New studies by the US-based Insurance Institute for Highway Safety (IIHS) and the affiliated Highway Loss Data Institute (HLDI) show crash rates spiked with the legalisation of recreational marijuana use and retail sales in California, Colorado, Nevada, Oregon and Washington.
However, despite the increase in crash rates, IIHS says studies of whether marijuana makes drivers more likely to crash are inconsistent. The IIHS study, which used data collected from injured drivers in three emergency rooms in Denver, Colorado; Portland, Oregon; and Sacramento, California, showed no increased crash risk associated with the drug except when combined with alcohol.
The preliminary results of a separate IIHS study of injured drivers who visited emergency rooms in California, Colorado and Oregon showed that drivers who used marijuana alone were no more likely to be involved in crashes than drivers who hadn’t used the drug. The IIHS says this is consistent with a 2015 study by the National Highway Traffic Safety Administration (NHTSA) that found a positive test for marijuana was not associated with increased risk of being involved in a police-reported crash.
The IIHS said increased crash numbers identified by the new study could possibly be because legalisation of marijuana encouraged more people to drink and use marijuana together. Studies comparing the simultaneous use of alcohol and marijuana in states where marijuana is legal with states where it is illegal will be needed to test this hypothesis. Some early evidence has emerged showing that combined use of marijuana and alcohol has increased while the reported use of alcohol alone has decreased, especially in states where recreational use of marijuana is legal.
To better understand the net impact on safety, researchers at the IIHS and the HLDI have conducted a series of studies since 2014 examining how legalisation has affected crash rates and insurance claims in the first states to legalise recreational use. The most recent IIHS study shows injury and fatal crash rates in California, Colorado, Nevada, Oregon and Washington jumped in the months following the relaxation of marijuana laws in each state.
A nationally representative survey, conducted recently by the AAA Foundation for Traffic Safety, also found that drivers who self-reported using both alcohol and marijuana were more likely than those who had only consumed alcohol to say they had driven while impaired and engaged in dangerous driving behaviours such as making aggressive manoeuvres or speeding on residential streets.
The IIHS says other factors related to how legalisation has affected the way people use marijuana, rather than the physiological effects of the drug, may also be at play. For example, the larger spike in crash rates in Colorado – the first state to legalise recreational use – suggests a burst of enthusiasm that levelled off as the drug’s new status became more commonplace. The first few states to legalise marijuana even used the legalisation as part of their tourism promotions.
According to the IIHS, it’s also possible that disparities in state and local regulations might encourage more travel by marijuana users. For example, marijuana users in counties who do not allow retail sales may drive to counties that do. This increased travel could lead to more crashes even if their crash risk per mile travelled is no higher than that of other drivers.
“Our latest research makes it clear that legalising marijuana for recreational use does increase overall crash rates,” said IIHS-HLDI President David Harkey. “That’s obviously something policymakers and safety professionals will need to address as more states move to liberalise their laws – even if the way marijuana affects crash risk for individual drivers remains uncertain.”
The IIHS says simulator tests show drivers who are high on marijuana react slower, find it harder to pay attention, have more difficulty maintaining their car’s position in the lane and make more errors when something goes wrong. However, the tests also showed marijuana-impaired drivers are likely to drive at slower speeds, make fewer attempts to overtake, and keep more distance between their vehicle and the one ahead of them.