Bill To Combat Marijuana-Related Impaired Driving Should Be Rejected

There is no blood, saliva, or breathalyzer analysis that can ascertain whether a person who has used marijuana is actually impaired.


This Op-Ed piece appeared in the Boston Globe, Tuesday January 18th, 2022. It was written by Michael Milburn, and William DeJong.


Prompted by the Commonwealth’s legalization of marijuana and growing concerns about an increase in impaired driving, Governor Charlie Baker has refiled the Trooper Thomas Clardy Law, which states that the presence of any intoxicating substance or its metabolites in a driver’s system, as indicated by breath analysis or a chemical test of blood or “oral fluid,” shall be admissible as evidence of that individual being under the influence. Mitigating marijuana-impaired driving is a public safety priority, but testing for marijuana use rather than actual impairment is a wasteful and scientifically invalid approach that would be successfully challenged in the courts.


These tests can indeed detect THC, the primary psychoactive chemical in marijuana, but in fact, there is no blood, saliva, or breathalyzer analysis that can ascertain whether a person who has used marijuana is actually impaired, and there is no level of THC in blood or oral fluid that can discriminate between an impaired and unimpaired person. As reported in a 2017 US Department of Transportation report to Congress, “the level of THC in the blood and the degree of impairment do not appear to be closely related.” Moreover, THC collects in fat and other body tissues, but then slowly reenters the bloodstream. Thus, even after a few weeks of non-use, a drug test for THC may still show evidence of past marijuana use, long after any impairment has passed.


Notably, courts in Oklahoma (Rose v. Berry Plastics Corp.) and Arizona (Whitmire v. Wal-Mart Stores Inc.), persuaded by the scientific evidence, have ruled that a positive drug test does not justify firing an employee who is a medical marijuana patient unless there is also evidence of impairment. It seems inevitable that judges will apply this same line of reasoning to roadside testing of suspected impaired drivers.


Should law enforcement officers instead use the Standardized Field Sobriety Test, as they do with drivers suspected of being alcohol-impaired? No, because researchers have shown that this method does not reliably measure impairment from marijuana. Its key component is the horizontal gaze nystagmus test, a procedure approved by the National Highway Traffic Safety Administration for observing whether drivers have jerky, uncontrollable eye movements as they try to track a small object that is moved from side to side. Based on research, the proposed law’s acceptance of the horizontal gaze nystagmus test as a reliable field sobriety test is badly misinformed.


But what would be better to address impaired driving — whatever the cause — is a fast, reliable, and objective measure of impairment that can be used during a traffic stop. In short, effective law enforcement requires assessing impairment, not substance use. This is why our company, Impairment Science Inc., developed an app to assess impairment. Used on a smartphone or tablet and taking only three minutes, the Druid app features four game-like tasks that measure reaction time, decision-making accuracy, eye-hand coordination, balance, and the ability to perform divided-attention tasks. Independent researchers have validated Druid’s ability to detect impairment due to alcohol and marijuana. Presently, Druid can be used by employers with workers in safety-sensitive jobs, but work to adapt the app for use during roadside stops is only now underway.


Developing a reliable and objective measure of impairment for roadside stops is essential, but calls to use the mere presence of THC in a driver’s system to define legal impairment must be resisted. The Legislature should reject Baker’s bill.


Michael Milburn, a retired professor of psychology at UMass Boston, is founder and chief science officer of Impairment Science Inc. He conceived the idea of the Druid app. William DeJong, a retired professor of public health at Boston University and a former member of the board of directors of MADD, is the director of research and evaluation at Impairment Science.

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