Jacob Sullum

Last week, when the Colorado General Assembly passed groundbreaking legislation aimed at taxing and regulating marijuana, it also passed a bill redefining when cannabis consumers are considered too stoned to drive.

The revised rule seems consistent with the voter-approved policy of treating marijuana like alcohol and therefore may be copied by other states that decide to follow Colorado down the path to legalization. But the standard has little scientific basis, and it creates unfair legal risks for people who pose no threat to public safety.

The new law allows a jury to convict someone of driving under the influence of a drug (DUID) based on nothing more than a test indicating that his blood contained 5 nanograms or more of marijuana's main active ingredient, tetrahydrocannabinol (THC), per milliliter. The Colorado legislature had rejected the 5-nanogram cutoff on five other occasions based on concerns that it is a poor measure of impairment.

Those concerns are well-founded because there is wide variation in how people respond to a given dose of THC. Although some people may be dangerously impaired at 5 nanograms, regular consumers, including patients who use marijuana as a medicine, can drive competently at much higher THC levels because they develop tolerance to the drug's effects and learn how to compensate for them.

Since THC accumulates in fatty tissue, it can be detected in the blood of frequent users days after their last dose. But that does not mean regular pot smokers can never drive safely.

In a 2012 experiment by KDVR, the Fox station in Denver, a medical marijuana user arrived with a THC level of 21 nanograms per milliliter, even though he had not consumed any cannabis that day. He performed fine on a driving simulator both before and after smoking marijuana, which raised his THC level to 47 nanograms.

This year KIRO, the CBS affiliate in Seattle, enlisted three volunteers -- a daily medical marijuana user, a weekend smoker and an occasional smoker -- to navigate a car through a test course under the watchful eyes of a driving instructor and a police drug recognition expert. The volunteers completed the course satisfactorily both before and after smoking various amounts of marijuana, at THC levels ranging from four to seven times as high as 5 nanograms. The daily user smoked 1.4 grams of pot, reaching a THC level of 58.8 nanograms, before she was clearly too stoned to drive.

Jacob Sullum

Jacob Sullum is a senior editor at Reason magazine and a contributing columnist on Townhall.com.
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