficit in the country and said the fear of positives over marijuana metabolites—which can remain in a person’s blood far long after active impairment—may be keeping would-be drivers out of the industry.

The record-high number of refusals came as the transportation industry faces a nationwide shortage of drivers, which some trade groups have said has only been made worse by drug testing policies that risk flagging drivers even when they’re not impaired on the job.

Current federal law mandates that commercial drivers abstain from cannabis, subjecting them to various forms of drug screening, from pre-employment to randomized testing.

In June 2022, meanwhile, an ATRI survey of licensed U.S. truck drivers found that 72.4 percent supported “loosening” cannabis laws and testing policies. Another 66.5 percent said that marijuana should be federally legalized.

Cannabis reform advocates, meanwhile, have also called on federal officials to change what they call “discriminatory” drug testing practices around the trucking industry.

A top Wells Fargo analyst said in 2022 that there’s one main reason for rising costs and worker shortages in the transportation sector: federal marijuana criminalization and resulting drug testing mandates that persist even as more states enact legalization.

Then-Rep. Earl Blumenauer (D-OR) sent a letter to the head of DOT in 2022, emphasizing that the agency’s policies on drug testing truckers and other commercial drivers for marijuana are unnecessarily costing people their jobs and contributing to supply chain issues.

The 2022 ATRI report noted that research into the impact of cannabis use on driving and highway safety is currently mixed, complicating rulemaking to address the issue. A separate 2019 report from the Congressional Research Service (CRS) similarly found that evidence about cannabis’s ability to impair driving is inconclusive.

A study published in 2019 concluded that those who drive at the legal THC limit—which is typically between two to five nanograms of THC per milliliter of blood—were not statistically more likely to be involved in an accident compared to people who haven’t used marijuana.

Separately, the Congressional Research Service in 2019 determined that while “marijuana consumption can affect a person’s response times and motor performance … studies of the impact of marijuana consumption on a driver’s risk of being involved in a crash have produced conflicting results, with some studies finding little or no increased risk of a crash from marijuana usage.”

Another study from 2022 found that smoking CBD-rich marijuana had “no significant impact” on driving ability, despite the fact that all study participants exceeded the per se limit for THC in their blood.

Evan as far back as 2015, a U.S. National Highway Traffic Safety Administration (NHTSA) concluded that it’s “difficult to establish a relationship between a person’s THC blood or plasma concentration and performance impairing effects,” adding that “it is inadvisable to try and predict effects based on blood THC concentrations alone.”

In a separate report last year, NHTSA said there’s “relatively little research” backing the idea that THC concentration in the blood can be used to determine impairment, again calling into question laws in several states that set “per se” limits for cannabinoid metabolites.

“Several states have determined legal per se definitions of cannabis impairment, but relatively little research supports their relationship to crash risk,” that report said. “Unlike the research consensus that establishes a clear correlation between [blood alcohol content] and crash risk, drug concentration in blood does not correlate to driving impairment.”