Prompted by the state’s legalization of marijuana and concerns about an increase in impaired driving, Governor Baker this year proposed legislation to enact recommendations of the Special Commission on Operating under the Influence and Impaired Driving. This bill (H.71), the subject of a recent joint committee hearing on Beacon Hill, is seriously flawed and should be rejected.

The bill states that the presence of any intoxicating substance or its metabolites in a driver’s system, as indicated by a breath analysis, a chemical test of blood or “oral fluid,” shall be admissible as evidence of that individual being under the influence. Keep in mind that the public safety concern here is not marijuana use itself, but whether that use has resulted in intoxication while driving. These tests can indeed detect ∆9-tetrahydrocannabinol (THC), the primary psychoactive constituent of cannabis, 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.

Why this is the case is well understood. A primary factor is that THC does not linger very long in the bloodstream. Occasional users may be impaired for several hours after their THC blood level has dropped to near zero, creating a problem if, as often happens, blood and saliva samples are taken by a Drug Recognition Expert (DRE) two or more hours after a traffic stop.

For frequent users, there’s a different problem. THC collects in fat and other body tissues, but then slowly reenters the bloodstream. Thus, even after a month of non-use, a test for cannabinoid-related substances might still show evidence of THC ingestion, long after any impairment has passed. 

Should law enforcement officers instead use the Standard Field Sobriety Test, as they do with drivers whom they suspect of being alcohol-impaired? No, because researchers have shown that this method does not reliably measure impairment from cannabis. Reflecting this fact, H.71 states that a horizontal gaze nystagmus (HGN) test, a method approved by the National Highway Traffic Safety Administration, can be entered into evidence when a defendant is alleged to be under the influence of alcohol or several other drugs, but HGN cannot be used as evidence of cannabis impairment.

Notably, courts in Oklahoma (Rose v. Berry Plastics Corp. et al., 2019 OK Civ. App. 55 (Ok. Civ. Ct. App. Oct. 16, 2019) 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 cannabis patient unless there is also evidence of impairment. Inevitably, judges will apply this same line of reasoning to roadside testing of suspected impaired drivers, and cases brought under H.71 will likewise be set aside.

There needs to be an objective measure of impairment that can be used by the roadside.  Until then, officers must continue to convince a judge or jury that their observations alone of a driver’s behavior and appearance constitute evidence of impairment beyond a reasonable doubt.

Baker’s bill is the wrong answer to a serious problem and will waste potentially millions of dollars on useless drug tests. It should be rejected.

Michael Milburn, PhD

William DeJong, PhD

DRUIDapp. Inc., Cambridge, MA

Dr. Milburn was a professor of psychology at UMass/Boston for 40 years. Dr. DeJong was a professor of public health at Boston University for 17 years and previously served on MADD’s national board of directors.