Last year, HLDI released a groundbreaking analysis of insurance losses in Colorado, Oregon and Washington that found that legalizing recreational marijuana use in the three states was associated with a combined 2.7 percent increase in the frequency of collision claims per insured vehicle year relative to nearby control states (see "High claims: Legalizing recreational marijuana is linked to increased crashes," June 22, 2017).
Collision coverage insures against physical damage to a driver's vehicle sustained in a crash with an object or other vehicle, generally when the driver is at fault. An insured vehicle year is one vehicle insured for one year or two vehicles insured for six months each.
In a new report, HLDI analysts estimate that the frequency of collision claims rose a combined 6 percent following the start of retail sales of recreational marijuana in Colorado, Nevada, Oregon and Washington, compared with the control states of Idaho, Montana, Utah and Wyoming.
The new combined-state analysis adds another year of collision loss data (January 2012 through October 2017) and accounts for the 2017 start of retail marijuana sales in Nevada, which was used as a control state for Oregon in the prior report.
A separate IIHS study examined 2012–16 police-reported crashes before and after retail sales began in Colorado, Oregon and Washington. IIHS estimates that the three states combined saw a 5.2 percent increase in the rate of crashes per million vehicle registrations, compared with neighboring states that didn't legalize marijuana sales.
IIHS researchers compared the change in crash rate in Colorado, Oregon and Washington with the change in crash rates in the neighboring states that didn't enact recreational marijuana laws. Researchers compared Colorado with Nebraska, Wyoming and Utah, and they compared Oregon and Washington with Idaho and Montana. The study controlled for differences in demographics, unemployment and weather in each state.
The size of the effect varied by state. Although the study controlled for several differences among the states, the models can't capture every single difference. For example, marijuana laws in Colorado, Oregon and Washington differ in terms of daily purchase limits, sales taxes and available options for home growers. These differences can influence how often consumers buy marijuana, where they buy it and where they consume it.
The 5.2 percent increase in police-reported crash rates following legalization of recreational marijuana use is consistent with the 6 percent increase in insurance claim rates estimated by HLDI.
"The new IIHS-HLDI research on marijuana and crashes indicates that legalizing marijuana for all uses is having an impact on the safety of our roads," says David Harkey, president of IIHS and HLDI. "States exploring legalizing marijuana should consider the highway safety impact."
Marijuana is still an illegal controlled substance under federal law.
Under the influence
Impaired driving is a longstanding problem, with about a third of all fatally injured drivers in crashes having a blood alcohol concentration (BAC) of 0.08 percent or higher (0.08 g alcohol per 100 mL blood). All 50 states and the District of Columbia have per se laws making it a crime to drive with a BAC at or above 0.08 percent. Effective in December, Utah will lower the threshold to 0.05 percent or above.
Driving under the influence of marijuana is illegal in all 50 states and D.C., but determining impairment is challenging. Unlike alcohol, the amount of marijuana present in a person's body doesn't consistently relate to impairment. THC, or Tetrahydrocannabinol, is the primary psychoactive component of cannabis. A positive test for THC and its active metabolite doesn't mean the driver was impaired at the time of the crash. Habitual users of marijuana may have positive blood tests for THC days or weeks after using the drug.
Eleven states have zero tolerance per se laws for marijuana, which make it illegal to drive with any amount of THC or a metabolite in a person's body. South Dakota has a zero-tolerance law for drivers younger than age 21. In five states, it is illegal to drive with specified amounts of marijuana in a person's body. Of the nine states where recreational use of marijuana is already legal, only Nevada and Washington have per se laws for marijuana and driving. Nevada sets the THC limit at 2 ng/mL blood, and Washington sets it at 5 ng/mL blood, according to the Governors Highway Safety Association. In Colorado, judges or juries can infer that drivers with a THC limit of 5 ng/mL blood are impaired; drivers can rebut the inference with evidence that they weren't impaired.
While driving under the influence of alcohol is taboo, attitudes about driving after using or consuming marijuana are more permissive, and more people admit to using the drug.
Self-reported marijuana use among people 12 and older within the past month rose to 9 percent in 2016 from about 6 percent between 2002 and 2008, a Center for Behavioral Health Statistics and Quality survey found. In national roadside surveys, the proportion of nighttime weekend drivers who were positive for marijuana rose from 9 percent in 2007 to 13 percent in 2013–14.
Some studies have found that consuming THC just prior to driving can increase reaction time and impair distance estimation and lane tracking in both simulator and on-road studies. A study conducted using the National Advanced Driving Simulator found that drivers under the influence of marijuana had trouble maintaining constant lane position, but they tended to drive more slowly and with more headway than drivers not under the influence (see "More drivers use marijuana, but link to crashes is murky," May 12, 2015). However, other studies failed to find such differences in reaction time and lane position variation.
Marijuana's role in crashes isn't as clear as the link between alcohol and crashes. Many states don't include consistent information on driver drug use in crash reports, and policies and procedures for drug testing are inconsistent. More drivers in crashes are tested for alcohol than for drugs. When drivers are tested, other drugs are often found in combination with alcohol, which makes it difficult to isolate their separate effects.