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Clardy Law would keep roads safe

Next to alcohol, marijuana is the second-most commonly found substance in the bodies of drivers involved in fatal crashes.

Mourners lay flowers on the casket during burial services for Massachusetts State Trooper Thomas Clardy at St. Michael's Catholic Parish in Hudson in 2016.Keith Bedford/Globe Staff

On March 16, 2016, Massachusetts State Trooper Thomas Clardy was killed when his cruiser was struck on the Massachusetts Turnpike by a speeding motorist swerving across three lanes of traffic. The motorist was found guilty of involuntary manslaughter, but not driving under the influence of marijuana, despite having THC, the psychoactive chemical in marijuana, in his blood at the time of the crash. He had purchased marijuana at a local dispensary only hours earlier, and police found burnt marijuana joints inside his car. Governor Baker recently filed a bill to create the Clardy Law to treat driving impairment from alcohol and marijuana in the same way, thereby protecting the public from those who choose to drive after using marijuana.

Marijuana remains a public health concern with regard to motor vehicle crashes. Driving requires a complex combination of behaviors and skills, many of which are affected by marijuana. Marijuana and alcohol both affect driving, but in different ways. For several hours after use, marijuana affects one’s ability to estimate time and distance, memory, reaction time, concentration, information processing speed, and the processing of multiple pieces of information at the same time. For most people, there is no amount of marijuana use that is compatible with safe driving.


Next to alcohol, marijuana is the second-most commonly found substance in the bodies of drivers involved in fatal crashes. The proportion of drivers in fatal motor vehicle accidents has increased in states with medical marijuana laws while remaining unchanged in states without medical marijuana laws. In Massachusetts, arrests and summonses for operating under the influence of marijuana have not changed much since recreational marijuana stores opened in January 2019. Law enforcement officials say that is because they don’t have the proper tools to enforce the law and that they have encountered more drivers who use marijuana in recent years.

Baker’s bill creates a framework that treats marijuana impairment like alcohol impairment. After establishing probable cause that a person is operating under the influence of marijuana, often due to speeding or illegal driving maneuvers, specially trained law enforcement officers called drug recognition experts can conduct roadside assessments to determine if the person is impaired. The roadside assessments measure balance, coordination, and the ability to divide one’s attention among multiple tasks, just like standard field sobriety tests used for alcohol impairment. Saliva may be collected to confirm the presence of marijuana in the driver’s system. Just as with cases involving alcohol, refusal to submit a fluid sample — blood or urine, and/or submitting to a breathalyzer test — would lead to automatic suspension of the driver’s license. Keep in mind that currently, instead of making an arrest or offering a test, police officers may determine that a driver is an immediate threat to public safety and automatically revoke their license.


While the science of driving and blood alcohol content breathalyzer readings is clear, there are no clear-cut correlates between THC levels and impairment. The presence of THC in the body alone does not mean that the driver was driving while impaired. However, this bill combines roadside assessments and biochemical verification of recent use to create multiple data points in order to allow the officer to determine a driver’s impairment and for the justice system to hold drivers accountable for bad choices.


The science of marijuana is evolving. Many groups are working to develop devices to detect marijuana impairment as accurately as alcohol breathalyzers, but it seems clear that such devices will not be ready soon. States have responded in a variety of ways — some have zero-tolerance laws where it is illegal to drive with any measurable amount of marijuana in the body and others have no specific laws on marijuana impairment.

There are drivers in Massachusetts who are driving under the influence of marijuana, and law enforcement does not have effective methods to hold them accountable for risking lives on our roads. The Clardy Law provides the best solution to this problem given the available tools.

Dr. Kevin P. Hill is director of the Division of Addiction Psychiatry, Beth Israel Deaconess Medical Center. John F. Carmichael Jr. is chief of police in Newton.