--- title: How Much Does Marijuana Impact Your Driving? description: A new federal study has answers canonical: https://time.com/3930541/marijuana-impact-driving/ author: Eliza Gray article:opinion: false article:content_tier: free article:published_time: 2015-06-23T10:00:51.000Z article:modified_time: 2026-05-11T05:02:54.354Z article:section: Health og:title: How Much Does Marijuana Impact Your Driving? og:description: A New Federal Study Has Answers og:url: https://time.com/3930541/marijuana-impact-driving/ og:site_name: TIME og:image: https://static.time.com/v3/assets/bltea6093859af6183b/blta5c2bff735b349df/698699c2adae4416fc543ee3/469030421.jpg?branch=production&width=1000&quality=75&auto=webp&crop=16:9 og:image:width: 1000 og:image:height: 563 og:image:alt: Marijuana og:type: article twitter:card: summary_large_image twitter:title: How Much Does Marijuana Impact Your Driving? twitter:description: A New Federal Study Has Answers twitter:image: https://static.time.com/v3/assets/bltea6093859af6183b/blta5c2bff735b349df/698699c2adae4416fc543ee3/469030421.jpg?branch=production&width=1000&quality=75&auto=webp&crop=16:9 ---  # How Much Does Marijuana Impact Your Driving? > Sponsored content. 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Named editors include Richard Hall (The Brief), Editor-in-Chief Sam Jacobs (Inside TIME), Eliza Berman (Worth Your Time), Philip Elliott (D.C. Brief), Justin Worland (Future Proof) and Harry Booth (In the Loop). Health Matters is written by TIME's health desk. #### How do I sign up for a TIME newsletter? Go to time.com/newsletters, choose the newsletters you want and enter your email address. There is no cost and no subscription requirement. by [Eliza Gray](https://time.com/author/eliza-gray/) ## Eliza Gray Jun 23, 2015 10:00 AM UTC  Marijuana leaf on a white background Marijuana leaf on a white background Getty Images by [Eliza Gray](https://time.com/author/eliza-gray/) ## Eliza Gray Jun 23, 2015 10:00 AM UTC A rigorous federal research study by the National Institute on Drug Abuse offers new data on the effects of marijuana on driving performance. The exact impact of marijuana on driving ability is a controversial subject—and it’s become more important states continue to loosen their drug laws. And, while drunk driving is on the decline in the U.S., driving after having smoked or otherwise consumer marijuana has become more common. According to the most recent national roadside survey from the National Highway Traffic Safety Administration of weekend nighttime drivers, 8.3 percent had some alcohol in their system and 12.6 percent tested positive for THC—up from 8.6 percent in 2007. It is illegal in all states to drive under the influence of anything, but years of work went in to establishing the .08 breathe alcohol limit that exists in most states. The question is whether we can establish a similar threshold for pot. To find out, the study recruited 18 occasional cannabis smokers, 13 of them men, between the ages 21 and 37\. The participants took six 45-minute drives in a [driving simulator—](http://www.nads-sc.uiowa.edu)a 1996 Malibu sedan mounted in a 7.3 diameter dome—at the University of Iowa. Each drive tested a different combination of high or low concentration THC, alcohol, and placebos (to create a placebo, participants were given fruit juice with alcohol swabbed in the rim, topped of with 1ml alcohol, to mimic alcohol’s smell and taste). ### Go Inside the Harvest of Colorado's Most Controversial Marijuana Strain  Charlotte's Web grows on the Stanley brother's farm near Wray, Colo. on Sept. 22, 2014\. The Stanley Brothers have developed a popular strain of cannabis that has been found to be helpful in reducing seizures in some children. Charlotte's Web is high in a compound called cannabidiol (CBD) and low in tetrahydrocannabinol (THC) the compound that causes a high. Matt Nager for TIME  Charlotte's Web grows on the Stanley brother's farm near Wray, Colo. on Sept. 22, 2014. Matt Nager for TIME  From left: Zachary Sobol, Michael Atchley, and Jared Stanley walk to the farm to prepare for harvest of Charlotte's Web near Wray, Colo. on Sept. 22, 2014. Matt Nager for TIME  Rows of Charlotte's Web await harvest at the Stanley brother's farm near Wray, Colo. on Sept. 22, 2014. Matt Nager for TIME  Jared Stanley helps in the harvest of Charlotte's Web on the Stanley brother's farm near Wray, Colo. on Sept. 22, 2014. Matt Nager for TIME  Colin Sparks searches for seeds during the harvest of Charlotte's Web on the Stanley brother's Charlotte's Web farm near Wray, Colo. on Sept. 22, 2014. Matt Nager for TIME  Discarded leaves of Charlotte's Web at a farm near Wray, Colo. on Sept. 22, 2014. Matt Nager for TIME  Tim Knight helps in the harvest of Charlotte's Web on the Stanley brother's farm near Wray, Colo. on Sept. 22, 2014. Matt Nager for TIME  Rows of Charlotte's Web await travel to a drying facility after being harvested on a farm near Wray, Colo. on Sept. 22, 2014. Matt Nager for TIME  Zachary Sobol catches a ride on a trailer to the Stanley brother's farm during harvest of Charlotte's Web near Wray, Colo. on Sept. 22, 2014. Matt Nager for TIME  Rows of Charlotte's Web await travel to a drying facility after being harvested from a farm near Wray, Colo. on Sept. 22, 2014. Matt Nager for TIME  One of the Stanley brother's greenhouses growing cannabis near Wray, Colo. on Sept. 22, 2014. Matt Nager for TIME  Zachary Sobol hangs rows of Charlotte's Web to dry after harvest in Wray, Colo. on Sept. 22, 2014. Matt Nager for TIME  Penn Mattison, left, and Zachary Sobol, right, hang rows of Charlotte's Web to dry after being harvested from a farm near Wray, Colo. on Sept. 22, 2014. Matt Nager for TIME  The Stanley brothers' new company laboratory in Boulder, Colo. on Sept. 23, 2014. Matt Nager for TIME  A petri dish full of cannabis explants await transfer and transplanting at the Stanley brothers' lab in Boulder, Colo. on Sept. 23, 2014. Matt Nager for TIME  Molecular plant biologist and lab manager Bear Reel awaits the cooling of agar to help induce rooting of transplanted cannabis plants at the Stanley brothers' company laboratory in Boulder, Colo. on Sept. 23, 2014. Matt Nager for TIME  The lab notebook of molecular plant biologist and lab manager Bear Reel at the Stanley brothers' company laboratory in Boulder, Colo. on Sept. 23, 2014. Matt Nager for TIME  A flask of agar cools before explants of hemp are transplanted to help induce rooting and new growth at the Stanley brothers' company laboratory in in Boulder, Colo. on Sept. 23, 2014. Matt Nager for TIME  Tissue cutter technician Meg Regan holds a vial of explants before sanitizing and transplanting at the Stanley brothers' laboratory in Boulder, Colo. on Sept. 23, 2014. Matt Nager for TIME The researchers looked at 250 parameters of driving ability, but this paper focused on three in particular: weaving within the lane, the number of times the car left the lane, and the speed of the weaving. While alcohol had an effect on the number of times the car left the lane and the speed of the weaving, marijuana did not. Marijuana did show an increase in weaving. Drivers with blood concentrations of 13.1 ug/L THC, the psychoactive ingredient in cannabis, showed increase weaving that was similar to those with a .08 breath alcohol concentration, the legal limit in most states. For reference, 13.1 ug/L THC is more than twice the 5 ug/L numeric limit in Washington and Colorado. Dr. Marilyn Huestis, the principal investigator in the study, says it is important to note that the study looked at the concentration of THC in the driver’s system _while_ they were driving. This is quite different from the concentration typically measured in a drugged driver out on the road, whose blood may not be checked until several hours after an arrest, allowing the THC level to drop considerably from the time they were driving. Huestis says the researchers are looking at how to estimate how long it takes for THC concentrations in the blood to drop. Huestis believes that the 5 ug/L limit is not strict enough, particularly when you take into account those with low tolerance. The study also found that pot and alcohol have more of an impact on driving when used together. Drivers who used both weaved within lanes, even if their blood THC and alcohol concentrations were below the threshold for impairment taken on their own. “We know cannabis is primarily found with a low dose of alcohol,” Huestis says. “Many young people have a couple beers and then cannabis.” Smoking pot while drinking a little alcohol also increased THC’s absorption, making the high more intense. Similarly, THC delayed the peak of alcohol impairment, meaning that it tended to take longer for someone using both to feel drunk. Such data is important to educate the public about pot’s effects before they get on the road. “I think this has added really good knowledge from a well-designed study to add to the current debate,” on marijuana’s effects on road safety, says Dr. Marilyn Huestis, the principal investigator in the study, which was conducted by researchers at the National Institute on Drug Abuse. _Read next:_ [_Here’s Why Marijuana Prices Appear to Be Dropping in Colorado_](http://time.com/3930610/marijuana-prices-colorado/?iid=time%5Freadnext) [_Listen to the most important stories of the day_](http://time.com/newsletter/the-brief/autoplay/?xid=spl-fullbrief-article) ```json [{"@context":"https://schema.org","@type":"NewsArticle","@id":"https://time.com/3930541/marijuana-impact-driving/","mainEntityOfPage":{"@type":"WebPage","@id":"https://time.com/3930541/marijuana-impact-driving/"},"headline":"How Much Does Marijuana Impact Your Driving?","datePublished":"2015-06-23T10:00:51.000Z","dateModified":"2026-05-11T05:02:54.354Z","description":"A new federal study has answers","url":"https://time.com/3930541/marijuana-impact-driving/","keywords":["Drugs","Marijuana","Pot","Drunk-driving","health"],"thumbnailUrl":"https://static.time.com/v3/assets/bltea6093859af6183b/blta5c2bff735b349df/698699c2adae4416fc543ee3/469030421.jpg?branch=production&width=1200&quality=75&auto=webp&crop=1200:675&height=675","author":[{"@type":"Person","name":"Eliza Gray","jobTitle":null,"url":"https://time.com/author/eliza-gray/"}],"articleSection":"Health","image":[{"@type":"ImageObject","url":"https://static.time.com/v3/assets/bltea6093859af6183b/blta5c2bff735b349df/698699c2adae4416fc543ee3/469030421.jpg?branch=production&width=1200&quality=75&auto=webp&crop=1200:675&height=675","width":1200,"height":675,"headline":"Marijuana","caption":"Marijuana","creditText":"Getty Images","representativeOfPage":true}],"publisher":{"@type":"Organization","name":"Time","url":"https://time.com/","logo":{"@type":"ImageObject","url":"https://time.com/images/logo.png","width":528,"height":156},"foundingDate":"March 3, 1923","sameAs":["https://www.facebook.com/time","https://www.instagram.com/time/?hl=en","https://twitter.com/time","https://www.pinterest.com/timemagazine"]}},{"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"item":{"@id":"/section/health/","name":"Health"}},{"@type":"ListItem","position":2,"item":{"@id":"/tag/drugs/","name":"Drugs"}},{"@type":"ListItem","position":3,"item":{"@id":"https://time.com/3930541/marijuana-impact-driving/","name":"How Much Does Marijuana Impact Your Driving?"}}]}] ```