Say What? The Problem with Per Se Laws for Cannabis-impaired Driving

Published on April 5, 2019

Most people know about per se laws to test for alcohol-impaired driving.

A blood alcohol of .08 or higher allows police to infer that you are impaired from alcohol. If you are driving and your blood alcohol level exceeds .08 level, then you are in trouble.

For cannabis, you may also know that seven states like Washington and Colorado have per se laws that specify legal limits of THC in the blood, typically 5 ng/mL. The entire nation of Canada has the same per se limit, and even more U.S. states have “zero tolerance” laws.

Any biological evidence of the past use of cannabis found in a driver, regardless of impairment or the lack of it, can land a person in jail. If you are interested in what states have what  laws for THC, you can check here at the Governors’ Highway Safety Association.

What Happened in Michigan

Now, you might or might not have probably heard about the recommendations of the Impaired Driving Safety Commission in Michigan.

The outgoing Republican Governor Rick Snyder appointed the Commission. The Commission recommended that Michigan’s Legislature not specify a per se THC blood limit that would constitute “cannabis impaired driving.”

The main reason the Michigan Commission concluded against per se laws are that they are not based on science. The 2017 U.S. National Highway Traffic Safety Administration report to Congress on driving and cannabis impairment (p. 28) acknowledges that THC blood levels do not predict impairment and have no scientific basis.

The results from saliva tests have a similar problem—they can show recent use, but not impairment. This research study from France found that there is “no legal driving limit for cannabis that can catch impaired recreational users without unfairly penalizing unimpaired regular or medicinal users.” 

Some Science

THC molecules in cannabis are different than the composition of other drugs.  A chemist friend at Washington State University who has developed a THC-saliva analysis device told me that there is just not enough THC in breath to make a “marijuana breathalyzer” work.

THC is also lipophilic, so that THC metabolites end up being stored in fat cells.  These molecules dissipate gradually over time, long after impairment has passed. All drug testing for cannabis just tests for past use, not impairment.

The Legal Situation

I testified a year and a half ago in front of Washington state legislators on the Public Safety Committee about testing for impairment.

When I finished, one representative exclaimed, “We need to change the law!”  Despite the science, however, multiple states still have per se or zero-tolerance of THC laws.  The Republican Governor of Massachusetts Charlie Baker has proposed new legislation suspending a person’s license if they refuse a drug test when requested by an officer.

Since Baker strongly opposed ending cannabis prohibition, he received large campaign donations from the Massachusetts Corrections Officers Federal Union. Prison guard unions, along with the private prison industry, oppose cannabis legalization.

Presumably, this way they can keep their prisons filled, not lose union members, and maintain profits.  Individuals who think science should underlie our public safety laws should  should oppose pe se laws.

Looking for more from Michael on Grit Daily? Check out his column. 

Michael Milburn is Founder and Chief Scientific Officer for DRUIDapp, Inc.  The DRUID® app calculates the level of impairment from cannabis or any other source in 2 minutes by measuring reaction time, decision making, hand-eye coordination, balance and time estimation.  He has just been awarded a substantial Small Business Research Grant from NIH for research on DRUID.  Dr. Milburn holds degrees from Stanford (AB) and Harvard (PhD), and he was Professor of Psychology at the University of Massachusetts/Boston for 40 years, with a specialty in research methods, measurement and statistics.  His unique set of skills enabled him to develop the DRUID® impairment measurement app, now in the App Store and Google Play.

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