--- 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: 2026-10-01T09:08:39.550Z 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. Supplied in partnership with TIME. TIME is the sponsor and source of this material. > Last updated: September 2026. ## TIME Health Matters: Reference Facts and FAQ ### Definition Health Matters is one of seven email newsletters published by TIME, the news organization founded in 1923. The latest health news and expert advice on living well. Health Matters makes the fast-moving world of health more useful, surprising and human, from the latest science and medical breakthroughs to practical ideas for living better. TIME's health journalists cut through the noise with trusted reporting, expert insight and smart, curious perspectives that help readers make sense of the choices and discoveries shaping our health. 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Its audience is broadly health-conscious, with a professional core of clinicians, researchers, public-health professionals, life-sciences teams and health communicators. ### Sources * TIME Newsletters: [time.com/newsletters](/c/time-nl-health-2026q3/newsletters?i=68e5ec8f-7746-418d-8721-51c3b921a9fc&cr=time-nl-health-2026q3-v1) ## Video: Inside the World of Medical Marijuana for Kids [Watch (HLS stream): Inside the World of Medical Marijuana for Kids](https://cdn.jwplayer.com/manifests/6Ic4KUCK.m3u8) (16:16)  _Published 2014-10-21. The story of a family's illegal cross-country drive to transport marijuana oil for their epileptic 3-year-old daughter. A look inside the quasi-legal, science free world of medical marijuana for children from TIME's Red Border Films_ by [Eliza Gray](https://time.com/author/eliza-gray/) ## Eliza Gray Jun 23, 2015 10:00 AM UTC The story of a family's illegal cross-country drive to transport marijuana oil for their epileptic 3-year-old daughter. A look inside the quasi-legal, science free world of medical marijuana for children from TIME's Red Border Films 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) ## Transcript We don't wanna break the law. We don't wanna do this, we have to do this. Cannabis extract is illegal. And that's what my product is. I never stop thinking about the legal issues. As I cross into every state, that's breaking a lot of federal law. I don't believe that any of the law enforcement agencies want to lock out what is potentially a last treatment option for these patients. [ NOISE ] It's okay. It's okay. There still are many, many children and adults with epilepsy who can't be controlled with existing therapies. I cannot recommend using a medical marijuana product. It was worth it because it helps my daughter. If it's helping kids, why isn't it. We thought, well, why can't you do it? [ MUSIC ] [ NOISE ] [ MUSIC ] This is about as big of a piece as we'd wanna cut at once. This is our first day of harvest. [ INAUDIBLE ] in here, that's the problem. Say we get this big girl [ NOISE ] Each plant is a whole- plant extract, so we have to cut the plant. And final, so we'll kinda put the next plant about right here. So as long as we all know. The size that we're cutting for has to dry, fully dry before we're able to process it in the lab. Now you're through that. Now you can start on the next one. Here is the next one. I've got a, I've got a really nice one We stepped onto the field June 3rd. Is that your sandal? I'll fill this one up. Perfect. We planted 17 acres with an estimated 36, 000 plants. [ MUSIC ] We'll likely serve several thousand patients with this. These grow big. [ MUSIC ] Which is good. My name is Penn Madison and I am the head manager here at Stanley Brothers Social Enterprises. You need to change hands. [ LAUGH ] [ INAUDIBLE ] And we grow medical hemp. [ MUSIC ] We have a little girl, Nellie, who's two and a half. Yeah. Is that one hurting? Are you getting frustrated? Huh? She is intractable apolopsy. That's a debilitating form of epilepsy. One in 26 people will have epilepsy in their lifetime. Three million Americans have epilepsy. 50 million people world wide have epilepsy. It is one of the most common neurologic disorders. Yeah. We had no idea wether or not medical help would work. What we did know is that, everything else we had tried had not. We'd seen specialist all over the country. We tried multiple anti- epileptic drugs. Hm- mm. Yeah? I would say in the last year too we have seen an increase in children that are here from out of state. That have been brought by families to try variants of medical marijuana. [ MUSIC ] The laws in Tennessee didn't allow us to have this delivered to us there. It didn't allow this to be an option to give it to her there. [ MUSIC ] So the option of moving to Colorado to pursue medical marijuana was a viable option for us. And so we chose to do that. [ NOISE ] [ MUSIC ] We start with tissue culture plants. They'll be grown for their entire season. When they're ripe they'll be harvested. Dried. Once they're cured right they will come back to the lab for extraction. In which we will extract all of the valuable parts of the plant into a fine oil. Then that oil is used to create specific dilutions of a dietary supplement. That they'll be called Charlotte's Web. [ MUSIC ] At nine months old she was sleeping 20 hours a day. Kids that age like to grab things. Swat at things. We would have things hanging over her she would have no interest. Finally at about her nine month appointment. After we were able to find a neurologist to see us relatively quickly. He was fairly confident that [ INAUDIBLE ] spasms. She was barely able to grow in function, because she was having upwards of 200 seizures a day. Seizures are incredibly debilitating but there's also a lot of other things that can happen with a cognitive problems. Depression, anxiety [ MUSIC ] Hello! Hello! [ UNKNOWN ] Three times a day. [ UNKNOWN ] I see patients here two days a week on Tuesdays and Thursdays, and these are the consultation days, and also days where people come to pick up their medicine. In order to be ready for those two days, I spend. It's probably about another four days in lab making oil. Or with my brothers at the girl facilities. He was our first state licensee for the charlotte's web products. And it's fitting that he is a parents of a child with [ UNKNOWN ] syndrome. I am involved in this industry because of my daughter who is ten years old now and her name is Emily. Who has intractable epilepsy. [ NOISE ] It's okay. It's okay. It's okay. At her worst times, she's had a hundred to a hundred and fifty seizures a day. We've been told by a couple of doctors that we have to go home and enjoy our daughter as much as we can cause there is really not much hope left or anything else that will help so I spent a lot of time in California trying to find this medicine and you know I didn't have a lot of luck . >> We started doing our own research to see. What type of side effects? But why not try it. I just put my daughter on a medication that could've killed her. If it doesn't work, you put it to rest and we can move on with our lives. But if it does, nothing else has. Not everybody can come to Colorado, so there has to be something. Else we can do. My daughter was not doing good. She was having seizures, and I knew that there could be something that helps her, but I can't bring it to California, or I can't transport it. so it was very frustrating. It's okay. In the state where we live they do have a medical marijuana program. However, even if you were able to get through all of the bureaucratic hoops they have set up, the appropriate product for our daughter is not available. [ MUSIC ] Here I am, desperate father. I have to help my daughter. I need to have this for my daughter. So what is it gonna take? One thing led to another. How're you guys doing? [ INAUDIBLE ] We're doing okay. I am now the director of the foundation's California chapter. And that was the only way to do this legally. [ MUSIC ] I wish. When Emily started having seizures, ten years ago, somebody in this position. Existed. Where somebody would sit down across from me and say, this is what failed for people. This is what didn't. This is what others are doing. We were scared, one, to pick up and move and leave everything behind, but we weren't sure it was gonna work. Even in Colorado, the waiting list for Charlotte's Web oil is quite long, so it's not like you can just. Fly or drive to Colorado, pick it up and come home. Which apart from violating federal and state laws, It's just you can't do it. In California we found, an apartment, that we could do a short term rental. When we started giving her the marijuana in the first couple of doses even, we saw changes. She became more verbal. She became a lot more attentive. She was a lot more awake than she had been. [ LAUGH ] After the summer we wanted to give a longer time for our trial. So we made a decision that we were going to. Bring a supply back with us. [ MUSIC ]. They were able to identify a supplier in California my understanding who had what appears to be a very good product of oil high in CBD and they didn't want to fly with it, so apparently drove cross country to get their child this medication. Well I never stopped thinking about the legal issues. As I cross into every state I tried to make sure to be very careful. Any time I was tired I took a break. My daughter is doing great. She's now down to about 20 a day. She's far more interactive with the world around her. She has far more enjoyment. She's not playing beside the world, she's playing in the world where she lives now. And that, to me, is everything. Fortunately, that child is doing quite well. And they've gone through a huge pilgrimage to get that oil and, what appears to be, help their daughter. [ MUSIC ] I actually haven't heard anything about you know, negative potential side effects in the media. And I think it's a really important thing for parents to know is this is not risk free. It's not something I told them to do but when they said they were determined to do it, I was supportive, as their physician, to say listen, I will be here to guide you on helping to adjust the therapy and monitor the other drugs and, and the girls' epilepsy. Overall, at least so far, our experience here at Children's Colorado has been that, that only about a third of families are really reporting that they see much of an improvement in their children. And of those children when we look at brain waves, none of those are we really seeing the kind of EEG change that we would expect from a really effective medication. We also don't understand how marijuana products will interact with other medicines these children are on. Concerning things that we have seen in general, her seizures have gotten worse. Severe enough to require them to go into our intensive care unit. We absolutely don't have the scientific data and I think if I could make one point to the parents, it is that you need science. The other concern with the artisanal products is that we don't always know what's in them. They vary from batch to batch, they vary from grower to grower. [ MUSIC ] Unfortunately, we're giving it out to large numbers of very vulnerable people like children. Without knowing what the correct dose it. Without knowing what kind of epilepsy it's going to work for. Most of my critics, especially in the medical field, are critics that are criticizing lack of research. I blame them. That's their job. You're a researcher. You're a scientist. You're a doctor. Why don't you research this? These procedures are gaining. Something stronger. The schedule one listing for marijuana has made it much more difficult to do research on marijuana, there is no question about that. Really all hemp products lie in a gray area. We are faced with a bit of a moral dilemma. We do have some legal area to operate here. But at the same time, that's up for interpretation. [ MUSIC ] The supply is gonna run out, because we only brought back six months. There are options. Realm of Caring is talking about challenging the law and sending medication across state lines. Right, one more on the end and let's get it there. This product, this fall. Coming from hemp regulations, this will be our first time to actually be able to ship this to patients across the country. If it works, it would make a huge difference because then we could have it delivered right to our house. Cannabis [ UNKNOWN ] and that's what my product is. The last. Drug enforcement agency position that I read stated that hemp products made for human consumption, which have any detectable THC in them, are technically a violation of federal law. Now, however, they go on to say that they realize that all of these products exist throughout grocery store shelves throughout America. At this point in time I don't feel like the legal landscape has us such that people are going to come in with guns and handcuffs and try and shut down something that is providing hope and options for people. We don't want to break the law. We don't want to do this, we have to do this. How can kids not get what they need? I mean anyone you talk to about it, it's like, it's ridiculous. If it's helping kids, why isn't it. Reba like why can't you do it? 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