A new study by researchers at the National Institute on Drug Abuse (NIDA) confirms that marijuana has a less dramatic effect on driving than alcohol does and casts further doubt on the standard that Colorado and Washington use for determining when someone is too stoned to drive. In the double-blind study, 18 occasional cannabis consumers—defined by marijuana use at least once in the previous month, but no more often than three days a week—took tests at the National Advanced Driving Simulator in Iowa City after vaping marijuana, drinking alcohol, or taking a placebo. The researchers, led by NIDA senior investigator Marilyn Huestis, found that a THC level of 13.1 nanograms per milliliter of blood had an impact on weaving within the lane (a.k.a. standard deviations of lateral position, or SDLP) similar to a blood alcohol concentration (BAC) of 0.08 percent, the cutoff used for most drivers in every state. 
 
 Notably, that THC level is more than twice as high as the five-nanogram standard established by Colorado and Washington when they legalized marijuana for recreational use. This result is especially striking because the subjects were occasional users. People who use cannabis every day, including patients who use it for medical reasons, may develop enough tolerance that they can drive safely at THC levels even higher than 13 nanograms. Yet according to Time, "Huestis believes that the 5 ug/L limit is not strict enough, particularly when you take into account those with low tolerance." That's another way of looking at it, I suppose, but the five-nanogram cutoff in practice means that many regular cannabis consumers can never legally drive, even when they're not impaired, which hardly seems sensible or fair. 
 
 Huestis and her colleagues found that alcohol and marijuana together had an additive effect on SDLP, with five nanograms of THC per milliliter plus a 0.05 percent BAC having an impact similar to a 0.08 percent BAC. In addition to SDLP, Huestis and her colleagues measured lateral acceleration (weaving speed) and lane departures per minute. Both were affected by alcohol but not by marijuana. This particular study did not consider any of the many other indicators used to measure driving impairment, although Huestis et al. plan to publish more on this subject to help illuminate the question of how best to define stoned driving. 
 
 Another result of this study that is relevant to enforcing such limits: Saliva tests accurately confirmed exposure to cannabis, but "with greater THC concentration variability than paired blood samples," which "poses challenges in concentration-based effects interpretation." In other words, saliva tests may be easier and less invasive, but they are less reliable in determining whether someone has exceeded whatever the legal threshold might be. 
 
 Also interesting, especially for people who have heard the frequent warnings from anti-pot prohibitionists about the dangers posed by increased potency: Most of the subjects, who were allowed to vape at will for 10 minutes, achieved about the same blood THC levels whether they were given weak cannabis (with a THC content of about 3 percent) or stronger cannabis (with a THC content of about 7 percent). As the researchers put it, "smokers can self-titrate cannabis dose to achieve desired pharmacological response." 
 
 Although this study provides additional evidence that marijuana poses less of a road hazard than drug warriors often claim, Time health reporter Eliza Gray manages to obscure that point. "While drunk driving is on the decline in the U.S.," she writes, "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." Gray neglects to mention that a landmark NHTSA crash risk study released the same day found that, once the data were adjusted for confounding variables, cannabis consumption was not associated with an increased probability of getting into an accident. ||||| TIME Health For more, visit TIME Health 
 
 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—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 Matt Nager for TIME Matt Nager for TIME Matt Nager for TIME Matt Nager for TIME Matt Nager for TIME Matt Nager for TIME Matt Nager for TIME Matt Nager for TIME Matt Nager for TIME Matt Nager for TIME Matt Nager for TIME Matt Nager for TIME Matt Nager for TIME Matt Nager for TIME Matt Nager for TIME Matt Nager for TIME Matt Nager for TIME Matt Nager for TIME Matt Nager for TIME Matt Nager for TIME 1 of 20 Advertisement 
 
 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 
 
 The Brief Newsletter Sign up to receive the top stories you need to know right now. View Sample Sign Up Now 
 
 Listen to the most important stories of the day
– When it comes to smoking marijuana and driving, many variables affect impairment—the potency of the drug, the tolerance of the toker, when the drug was taken, how the drug was taken—making guidelines difficult to implement. But with some states decriminalizing the possession of weed, the feds are investigating a pot version of the .08 breath alcohol concentration set as the legal driving limit in most of the country. A new study by the National Institute on Drug Abuse finds that drivers with blood concentrations of 13.1 ug/L THC, which is more than twice the 5 ug/L limit in Washington and Colorado, exhibit impairment similar to those with a .08 breath alcohol concentration, reports Time. The small study of 18 occasional pot smokers between 21 and 37 involved a 45-minute driving simulator in a 1996 Malibu sedan at a University of Iowa dome. Not only did researchers test different combinations of high or low concentrations of THC, alcohol, the combination of the two, and placebos, but they also looked at 250 parameters of driving, with a focus on weaving. While alcohol affected both lateral acceleration (weaving speed) and lane departures per minute, marijuana alone did not, reports Reason Magazine, which takes issue with current limits: "The five-nanogram cutoff in practice means that many regular cannabis consumers can never legally drive, even when they're not impaired, which hardly seems sensible or fair." (What happens when mixing alcohol and marijuana?)