Ted S. Warren / AP Marijuana is weighed and packaged for sale at the Northwest Patient Resource Center medical-marijuana dispensary in Seattle on Oct. 10, 2012.

Less than two weeks after recreational pot stores opened their doors in Colorado, proponents of legalization appeared to have their first black eye. A 23-year-old named Keith Kilbey, seemingly under the influence of drugs, crashed his car into a state patrol car—which then crashed into another state patrol car. A day after the incident, an official spokesperson said they suspected that the driver had been using marijuana.

A month later, it’s unclear whether Kilbey was actually stoned, and even less clear whether he purchased marijuana at a new pot shop. And a young man poised to be a harbinger of all the stoned drivers to come is now a reminder of how long it’s going to be before Colorado and Washington can really tell what impact legal weed is making on their states.

The potential for legal marijuana to yield more impaired drivers on the road has been a talking point throughout the political process in both states—as well as states like Alaska that may soon be voting on legalization—but the reality of that risk and how to assess it are pretty hazy. “It’s generally part of this conversation everywhere,” says Mason Tvert, communications director for the Marijuana Policy Project. “It’s not an easy question.”

While alcohol regulations have given officials a starting point for developing some marijuana-related rules, like how far vendors have to be from schools, there’s no simple parallel when it comes to driving. “It’s not like alcohol,” says Emily Wilfong, spokesperson for the Colorado Department of Transportation. “People metabolize it differently. There are different potencies. So there’s really no solution in terms of saying ‘you’re now at the limit.’”

Colorado will start spending $430,000 of federal grant money in March on an awareness campaign driving home the message that driving while high is illegal; the state will put up posters in pot dispensaries and broadcast ads with the tagline, “Drive high, get a DUI.” Wilfong says that the campaign won’t, however, include any advice like how long someone should wait to drive after smoking weed or eating a pot brownie. “I just don’t think there’s enough research that we can say, ‘Wait x amount of hours before getting on the road,’” says Wilfong. “I don’t know whether it’s five hours or 10 hours or the next day. We just don’t know.”

Both Colorado and Washington have, technically, settled on limits for how much THC a driver can have in his or her system while behind the wheel: a maximum of five nanograms per milliliter of blood. But there’s no handy chart that translates that scientific gobbledygook into as easy self-assessment. Various studies have tried to suss out how long marijuana remains in the system but reached no single conclusion that could be used as the basis for new DUI laws. Brian Vicente, a lawyer at a Denver firm that deals solely in marijuana-related issues, points out that research on high driving conducted in pre-legalization days often lacks the necessary context, like distinguishing between active THC (which might stay in someone’s blood for a few hours) and metabolized (which can remain for weeks).

Confusion about exactly how high is too high to drive—particularly for habitual users like medical marijuana patients, who may always have significant levels of active THC in their blood—also puts law enforcement in a precarious position, in part because administering a breathalyzer to determine a driver’s blood-alcohol content is a far cry from performing the blood test needed to measure THC levels. The last time Jason Hicks, a sergeant with the Washington State Patrol, sought a search warrant required to get such a test, he had to wait 45 minutes.

Hicks emphasizes that it is not illegal to smoke marijuana and drive; it is illegal to drive once you’ve smoked enough marijuana to be impaired. Still, police have little doubt that ingesting marijuana affects driver judgement, he says. “Operating a motor vehicle is a complex, attention-dividing task,” Hicks says. “Any time you introduce a substance into your body that hinders your ability to divide that attention, well, now you’re not your hundred-percent best behind that 4,000 pound weapon.” Studies have shown that marijuana use can cause drowsiness, lethargy and an altered sense of time, as well as inhibiting people’s ability to perform divided-attention tasks.

In Washington, state troopers like Hicks are generally more interested in the straightforward signs of impairment than measurements of THC. “I could give you study after study after study that shows that at [a blood-alcohol content of] .08, there are levels of impairment with just about everybody,” he says. “I couldn’t produce those same studies when it comes to the five nanogram limit for marijuana.”

Troopers have all been trained to spot the effects of various drugs on drivers, including cannabis. Many signs of highness are the same as drunkenness: poor balance, slurred speech, watery eyes. And troopers are often tipped off by the same behaviors, like slipping across dividing lines, failing to maintain a steady speed or just sitting there when a red light turns green.

There are other red flags, like a green tongue or inability to cross one’s eyes, that are potential reactions to pot that alcohol doesn’t share. Troopers sniff the air, perform the same field sobriety tests they do for suspected drunkenness, like asking drivers to stand on one leg, and make their best assessment. “It’s the totality of the circumstances,” Hicks says.

Statistics released by the Washington State Patrol last November suggested that the number of drivers who used weed may be on the rise; while about 1,000 drivers tested positive for marijuana in each of 2011 and 2012, 745 tested positive in just the first six months of 2013. Hicks says, however, that the total number of arrests made for driving under the influence of drugs decreased during the first year pot was legal in the Evergreen State, from 1,621 in 2012 to 1,357 in 2013.

The rub is that the Washington authorities don’t break down those statistics into how many drivers tested positive for cocaine or marijuana or speed—so the number of pot-related arrests is unknown. Colorado, similarly, conflates arrests for driving while high with arrests for driving under the influence of alcohol, meaning that hard answers to the question of whether legal marijuana leads to more impaired driving are going to be hard to come by for some time.

So far, Denver lawyer Vicente says, he hasn’t seen any uptick in clients flocking to his firm because they got arrested for driving while high. The state has been aggressive in getting out the message that it’s illegal to consume weed in a vehicle or drive while stoned, and marijuana users may be aware that their behavior could affect pot’s chances of being legalized elsewhere. “People realize that Colorado is really under the microscope,” Vicente says. “And it’s our duty as a state to be responsible.”