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Leah Thomas May 18, 2015

A federal agency – the National Highway Traffic Safety Administration (NHTSA) – published a new study that shows there is no increased risk of an accident when a driver smokes weed prior to driving when adjusted for age, gender, ethnicity, and alcohol concentration. However, a driver with a blood alcohol concentration over .05 is nearly 7 times more likely to have an accident.

Wondering if it's really true? Well NHTSA is the same agency that studied the effects of alcohol on drivers years ago, and "developed" the standardized field sobriety tests that most police departments across the country use: the one-leg stand, the walk-and-turn, and "that eye test" that is called the horizontal gaze nystagmus (HGN). While we as defense attorneys challenge the validity of these field sobriety tests routinely, the marijuana study's conclusion is logical.

In Wisconsin, operating a vehicle with a restricted controlled substance is a strict liability offense, such that any detectable amount of marijuana is a violation. However, the problem is that marijuana can be found in the blood long after the high has worn off – even days or weeks later. Just like alcohol, your body metabolizes or processes the marijuana out of your system. Other compounds are created as a result of this process, and they remain detectable but do not have any effect. This means drivers are being charged without any impairment of their driving.

There are a whole host of factors why detectable drug presence doesn't indicate impairment the way it does with alcohol. "Most psychoactive drugs are chemically complex molecules, whose absorption, action, and elimination from the body are difficult to predict," the report authors write, "and considerable differences exist between individuals with regard to the rates with which these processes occur. Alcohol, in comparison, is more predictable." In heavy marijuana users, measurable amounts of THC can be detectable in the body days or even weeks after the last use, and long after any psychoactive effects remain.

Read more about this issue in the Washington Post. This is an area of the law that will continue to change as more states legalize marijuana. Colorado now defines a violation as over a very low level of marijuana, rather than any detectable amount. Time will tell whether this is a workable model.