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5 things to know about marijuana and its effects on driving

Posted: September 2, 2014 - 12:03am
FILE - This  Oct. 8, 2012 file photo shows the wrecked Subaru Impreza in which four people died as it is loaded onto a flatbed truck on the Southern State Parkway in West Hempstead, N.Y., after and early-morning accident. At the wheel was a New York teenager, Joseph Beer, who had smoked about $20 worth of marijuana, before getting into the car with four friends, and driving over 100 mph before crashing into trees with such force that it split the car in half. As states liberalize their marijuana laws, public officials and safety advocates worry that more drivers high on pot will lead to a spike in traffic deaths. Researchers who have studied the issue, though, are divided over whether toking before taking the wheel in fact leads to more accidents. (AP Photo/Frank Eltman, File)  Frank Eltman
Frank Eltman
FILE - This Oct. 8, 2012 file photo shows the wrecked Subaru Impreza in which four people died as it is loaded onto a flatbed truck on the Southern State Parkway in West Hempstead, N.Y., after and early-morning accident. At the wheel was a New York teenager, Joseph Beer, who had smoked about $20 worth of marijuana, before getting into the car with four friends, and driving over 100 mph before crashing into trees with such force that it split the car in half. As states liberalize their marijuana laws, public officials and safety advocates worry that more drivers high on pot will lead to a spike in traffic deaths. Researchers who have studied the issue, though, are divided over whether toking before taking the wheel in fact leads to more accidents. (AP Photo/Frank Eltman, File)

WASHINGTON — The legalization of recreational marijuana in two states — Colorado and Washington — and medical marijuana in more than 20 others has raised concern that there will be more drivers stoned behind the wheel. What’s not clear is whether that will translate into an increase in fatal crashes.

WHAT WE KNOW

While marijuana users can perform simple tasks well while they are high, brain imaging has shown that they have to use more of their brain to do so. Their reaction times are slower, peripheral vision is decreased and multitasking impeded. As a result, when sudden or surprising things occur to complicate those tasks — such as when a pedestrian steps in front of a car — they cannot respond as well. On the other hand, marijuana users tend to be aware they are impaired and try to compensate for it.

WHAT WE DON’T KNOW

It’s not clear how much marijuana use contributes to crash risk. Some studies have found that marijuana can double crash risk, but others have found virtually no increase.

HOW HIGH IS TOO HIGH

Traces of THC, the psychoactive ingredient in marijuana, can be detected in the blood of some habitual marijuana users days or weeks after they last used the drug, making it hard to use blood tests to discern a current level of impairment. Most states haven’t set a THC threshold for impairment, but Colorado and Washington have settled on an intoxication blood level of five parts per billion. There’s no roadside test for THC like those for alcohol, but some states are experimenting with a saliva test.

REAL-WORLD EXPERIENCE

In 2013, the first full year after Washington state legalized pot, nearly 25 percent more drivers tested positive for marijuana than before legalization. But there’s been no corresponding jump in accidents or arrests for intoxicated driving. A University of Colorado Medical School study found that the share of drivers involved in fatal motor vehicle crashes in Colorado who tested positive for marijuana more than doubled between 1994 and 2011. A National Safety Council study looked at the prevalence of drivers involved in fatal crashes who tested positive for marijuana in 12 states from 1992 to 2009 — before and after implementing medical marijuana laws. Only three states showed an increase — California, Hawaii and Washington — and those appeared to be a one-time increase possibly associated with differences in testing.

WHAT NEXT?

The National Highway Traffic Safety Administration is conducting research to get a better idea of how pot affects driving. NHTSA and Washington state officials have also teamed up to assess change in marijuana use by drivers before and after the state allowed retail sale of the drug, with results due next year.

• Associated Press writer Gene Johnson in Seattle contributed to this report.

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Karl Ashenbrenner
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Karl Ashenbrenner 09/02/14 - 09:19 am
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Interesting that a simple Google

search finds lots of "studies" that purport an increase in crashes from stoned driving. But further research (actually reading the study conclusions) show that there is no real statistics and no real world test to show if a driver was stoned at the time...only blood tests taken hrs after a crash. In fact some studies conclude that the increase in accidents from being stoned while driving will be so small that the better use of resources would be to continue to address drunk driving instead. Makes one wonder. And the U of Colorado study? Showed that marijuana use stayed the same or went down from 2011 to 2013. Although a Dr Thurstone from UC Med disputed the findings which only looked at adolescent drug use. And the "crash study"? Couldn't be found in my research on the University of Colorado Medical School web site or otherwise.

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