Michigan recognizes that THC levels in the blood are not a reliable indicator of driving under the influence

The question cannabis on the roads remains a real headache for countries and states that have legalized it, as well as for those that haven’t. In fact, the tests currently available are unreliable and, more broadly, the effects of cannabis on driving are difficult to assess.

Should they be tested for THC levels in their blood? A Michigan commission tasked with studying the issue does not believe so. In a report released last week, the commission recommended against imposing THC limits in the blood.

Too weak a correlation

After two years of studying the issue, the six members of the’Impaired Driving Safety Committee experts appointed by the state governor concluded that the correlation between blood THC levels and impaired driving was too weak. «Blood THC levels are an indicator of exposure but are not a reliable indicator of an individual’s impaired consciousness,» writes the report. However, this is the standard test used in traffic stops (saliva tests are always confirmed by blood tests).

The commission cites several reasons for this conclusion. First, when cannabis is smoked or vaporized, THC levels rise rapidly and fall just as quickly: they are reduced by half within 10 minutes. This fluctuation is problematic in itself. Second, it is possible to have THC in the blood without being under the influence of cannabis. In fact, THC remains in the bloodstream for about a week after the last use, and in regular users, it can remain in the system for more than a month. THC therefore remains in the blood long after its effects have worn off.

Despite these facts, some U.S. states continue to impose arbitrary blood THC limits. This is the case in Colorado, Washington, Montana, Nevada, and Ohio based on High Times. Under current Michigan law, no level of THC in the blood is permitted, but with the recent legalization of recreational cannabis and the legalization of medical cannabis in 2008, the previous governor decided it was appropriate to address the issue and commissioned this report. It is now up to the legislature to establish new rules regarding driving under the influence in accordance with the report’s recommendations.

By analyzing simulations, the commission also made an interesting discovery: it found that drivers under the influence of cannabis tended to adjust to the subjective effects of the drug. They therefore drive more slowly, maintain safe following distances, and take fewer risks than when they are sober—which is typically not the case for drivers under the influence of alcohol. The latter tend to drive faster, follow more closely, and take more risks than when they are sober. The commission therefore concluded that drivers under the influence of cannabis are less dangerous than those under the influence of alcohol.