As more states consider rescinding bans on recreational marijuana use, lawmakers, police officers and transportation officials are urging each other to prepare for an increase in drugged drivers. Exactly how should they brace for a siege of stoned drivers? No one is quite sure.

Alcohol may be a well-defined enemy on American roads, but when it comes impairment caused by other drugs, there's no clear scope of the problem, no scientific consensus on what levels affect coordination and no one-size-fits-all approach for fixing inconsistent laws. "We haven't figured out how to cope with it," said Jim Hedlund, former chief of the mathematical analysis division at the National Highway Traffic Safety Administration and author of a report released Wednesday that tries to make sense of an ever-shifting landscape of drug types, traffic laws and attitudes toward drug use.

The report, prepared for the Governors Highway Safety Association, found drugged drivers may pose as great a threat as drunk ones. Among key findings in the report: drugs were present in 39.9 percent of fatally injured drivers with a known test result in 2013, the latest year for which data is available. That's an increase from 27.8 percent nearly a decade earlier, and close to the 40 percent of deceased drivers that had alcohol present in their bodies, according to Fatality Analysis Reporting System data.

"We cannot and should not try to compare marijuana and alcohol. They are two distinct drugs." - Joanne Thomka

Approximately 11,000 people are killed in alcohol-involved crashes every year on US roadways, so the prospect that drugs – illegal, prescription, or over-the-counter – may contribute to highway fatalities in similar levels is a revelation. Granted, some of those drivers may have partaken in both alcohol and drugs. But safety officials say they don't have much data on that overlap, nor how it translates from fatalities specifically toward a broader trend of drugged driving.

"In Colorado, it's sometimes difficult to get a true assessment, because alcohol is overwhelmingly the most-used drug," Glenn Davis, highway safety manager for the Colorado Office of Transportation Safety, said. "Law enforcement is highly trained for alcohol, so if someone is using both, that's what law enforcement will check for."

Even as Davis emphasizes that drugged driving encompasses much more than marijuana – FARS forms list 430 drugs that could impair drivers, 80 of which were used by fatally injured drivers in 2013 – he acknowledges that marijuana is one that many agencies across the country are focusing on right now because so many states are either considering decriminalizing recreational marijuana use or voters are petitioning to place the issue on ballots. This November, Ohio residents will vote on a referendum that, if enacted, would permit the sale of marijuana for recreational and medicinal purposes.

Welcome to Colorado (with marijuana leaf symbol) roadside sign

Colorado is already there. Voters passed a constitutional amendment that permitted use of recreational marijuana in 2012, and Davis has been at the forefront of developing a response for the traffic-safety ramifications. Other states call him for advice as they seek to confront drugged driving, but that can be difficult to dispense, as laws vary considerably state to state.

That legal landscape is convoluted. Fifteen states have zero-tolerance laws in effect for driving with any measurable amount of specified drugs in the body, while six states have per se laws in effect for one or more drugs, according to the GHSA. Per se laws set specified amounts of drugs in the body that cannot be exceeded. Some of the state laws conflict with federal law. (Click here for state-by-state list of laws.)

When it comes to marijuana in particular, 18 states have zero-tolerance laws, three have zero tolerance for THC, the active compound responsible for marijuana's psychological effects, but no restriction on metabolites, a compound that presents no psychological effects but one that can linger in the body – what shows up on traditional workplace screenings. Five states have specific per se limits and one – Colorado – has a "reasonable inference" law for THC.

"It's something we'll always look at. We're in the infancy of it." - Glenn Davis

For states setting per se limits, it's tempting to establish a line such as the .08 blood alcohol content limit that constitutes driving under the influence in all 50 states. But there's no scientific agreement on an equivalent number for marijuana. Six states set it at 5 nanograms, but in some cases, experts feel it's an arbitrary threshold. "Marijuana, we don't know what that level should be," said Joanne Thomka, director of the National Traffic Law Center. "We cannot and should not try to compare marijuana and alcohol. They are two distinct drugs."

The National Highway Traffic Safety Association conducted a roadside survey of drivers during weekday days and weekend nights in 2013, and found 22 percent of the drivers tested positive for some sort of drug or medication. Illegal drugs, including marijuana were more prevalent on weekend nights, with 15.2 percent of drivers testing positive for its presence. Compared with previous NHTSA roadside surveys, safety experts see the use of marijuana and other drugs rising on the roads, while the number of drivers drinking has fallen.

That suggests that motorists don't yet consider drugged driving to be as serious an offense as drunk driving – a finding bolstered by a Colorado survey, in which 28 percent of marijuana users told Davis' office they actually drive better while stoned. But they aren't the only ones. The GHSA report found many prosecutors and judges are not familiar with Driving Under the Influence of Drug cases and that they want an easy-to-understand equivalent of the .08 BAC. In some cases, they may not accept testimony from drug-recognition experts on impairment.

"Some judges and juries expect to find that same relationship with alcohol," Hedlund said. "What also often happens is if an officer finds alcohol is .08, then they don't look any further, because they know they have a case that's easy to prosecute and convict." Drugged-driving, he said, "are definitely harder work for law enforcement, judges and prosecutors."

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They're not the only ones who may need more training. As Davis has taken his education message to law-enforcement and judicial experts in Colorado, he's also reached out to representatives from the marijuana industry, in some cases the same people who fought to pass the amendment. They may seem unlikely allies, but Davis says such cooperation is essential for any states who are seeking to copy Colorado.

"We've made some great partnerships with the marijuana industry," he said. "Point of use and point of sale are where we do outreach, and they help get us in there. I think there are some things we actually agree on. We don't want people driving high, and they don't want people driving high. So it's an interesting partnership."

In addressing drugged driving overall, he's hoping to get more data and assess not only how many people are crashing in drugged-driving incidents, but he wants to see how many are arrested for driving under the influence of drugs and what sort of dent an influx of newly trained drug-recognition experts can make on those numbers. "But it's something we'll always look at," he said. "We're in the infancy of it."

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