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The Evergreen

Seattle Times coverage of pot policy, culture and lifestyle.

February 11, 2015 at 10:03 AM

Federal study: Drivers who have used pot aren’t more dangerous than sober drivers

Seattle police Officer Mike Lewis makes a routine traffic stop. (Photo by Dean Rutz / The Seattle Times)

Seattle police Officer Mike Lewis makes a routine traffic stop. (Photo by Dean Rutz / The Seattle Times)

A federal study of drug and alcohol crash risk finds drivers who have been drinking are much more likely to crash than drivers who have consumed pot.

The study also found, after adjusting for demographic factors like age, gender and race, as well as alcohol use, marijuana users were not at a greater risk to crash than sober drivers.

The National Highway Traffic Safety Administration (NHTSA) conducted the study in Virginia Beach, Virginia. It compared more than 3,095 drivers in the area who were involved in crashes, and twice as many controls (6,190) who were not.

The findings diverge from several studies on marijuana and driving, but the NHTSA believes this new study is the most comprehensive.

“We believe this to be the largest, the most closely controlled study of its kind that’s been conducted in the United States,” said Gordon Trowbridge, a spokesman for NHTSA.

Takeaways from the federal study and from Washington data sources: 

1 If you're above the legal limit for alcohol, you're at least four times more likely to crash

Source: NHTSA 

Drivers with a blood alcohol content (BAC) level of .08 were about four times as likely to crash, according to the new research. Those who had BAC levels double the legal limit (.16) were 14 times as likely to crash. This data hews closely to previous transportation safety research, said Trowbridge, which helped confirm some of the surprising data on marijuana.

“We assessed alcohol related crashes in this study largely as statistical contorl,” said Trowbridge. “We know clearly what the risk of alcohol represents.”


2 Study shows drivers using pot have the same chance of crashing as sober drivers


Source: NHTSA report

Nearly 8 percent of drivers who crashed had marijuana in their system, whereas about 6 percent of the control — those who didn’t crash — tested positive for THC.

But more young, male drivers consume marijuana. They’re also more likely to crash.

“It’s one of the fundamental truths of traffic safety: that young males are disproportionately prone to traffic accidents,” said Trowbridge.

After adjusting for demographics and alcohol use, researchers found that pot-consuming drivers had the same chance of crashing as sober drivers.

Trowbridge said the agency was just beginning to study and develop a research-based understanding of how marijuana affects drivers.

“We have decades of data on alcohol … and it’s easier to interpret. It has less complex effects than marijuana,” said Trowbridge.

He said more studies are in the works, including one that will rely on data from Washington state.




3 More drivers are being tested for THC DUIs in Washington


Source: WSP data

What about Washington, which legalized marijuana in 2012 and began state-regulated sales in July 2014?

“Even one DUI is too many … but the sky is not falling,” said Robert Calkins, a spokesman for the Washington State Patrol (WSP).

In impairment cases, the number of drivers given blood tests for marijuana has risen, while the percentage of those drivers who tested positive has not shown a consistent trend, according to figures provided by the state patrol.

In preliminary numbers from January through June last year, 3,072 drivers were given blood tests last year; 26.6 percent of drivers tested positive THC.

WSP: Still too early to understand data  

Calkins said it’s still too early to know what data is statistically relevant and determine how other factors might be affecting the data. Calkins said it’s possible law enforcement officers are better at recognizing impaired drivers than they have been in the past.

“We are still trying to be patient and understand the data before we start making significant changes or investments in training and equipment,” said Calkins.

In 2009, officers across the country began to receive training as part of the Advanced Roadside Impaired Driving Enforcement (ARIDE) program, which goes beyond what law enforcement academies teach.

4 In Washington, fewer drivers are testing above the legal "per se" limit for THC


Source: WSP data

When voters approved Initiative 502 to legalize marijuana they also signed off on a legal impairment limit of 5 nanograms of active THC per milliliter of whole blood. It’s illegal to drive with more THC in your system.

Plenty of factors cloud what that number actually means, though. Body type, means of consumption, metabolism and tolerance can contribute to varying effects for pot users.

Fifty-six percent of drivers who were given a blood test for pot in impairment cases during the first six months of 2014 had less than 5 ng/ml in their system, according to WSP data.

5 In a recent survey in Washington, most drivers said they'd tried pot

Last year, a research group called the Pacific Institute for Research and Evaluation (PIRE) conducted a three-week survey for the Washington Traffic Safety Commission (WTSC). Researchers stopped 955 vehicles randomly and asked their drivers to fill out a survey and give breath, blood and oral fluid samples. Data is forthcoming on the tests, but the survey results are rather interesting.

Sixty-nine percent of those surveyed said they’d used pot as least once. Forty-four percent said they had used pot in the past year within two hours of driving.




6 Few who drove shortly after consuming thought pot made their driving worse




Source: Preliminary PIRE survey results


Of almost 100 drivers who said they had consumed pot within two hours of driving during the past year, just 3 percent said it made their driving worse.

7 Fewer people are dying on Washington roads

Source: WSTC data

In 2005, 649 people were killed in crashes. Last year, just 436 people were killed.


8 About 40 percent of drivers tested after fatal crashes had drugs in their system


Source: WSTC data

Of 592 drivers involved in fatal crashes, about 25 percent of drivers had drugs in their systems. Of those tested (about 63 percent of drivers), almost 40 percent tested positive for drugs.

Note: Beginning in 2013, the  WSP’s toxicology lab ran full panels for all suspected DUIs. Previously, if someone met the “per se” limit for blood alcohol level, it wouldn’t be tested further. 

9 About 15 percent of drivers tested after fatal crashes had THC in their system


Source: WSTC data

Of drivers tested after a fatal crash (about 63 percent), more than 15 percent tested positive for marijuana. This figure include Carboxy-THC, which is an inactive, metabolite form of the chemical, so this doesn’t necessarily mean that 15 percent of drivers tested were high.

“We’re in a new frontier with marijuana not having enough research on its impacts mentally, physically,” said Jonna VanDyk, a program manager at the Washington Traffic Safety Commission.

VanDyk said the safety commission believes marijuana doubles your risk of being involved in a fatal crash.

“Marijuana impacts you physically in that it impacts your mental processes,” said VanDyk. “Driving, most of the time, is a very simple thing to do for most of us who have our license for many years. Once in awhile you need to think and move quickly. In those situations, being impaired on anything can be the difference between life or death.”

In 2013 in King County, 39 people were killed in fatal crashes involving an impaired driver. In 31 cases, a driver had been drinking. In 12 cases, a driver tested positive for marijuana.

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