For the 20 states in which marijuana is legal, either for medicinal or recreational purposes, there's a new challenge for law enforcement: judging whether stoned users are too impaired to drive.

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For the 20 states in which marijuana is legal, either for medicinal or recreational purposes, there’s a new challenge for law enforcement: judging whether stoned users are too impaired to drive.

On any given Friday and Saturday night in California, 7% of drivers on the road test positive for marijuana, according to the state’s first comprehensive roadside survey of alcohol and drug use while driving, conducted by the Pacific Institute for Research and Evaluation for the state’s highway department. That percentage is similar to the proportion of drivers who recorded some alcohol (not necessarily exceeding the legal limit for driving under the influence) in their blood.

While the survey does not address whether the positive tests resulted in impaired driving — more than 1,300 drivers were randomly pulled over and asked if they would participate in the study without any legal consequences resulting from the alcohol and drug testing — the study may help law enforcement officials to better understand how legalizing marijuana will impact driving decisions. Eighteen states, including California, currently allow marijuana for medical use, and Colorado and Washington became the first states to legalize recreational use — potentially creating a larger pool of stoned drivers.

MORE: Why Medical Marijuana Laws Reduce Traffic Deaths

Drivers in the survey were told that the results would be anonymous and would have no legal consequences, regardless of their blood alcohol levels (measured by a breathalyzer) or the results of the drug testing (from a saliva sample) for 50 drugs, including illegal substances, over-the-counter medications and prescription drugs. Participants also earned $20 for taking part, and those who had an unsafe blood alcohol concentration (BAC) or who seemed to be impaired were not arrested but were provided with alternative transportation for their safety. The survey targeted nine regions in the state where police officers pulled over potential participants between 10pm and midnight, and 1am to 3am during a Friday and Saturday night.

Although 7% of drivers tested positive for alcohol, most were not driving while drunk or impaired: 4.6% had a BAC less than the .05 cutoff for driving while impaired and only 1% had a BAC of over .08, the threshold for drunk driving.

However, because people who suspect they might be driving under the influence are far less likely to agree to stop for a survey, the research likely under-represents the real rate of potentially performance-impairing drug use by drivers.

MORE: Two States Become First to Legalize Marijuana

But the results provide a useful window into how changing marijuana laws will affect those who toke up and get behind the wheel. Research suggests that stoned users on the road are not as impaired as those who drink alcohol are, and there is some evidence that those who use marijuana, particularly for medical purposes, may be staying off the roads anyway. A national survey conducted in 2007 found that on average, about 7.7% of drivers may test positive for marijuana, and despite California’s booming medical marijuana industry, for example, fewer drivers tested positive for marijuana in 2012 than did in 2010, when a similar survey by the Pacific Institute found 8.5% of those pulled over had used before driving, compared to 7.4% in the current survey.

In addition, the results suggest that contrary to what some critics of legalizing marijuana have feared, pot users are not smoking marijuana and drinking at the same time, a potentially undesirable combination on the road, which is more dangerous than simple drunk driving. The 2010 survey found that 1.3% of people behind the wheel tested positive for both; the figure was 1% for 2012. Of those marijuana users who also drank, none was found to be legally drunk but 69% showed blood alcohol levels in the impaired range.

MORE: How Medical Marijuana Laws May Cut Traffic Deaths

The question of whether the people who test positive for marijuana are actually affected by the drug at the time they are driving, however, remains difficult to answer. The study used a cutoff level of 2 nanograms per milliliter of saliva of THC, the main psychoactive ingredient in marijuana. Driving within three hours of smoking pot is associated with a near doubling of the risk of fatal crashes. However, THC can remain in blood and saliva for highly variable times after the last use of the drug. Although the marijuana “high” only lasts three to five hours, studies of heavy users in a locked hospital ward showed THC can be detected in the blood up to a week after they are abstinent, and the outer limit of detection time in saliva tests is not known. While marijuana does not promote uninhibited and potentially reckless behavior like alcohol does, THC does have mild hallocinogenic and sedative effects, and studies show it can impair cognition and memory, making driving potentially more hazardous.

To address the question of how much pot makes driving under the influence of marijuana dangerous, Washington state’s law limits legal use in drivers who are pulled over to under 5 nanograms, regardless of any evidence of impairment. Opponents of the legalization, including some of those who use marijuana for medical reasons, felt the limit would unfairly snare many innocent medical users.

Washington, however, is an exception. Because of the scientific controversy over the link between marijuana fluid levels and impairing effects, few states have set standards for stoned driving detection. Colorado has debated them but legislators cannot come to an agreement.

They may have to soon, since the survey shows that drivers are toking up and getting behind the wheel. Studies clarifying how marijuana can affect decisions on the road will be more critical as more states grapple with legalization of the drug.

“Drugged driving poses a serious threat to public safety,” said Obama’s “drug czar” Gil Kerlikowske, in a statement released with the survey. “We commend the California Office of Traffic Safety for shedding light on this growing problem and for educating Californians about the prevalence of this danger.” And hopefully encouraging policy makers to support more research on better understanding marijuana’s effects on decision-making while driving.

MORE: Stoned Driving Nearly Doubles the Risk of a Fatal Crash