The Fairness of Marijuana Testing to Determine DUI Impairment

September 16, 2022 by Anthony Riccio

In 2012, Massachusetts legalized medical marijuana, and in 2016, recreational use became legal in the state for adults over the age of 21. But with these changes, law enforcement has not updated their standard operating procedures concerning a valid testing process for drivers stopped on suspicion of being impaired by marijuana.

Because Operating Under the Influence (OUI) includes marijuana, how has its legalization impacted OUI violations? That depends on who you ask in the state. Consulting data from the Massachusetts Executive Office of Public Safety and Security shows a 20% decrease in OUI drug violations statewide.

The numbers over this same period that are coming out of Western Massachusetts tell a different story. The western counties have seen an increase of approximately 4% in OUI violations, including marijuana. The combined counties report: 

  • 2019 had 295 violations
  • 2020 had 379 violations
  • 2021 had 306 violations

Local police departments attribute the bulk of these violations to marijuana, with area law enforcement data revealing that the legalization of recreational use led to a doubling of OUI drug arrests. OUI alcohol arrests also went up about 20%. The majority of these OUI violations occur with drivers under the age of 21.

These numbers may be subject to scrutiny because OUI arrests attributed to marijuana are more difficult to prove without testing that could determine the intoxicating substance is marijuana. There is no roadside physical exam or blood alcohol concentration (BAC) test for cannabis.

The State of an OUI Testing for Marijuana

Currently, Massachusetts has two drug-identifying tests for suspected impaired drivers. But even with these tests, arrest and prosecution can be successfully challenged. Typically, arrests occur because marijuana is seen or paraphernalia is found, or a driver might confess they are impaired.

Both swab and saliva tests have been discussed by researchers and legislatures, but as of now, the evidence is lacking for OUI cases where cannabis is named as the impairing substance.

In January, researchers at Massachusetts General Hospital (MGH) released a new study in brain imaging that uses functional near-infrared spectroscopy to objectively measure impaired brain activity and reliably distinguish it to be caused by cannabis and THC intoxication.

 The journal Neuropsychopharmacology reports that the researchers’ aim is to improve highway safety by creating small testing units that are battery-powered and can be carried in the field. The portable test would register an individual’s brain patterns to evaluate the correlation to THC intoxication and impairment.

But as of now, there are no breathalyzer-type tests for marijuana impairment. Nor is there a neural net that can be strapped to a driver’s head at a roadside stop. Right now, the arrest for OUI violations rests on judgment calls from law enforcement officers. Without objective evidence, this process is problematic.

A Lack of Scientific Testing 

Even before Massachusetts legalized medical and recreational marijuana, it had been decriminalized for years. The only real legal issue for most police forces is when someone is too high to drive.

In 2017, the supreme court of Massachusetts reviewed questions regarding investigative methods and admissible evidence in Marijuana OUI cases. Two of the specific issues challenged in the court were:

  1. The validity of using a police-administered roadside field sobriety test as admissible evidence during the prosecution of a Marijuana OUI.
  2. Whether or not it is valid to use a police officer’s opinion or testimony concerning a driver’s impairment on marijuana as admissible evidence in a Marijuana OUI case. 

The basis of a police investigation into cannabis intoxication relies on three factors: 

  1. Field Sobriety Tests which use physical and mental exercises to establish impairment
  2. Observation-informed opinions concerning impairment
  3. The evaluation by a Drug Recognition Expert (DRE)

Challengers have pushed back on both elements because of the lack of scientific integrity and are extremely subjective.

Expert testimony from a Drug Recognition Expert (DRE) will be needed to successfully prosecute a marijuana OUI. This is not intended to establish impairment as much as to determine which drug was taken and whether it was against the law.

In many cases, neither the prosecutor nor the DRE can prove what substance allegedly impaired a driver. This usually results from the high rate of false positives associated with DRE evaluations. This is especially true in marijuana OUI cases. The Atlantic recently offered research suggesting that the false positive rate in DRE evaluations ranges from 38% to 68%.

A Marijuana OUI Defense

In 2017, a Worcester District Court heard the case Commonwealth V. Gerhardt. The judge ruled field sobriety tests were not admissible in an OUI case involving marijuana. The trooper testified that when he stopped Gerhardt, he smelled marijuana and saw a partially smoked joint in the vehicle. Gerhardt was made to perform a few field sobriety tests and failed the one-leg stand exercise.

The judge reasoned that a failed field sobriety test proves nothing because plenty of people have coordination and balance issues because of a bad back or knees or feet. Age and weight also play a factor. Tests and opinions are irrelevant. To prosecute, THC intoxication needs scientific evidence to prove sufficient impairment. 

An individual high from marihuana does not stagger or slur their speech like someone impaired by alcohol or many other drugs. In the case of an individual high on cannabis, there are no universal indicators lay people can apply to determine if someone is high, much less impaired to the point of danger.

The smell of marijuana should not be a cause for a search. Because marijuana is legal, the smell or possession of it is not a crime. Proving THC impairment is an uphill battle for prosecution. The impairment from alcohol and other drugs is far more severe and has a higher degree of hindrance while operating a vehicle than Cannabis impairment. 

According to various studies, the use of marijuana has practically no proven correlation to vehicle crashes or fatalities. And there is no doubt that both drinking and driving, and distracted drivers pose a substantially higher threat on Massachusetts roadways than cannabis impairment.