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.'”
McNichol writes that “the substance-use community should take the initiative in formulating a marijuana impairment detection policy, rather than leaving this to law enforcement.”
McNichol says that as more jurisdictions begin to legalize marijuana and regulate it, minimising potential harm will depend on our ability to accurately and objectively detect dangerous marijuana-based impairments in both the workplace and on the road. The paper says that existing tools to make this medico-legal decision on the spot are insufficient, due largely to their lack scientific rigor.
The presence of cannabinoids, and their metabolites are not a reliable indicator that someone’s behavior is 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.”
One way of identifying impairments is to use so-called Drug Recognition Experts (DREs) and Workplace Impairment Recognisation Experts (WIREs).
McNichol writes that both DREs (and WIREs) claim their opinions are objective, reliable, based in science and scientifically backed, even though the methods they use weren’t designed to detect impairment.
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.
He says: “We need to act now on the things we have been pretending to do.” “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 should take the initiative, both in terms of the research which provides the basis for good laws and policies and in discussions about the law and policy that are necessary to establish these policies.
McNichol’s latest editorial was first published on the internet late last summer. It appears in this issue of Journal of Studies on Alcohol and Drugs. A pair of experts from the Center for Medicinal Cannabis Research at University of California at San Diego also call for the use of disinterested specialists to develop new approaches in the current issue of the Journal of Studies on Alcohol and Drugs.
The authors of the study from UC San Diego said that “developing more robust tools for identifying cannabis-impaired driving in an unbiased manner is crucial to keep our roads safe.”
Another wrinkle has emerged in the debate over cannabis testing: According to a researcher at the U.S. Department of Justice, THC levels may not be a good indicator of impairment.
Frances Scott a physical scientists at the National Institute of Justice Office of Investigation and Forensic sciences under DOJ questioned on a recent podcast the efficiency of setting THC driving limits “per se”. She said that there is no way, ultimately, to determine impairment based on THC levels as the law enforcement do for alcohol.
Scott explained one problem: “If you compare chronic and infrequent marijuana users, their THC concentrations are very different. They have effects that differ.” Scott said that “the same effect level will correlate with a different THC blood concentration in a chronic versus 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.
This paper concluded that “the consensus is there is no linear relation between blood THC and driving.” This is surprising, given that blood THC can be used to detect driving while impaired by cannabis.
The majority of states with legal cannabis measure THC intoxication based on whether someone’s THC blood levels fall below a specific cutoff. Findings from the study suggest that blood levels are not a reliable indicator of driving impairment.
Ten of the twelve papers reviewed in this review found that there was no relationship between the blood THC level and any driving measure, including speed. [standard deviation of lateral position (SDLP)]The two papers that did find a significant association were from the same study and found a significant relationship with blood THC, SDLP, speed and following distance. “The two papers which did show a significant correlation were both from the same research and showed a relationship between blood THC, SDLP and speed as well as 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.
According to a congressional report from 2023 for a Transportation, Housing and Urban Development, and Related Agencies Bill (THUD), the House Appropriations Committee continues to “support the development and implementation of an objective measure to determine marijuana impairment and related field sobriety testing to ensure road 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. A federal report published last year showed that the number of positive drug tests among commercial drivers fell in 2023 compared to the previous year, dropping from 57,597 in 2022 to 54,464 the prior.
In another survey, 65.4 percent said that they believed that the current methods of marijuana testing should be changed to ones that assess 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.
This record number of refusals comes at a time when the industry is facing a shortage of drivers nationwide. Some trade groups say that drug-testing policies are making the situation worse, as they can flag drivers who may not be impaired while on the job.
The federal law currently in place requires that all commercial drivers refrain from using cannabis. They are subjected to drug testing, ranging from random to pre-employment.
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. Sixty-six percent of respondents said federal legalization should also be considered.
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.
ATRI 2022 noted in its report that there is a lack of consensus on research regarding the effect of cannabis consumption on highway safety and driving. The issue has complicated the rulemaking process. 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.
The report noted that although several states had established legal per se cannabis impairment definitions, there was relatively little support for their link to accident risk. This report stated that “Unlike research consensus which establishes the clear correlation between [blood alcohol content] “Drug concentrations in the blood do not directly correlate with driving impairment.”