Marijuana And Driving Reaction Time: What You Need To Know

Nov 16, 2022 | Driving Safety

Since recreational marijuana was legalized in Colorado a decade ago, millions of consumers have had the opportunity to sample commercial cannabis products, including some strains and edibles with a level of potency that wasn’t widely available before. Unfortunately, some of those consumers have also gotten behind the wheel of a car while still under the influence, with tragic results.

Alcohol use remains the biggest factor in preventable car crashes; nearly a third of all fatal collisions are caused by drunk driving. But few people realize the extent to which marijuana, too, can be a source of serious driver impairment. Two European studies indicate that drivers with THC (the main psychoactive compound in marijuana) in their blood were twice as likely to cause a fatal crash than drivers who had no drugs or alcohol in their system.

Other studies present conflicting conclusions about the degree to which marijuana affects driving performance. But it’s clear that even low doses of THC can impair cognitive and psychomotor tasks associated with driving, while higher doses can produce many adverse effects that can lead to a crash, including the following:

  • Decreased car handling performance
  • Impaired time and distance estimations
  • Increased reaction times
  • Lateral travel (weaving, crossing lanes)
  • Lack of motor coordination
  • Impaired vigilance

Chronic marijuana use and combining the drug with alcohol can make things worse. “The more difficult and unpredictable the task, the more likely marijuana will impair performance,” concludes one U.S. Department of Transportation analysis.

Curiously, some researchers have found that marijuana-induced impairment can sometimes lead to compensation by the driver, such as driving more slowly and with more focus on simple tasks. But the impairment still poses a risk to that driver and others on the road, especially during the “acute phase” of intoxication — the first hour or so after smoking or ingesting marijuana. The degree of impairment subsides significantly after three hours, but some studies insist there is still a measurable decline in safe driving skills up to 24 hours after marijuana use.

The role that marijuana may have played in a crash can be difficult to determine, since the drug can be detected in body fluids for days or weeks after consumption — and because it is often combined with alcohol. The safest choice of all, of course, is not to drive high or drunk; if you’re going to consume substances that can affect your driving ability, plan on handing your keys to someone else who isn’t.


For more than thirty years the attorneys at Franklin D. Azar & Associates have helped thousands of injured people obtain complete and timely compensation for their losses. Our proven track record and expertise have allowed us to grow into the largest personal-injury law firm in Colorado, with offices in Denver, Aurora, Thornton, Fort Collins, Greeley, Grand Junction, Colorado Springs, and Pueblo. If you’ve been injured in a bus, car, truck, or motorcycle accident, you may be entitled to compensation.  Please call the car accident attorneys at FDAzar day or night at 800-716-9032, or contact us here for a free consultation and no-obligation evaluation of your case.