Take our quiz & meet the car you’ll love.

As Drugged Driving Increases, Hard Answers Still Elusive

CARS.COM — Three of today’s traffic-safety hot topics — drugged driving, distracted driving and pedestrian fatalities — are converging in an ongoing series of scientific studies that researchers hope will help shape future policymaking. Coincidentally, it’s coming together at a time when more than half of U.S. states have legalized medical or recreational marijuana, and when traffic fatalities in which drivers test positive for the drug are spiking.

Related: Marijuana’s Effect on Driving Not Cut-and-Dried Issue, Study Shows

November elections brought the number of states with legalized recreational weed to eight. Newcomers California, Maine, Massachusetts and Nevada joined Alaska, Colorado, Oregon and Washington, in addition to the District of Columbia. Meanwhile, the National Highway Traffic Safety Administration’s 2013-14 National Roadside Survey of Alcohol and Drug Use by Drivers showed that as the number of drivers testing positive for alcohol has greatly declined (by as much as 80 percent at peak times), those testing positive for THC, the active substance in marijuana, increased by nearly 50 percent between 2007 and 2014.

USA Today recently reported that in 2015, 12.4 percent of fatal crashes in Colorado — where voters approved recreational weed in 2012 — involved a driver who tested positive for marijuana alone, up 53 percent since 2013. In Washington, according to the AAA Foundation for Traffic Safety, the proportion of fatal crashes involving drivers who’d recently used marijuana more than doubled to 17 percent just a year after recreational use was legalized.

As for whether marijuana has contributed to the increase in fatal collisions or if it just means more people happen to be using it while driving (and crashing), the jury is still out. A 20-month study of 3,000 crash-involved drivers in Virginia Beach, Va., showed only a questionable causal effect from marijuana. But that’s not to say there’s zero causal effect.

Dr. Timothy Brown, who served as one of the lead investigators on a study by the University of Iowa’s National Advanced Driving Simulator in cooperation with the National Institute on Drug Abuse, the Office of the National Drug Control Policy and NHTSA, told that a measurable link was determined between marijuana inhalation and driving performance. The study found that 13.1 nanograms of THC per milliliter of blood had the equivalent effect of an 0.08 blood alcohol concentration with regard to “lateral vehicle control,” or in other words, weaving.

A BAC of 0.08, of course, is the legal driving limit for alcohol in all 50 states, whereas states that have set THC limits, such as Colorado and Washington, settled on 5 nanograms per milliliter. While scientists have abundant evidence linking a 0.08 BAC to specific levels of driver impairment and crash risk, any legal limit for THC at this time is effectively an arbitrary one due to limited study and wildly varying effects from person to person depending on factors like their metabolism, frequency of marijuana use and other variables.

“While the legality of marijuana is being debated at the state level, it’s not clearly understood how marijuana impacts a person’s ability to drive,” NADS states on its website. “Specific marijuana concentration levels cannot be reliably equated with a specific degree of driver impairment.”

NADS researchers tested 18 participants with half a dozen different combinations of alcohol and inhaled vaporized marijuana; among those combinations were placebos for both substances to control for the effects of each on its own. Subjects drove a 45-minute route in the sophisticated NADS simulator that included urban, suburban and rural environments that might be encountered on a typical evening commute, Brown explained. Their driving performance was observed and measured by speed, lane position, steering-wheel position, reaction time, headway to the lead vehicle and eye tracking.

Among takeaways from the study, Brown noted, was that degraded lane keeping under the influence of marijuana tended to be by a light-to-moderate user as opposed to a heavy, chronic user. And what will likely come as little surprise to marijuana advocates, researchers also observed that weed users tended to drive slower and alcohol users faster. Further — and contrary to popular belief — studies showed that the combined effects of alcohol and marijuana produce an “additive” effect as opposed to a “synergistic” one, meaning the drugs’ effects together don’t result in impairment greater than the sum of their individual parts.

As researchers continue examining data collected on the topic of drugged driving, two important areas of focus are emerging, Brown noted. One examines interactions between high drivers with pedestrians near the roadway or boundary line, or crossing the roadway: How much berth does the driver give the pedestrian? At what speed do they pass? How’s their reaction time?

This research becomes increasingly vital as pedestrian deaths have spiked in recent years. In 2015, 5,376 pedestrians were killed in vehicle collisions, nearly 500 more than the previous year for a nearly 10 percent increase — outpacing the 7 percent increase for traffic fatalities overall. Preliminary NHTSA figures for the first nine months of 2016, meanwhile, showed pedestrian deaths up another 8 percent over the same period the previous year.

The other aspect is people’s performance of “secondary tasks” while driving high, Brown said. Each year, more than 3,000 motorists are killed and more than 400,000 injured in crashes involving a distracted driver, and in the age of smartphones and in-car multimedia systems, drivers have more attention-dividing temptations than ever. Now add potential drug impairment to the mix.

“Very few people drive and just drive,” Brown said, noting that results of these studies should be available within the first half of this year. “They fiddle with the radio or grab something off the seat. What happens when drivers engage in a distraction while under the influence of cannabis … and how does that compare to when they’re not under the influence of cannabis?”

Beyond marijuana, the 800-pound gorilla in the room continues to be prescription-drug impairment. Brown noted that more than half of adults in the U.S. are now on two or prescription meds at any given time. In the long term — perhaps a decade or more — NADS researchers hope to develop a comprehensive model for how drugs, including prescription meds, alcohol and marijuana, interact together to impair driving. Brown said legislatures, pharmacists and doctors can make use of that information when crafting policy and writing prescriptions.