Marijuana-Impaired Driving A Report to Congress 8/17
An honest and accurate report. The poor correlation of THC concentrations in the blood with impairment is discussed, along with the implication that setting per se levels is not meaningful.
The unadjusted odds ratio for THC was 1.25, representing a significantly elevated risk of crashing by about 1.25 times or 25 percent. These unadjusted odds ratios must be interpreted with caution as they do not account for other factors that may contribute to increased crash risk. Other factors, such as demographic variables, have been shown to have a significant effect on crash risk. For example, male drivers have a higher crash rate than female drivers. Likewise, young drivers have a higher crash rate than older drivers. To the extent that these demographic variables are correlated with specific types of drug use, they may account for some of the increased crash risk associated with drug use.
In a study from The Orange County Crime Lab with nearly 5000 drivers suspected of driving under the influence of drugs there was no correlation of tests on psychomotor performance and THC blood concentrations.
Much has been bandied about in the press and legislatures about a AAA report showing an increase in traffic fatalities in WA and CO post-legalization. What is not often reported is:
• The number of fatal crashes in Washington increased only about 6%, from 401 to 429, between 2013 and 2014, according to FARS data. Note that 2013 had a low number of fatal crashes, “involving recent use of marijuana,” compared to 2012 (409) and 2011 (421). Colorado saw a 4% increase in fatal crashes involving marijuana between 2013 and 2014, according to FARS.
• The AAA study acknowledges that the “recent” marijuana use detected in crash victims in Washington could have happened a week earlier, and that “the data available cannot be used to assess whether a given driver was actually impaired, and examination of fault in individual crashes was beyond the scope of this study.” The increase could simply have been a result of increased prevalence of use, or increased scrutiny. The presence of blood levels as low as 1 ng/ml, in passengers and in drivers not at fault, were enough to designate a fatal crash as involving marijuana.
• The percentage of fatal crashes involving marijuana, or marijuana only, was quite low. Most fatal crashes also involved alcohol, and/or other drugs:
According to the Washington Traffic Safety Commission, of 592 drivers involved in fatal crashes in 2013, 38 tested positive for cannabis. In the following year, of 619 deadly crashes, the number testing positive for cannabis jumped to 75. However, as Staci Hoff, Research Director for WTSC, explained:
“Most of these drivers, these 75 drivers, also had alcohol or other drugs” in their systems. Over a five-year period, just 1.8 percent of fatal crashes involved drivers who tested positive only for cannabis.
“So, in our study, we looked at all five years of date, 2010 to 2014,” Hoff continued, “and there were never 3,000 drivers involved in these fatal crashes during that time period. Only 56 of them had THC and only THC, nothing else.”
• Impairment testing is the best answer and the only fair way to judge a driver. Testimony on the oral swab testing bill from law enforcement in California noted that DREs are expensive to train and deploy, but that the state would qualify for federal grants for that as-yet-unproven testing technology.
A recent NTHSA survey, the largest of its kind ever conducted, assessed whether marijuana use by drivers is associated with greater risk of crashes. The survey found that marijuana users are more likely to be involved in accidents, but that the increased risk may be due in part because marijuana users are more likely to be in groups at higher risk of crashes. In particular, marijuana users are more likely to be young men – a group already at high risk.
February 26, 2015 – An important new study shows high blood levels of THC can persist days after smoking following heavy use. Levels above 5 ng/ml, the DUI threshold in WA state, were detected as long as 4 days after use. This ought to put the nail in the coffin of proposals to establish mandatory per se THC blood thresholds for DUI. Highway safety researchers would do well to retarget their efforts towards developing better impairment tests, rather than pursue chemical testing for DUI.
A 20-month survey of drivers in 2013 and 2014 found that drinking and driving dramatically raises the chance of a crash, but didn’t find evidence that marijuana use is statistically significant in raising crash rates. See the NHTSA study online
December 19, 2014 – For the third consecutive year, Cal NORML was able to derail an ill-conceived “zero-tolerance” DUI bill that would have criminalized all drivers with detectable traces of THC in their system—a population that includes most daily users. NORML experts provided key expert testimony against the bill, and our e-list subscribers sent over 2,100 messages of opposition to legislators. Numerous accident studies have confirmed that marijuana is not a major risk factor in driving fatalities. In general, however, studies agree that the combination of alcohol and THC is particularly dangerous, if anything worse than “straight” drunken driving.
California posted an impressive 11% decline in fatal auto accidents in 2010. This continues a strong downward trend that started in 2006. Fatalities are down 35% since then. The entire West Coast has experienced similar declines. Colorado also did well, posting a 6.5% in fatal accidents in 2010. As in California, this represents the lowest number of accidents since statistics began in 1994.
Driving, Worker Safety Not Affected by Legalization
July 27, 2010 – A a careful review of the scientific evidence shows that fears about marijuana’s impact on road safety are unwarranted. There is no good scientific evidence that drug testing improves workplace safety.