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P1 Research: How marijuana legalization impacts highway safety

While there is insufficient data to determine the true impact of legalized marijuana on crime, the traffic data offers a clear correlation

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Detecting driving under the influence of marijuana is a significant challenge for law enforcement.

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In March 2020, Police1 and Louisiana State University (LSU) conducted an expansive survey capturing law enforcement attitudes toward marijuana use and enforcement. A total of 3,615 sworn LEOs weighed in on a range of topics, from the use of medicinal marijuana off duty to decriminalization.

Our special report features expert analysis of the survey findings, covering critical topics like police recruit marijuana use and how marijuana legalization impacts highway safety. Click here to access all our coverage.

As Executive Director of the Pennsylvania Chiefs of Police Association and a former member of the State Health Department’s Medical Marijuana Advisory Board, I would like to express my concerns, and those of the law enforcement community, about the legalization of marijuana and the relative effects on public and highway safety in our communities.

Driving impaired by any substance – alcohol or drugs, whether legal or illegal – is against the law in all 50 states and the District of Columbia.

I believe marijuana legalization in Pennsylvania and across the United States will pose significant challenges for law enforcement resulting from the unanticipated consequences it has on public safety. Those who drive under the influence of alcohol or drugs, whether obtained legally or illegally, pose a danger to themselves, their passengers and other motorists.

There is a movement in Pennsylvania and other states to either decriminalize or legalize the possession and use of recreational and medical marijuana. Marijuana has been legalized in 11 states and the District of Columbia. Decriminalization of marijuana possession is now the law in 23 states. Medical marijuana programs exist in 34 states. The argument in Pennsylvania has been, in part, that marijuana is a harmless drug that has led to unnecessary confinement, and the legalization of marijuana will result in increased tax revenues. The consequences to this change in policy drastically affect the safety of our highways.

To gauge the law enforcement view, Police1 and Louisiana State University (LSU) conducted a survey on “Policing in an Era of Legal Marijuana in March 2020. Data was collated from 3,615 law enforcement respondents across the United States. Approximately 93% of the respondents were assigned to law enforcement operations and/or patrol/traffic functions.


In response to the survey, 73% of respondents believed that marijuana was a less dangerous drug than other Schedule I drugs, while approximately 71% believed that marijuana was a “gateway drug.”

Determining impairment

One of the most salient concerns law enforcement officers have relates to the consequences of drug-impaired driving (DUID). We have all witnessed our share of crashes and traffic congestion, as well as vehicular, pedestrian and cyclist fatalities. Law enforcement officers are uniquely qualified to discuss the issues and concerns related to impaired driving.

Efforts to curb drunk driving have met with a great deal of success over the last decade, but drug-impaired driving is not the same as alcohol-impaired driving.

Alcohol is unique among impairing drugs in that there is a documented correlation between blood levels and levels of impairment. This does not exist for other drugs, and it has been shown to be nonexistent for THC in marijuana. It is not possible to currently identify a “valid” impairment.

We know that THC triggers receptors in parts of the brain that influence memory, thinking, concentration and coordination. Marijuana significantly impairs judgment and reaction time, and studies have found a direct relationship between blood THC concentration and impaired driving ability.

The role played by marijuana in crashes is often unclear because it can be detected in body fluids for days or even weeks after intoxication and because people frequently combine it with alcohol. Those involved in vehicle crashes with THC in their blood, particularly at higher levels, are three to seven times more likely to be responsible for the incident than drivers who had not used drugs or alcohol. The risk associated with marijuana in combination with alcohol appears to be greater than that for either drug by itself. [1]

Laws vary from state to state. Law enforcement attitudes and perspectives toward enforcement over the past decade has changed. Police1 asked law enforcement officers in states where marijuana use was illegal their opinion on decriminalization. There is an important distinction to be made here. Legalization of marijuana is the process of removing all legal prohibitions against it. Marijuana would be available to the adult general population for purchase and use at will, similar to tobacco and alcohol. Decriminalization is the act of removing criminal sanctions against an act, article, or behavior.

While some form of cannabis legalization is occurring in most states, law enforcement uncertainty exists.


The survey attempted to capture changes in law enforcement attitudes in the period after marijuana was legalized in their state. The survey suggests that of the 1,000 law enforcement respondents in those states, over 98% of officers believed that DUID had stayed the same or increased. This is a significant concern for law enforcement and public safety.


Determining the impact of legalized marijuana on crime and safety

There is insufficient data to determine the true impact of legalized marijuana on crime and safety. Many states have had difficulties caused by conflicting state legislation and local ordinances, policies and procedures. The situation is even more complex because marijuana remains a Schedule I controlled substance under federal law. While the National Highway Traffic Safety Administration (NHTSA) continues to conduct research to better understand the relationship between marijuana impairment and increased crash risk, studies in Colorado show:

  • Youth use and addiction rates have increased due to ease of accessibility, and there is great concern about the significant health impacts of chronic marijuana use on youth.
  • Difficulties in establishing what is a legal marijuana operation have created problems in conducting investigations, determining probable cause, and search and seizure procedures.
  • Detecting driving under the influence of marijuana is a significant challenge for law enforcement. Currently, there is no roadside test for marijuana intoxication.


Impaired driving poses a substantial risk to public and highway safety. While there is insufficient data to determine the true impact of legalized marijuana on crime, the traffic data offers a clear correlation.

For access to the full survey results, click here.


1. Hartman RL, Huestis MA. Cannabis effects on driving skills. Clin Chem, 2013; 59(3):478-492.

Scott L. Bohn is executive director of the Pennsylvania Chiefs of Police Association.