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It’s going to be really hard to figure out how many car crashes are caused by drivers under the influence of marijuana

In the last 3 years, 3 states—Colorado, Washington, and most recently Oregon—legalized the use of recreational marijuana.

During a congressional hearing on the potential threat posed by stoned drivers, Jeff Michael, a program administrator for the National Highway Traffic Safety Administration (NHTSA), was asked how many crash fatalities are caused by marijuana each year. His response:

“That’s difficult to say … We don’t have a precise estimate.”

He affirmed to the committee that the answer was “probably not” zero.

NHTSA is a federal agency that describes its mission as “Save lives, prevent injuries, reduce vehicle-related crashes.”

This is the government agency that tracks—down to a person—how many people in the U.S. are killed each year in drunk driving crashes, rollover accidents, unintended acceleration crashes, etc.

And yet, NHTSA officials have no idea how many people are injured or killed in crashes caused by stoned drivers.

Here’s why.

There’s no standard test for marijuana impairment.

The first NHTSA survey on the proportion of drivers with measurable blood alcohol content (BAC) was completed back in 1973.

In 1998, after years of studying state laws and car crash data, the NHTSA recommended that .08 become the national legal limit for a driver’s BAC.

Alcohol’s effect on the human body is predictable and measurable, compared to marijuana. That’s why there is—so far—no equivalent test for measuring marijuana impairment.

While imperfect, the BAC standard has worked well as a tool for measuring and prosecuting drunk driving. Combined with widespread public education, drunk driving crashes have been in steady decline for decades.

But it took 25 years for NHTSA to recommend a national standard for driving under the influence of alcohol. How long will it take to standardize a similar test for marijuana impairment?

Testing drivers for marijuana use

There are 3 ways that law enforcement officers can measure a driver’s impairment from marijuana: a blood test, a urine test, or the assessment of  police officers.

Blood tests and urine tests can measure the amount of THC from marijuana use. There are 2 main compounds:

  • Delta-9-tetrahydrocannabinol (THC): “Delta-9-THC” is the psychoactive compound that produces the immediate effects of marijuana use, including the “high.”
  • 11-nor-9-carboxy-delta-9-tetrahydrocannabinol (THC): “Carboxy-THC” is the inactive THC left over after the effects of the  delta-9-THC have dissipated.

Since the body’s fat cells can store THC, it can be hard to tell when the marijuana was consumed. See details: Marijuana pushes the limits on the inexact science of DUID.

A blood test can identify the presence of delta-9-THC, but that would only be present in the immediate aftermath of smoking marijuana. Blood tests are expensive: a lab has to process the results. In Oregon, blood tests for marijuana are currently only used in fatal crashes or potentially fatal crashes.

A urine test can identify carboxy-THC, which could be stored in the body for hours, days or even weeks.

“Urine testing can tell if they’ve smoked in the last 30 days. It doesn’t prove impairment at the time of arrest.”

– Dan Estes, the Oregon Department of Transportation Impaired Driving Manager. Courtesy of Oregon criminal attorneys Carini & Francis.

Some states have a zero tolerance policy: any detectable amount of marijuana in the bloodstream is grounds for DUI or DUII.

In Colorado and Washington,  the “per se” limit is 5 nanograms of THC per milliliter of blood. Any driver found at or above that level is considered over the legal limit—regardless of when the marijuana was used.

Oregon, where recreational marijuana use became legal as of July 1, 2015, has adopted neither a “per se” law or “zero tolerance” policy.

That leaves it up to officers to identify if a motorist is under the influence of marijuana.


The state of Oregon classifies driving under the influence of alcohol or drugs as the same offense: Driving Under the Influence of Intoxicants (DUII).

In Oregon, you could be prosecuted for DUII if you fail a field sobriety test—without a breathalyzer or blood test showing your BAC.

Since there is no equivalent test for marijuana, criminal prosecutions of Oregon drivers suspected of driving under the influence of marijuana will largely rely on field sobriety tests, and the testimony of police officers.

About 200 Oregon state and local police officers have undergone several weeks of training to identify people under the influence of marijuana: these officers have been certified as DREs: “Drug Recognition Experts.”

The problem with this method is that it provides no guidelines for motorists. “Don’t drive impaired” is good advice for everyone, but it’s not remotely quantifiable.

If we can’t quantify the number of motor vehicle accidents caused by stoned drivers, how can we educate the public and prevent these crashes?

Oregon will need to figure out what constitutes “driving under the influence of marijuana”

States are the test labs for federal laws. Other states, and the federal government, will look to the states that have legalized marijuana to measure the impact on costs, tax revenue—and safety.

Recently, Washington state officials did an analysis of blood tests of drivers in fatal traffic accidents. The analysis identified an increase in the percentage of blood samples showing delta-9 THC: indicating that the drivers had recently consumed marijuana.

The number of drivers involved in deadly crashes who tested positive for marijuana increased 48 percent from 2013 to 2014.

The Oregon Liquor Control Commission (OLCC) has been tasked with researching and presenting a report to the Oregon Legislature on driving under the influence of marijuana.

The OLCC may look to Washington’s analysis of blood tests that identified recent marijuana use in drivers involved in fatal crashes as a guide. But right now, blood tests are only standard after fatal or near-fatal crashes: that’s not useful for police officers looking to identify stoned drivers.

And it doesn’t provide any guidelines for drivers, who have no way to gauge how long to wait to get behind the wheel of a car after smoking or ingesting marijuana.

Marijuana isn’t quantifiable like alcohol. But stoned driving, like drunk driving, can only be addressed by a combination of both public policy and public education based on real data.

Right now, we have neither.

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