Getting busted for driving while high on marijuana is vanishingly rare in Southwest Colorado, a review of law-enforcement data has found.

Despite the proliferation of marijuana shops, few tickets were written for marijuana intoxication on local roads in 2014.

The Colorado State Patrol issued only four citations for marijuana-only driving under the influence in Montezuma County. La Plata County had two, and Archuleta County had one. In La Plata County, two other DUIs were classified as alcohol and marijuana, and one was for marijuana and another controlled substance.

Statewide, the numbers look a bit different. The agency issued 354 marijuana-only DUIs, and 674 citations where marijuana was one factor. Overall, marijuana was implicated in 12 percent of DUIs issued by the State Patrol.

There are a number of possible explanations: Law regarding marijuana DUIs is under debate and subject to change, while case law around alcohol DUIs is well-established. It’s not clear what level of marijuana intoxication makes a driver impaired.

Marijuana intoxication typically requires a blood test to confirm the driver has five nanograms of THC – marijuana’s active ingredient – per milliliter of blood, while alcohol is more readily detectable via a roadside breath test. Marijuana intoxication also is regarded as easier to challenge in court.

That leaves officers more likely to issue an alcohol DUI citation if the driver appears both drunk and high.

“When there’s a contributing factor with alcohol, typically we’ll go with that simply because there’s a quantitative level in statute that’s clear, and there really isn’t one for marijuana,” said Capt. Brett Mattson, a Colorado State Patrol spokesman.

With marijuana, “it’s certainly harder to tell than it is with alcohol,” Mattson said.

Meanwhile, policy makers are wrestling with how to enforce marijuana DUIs. State law relies on a five-nanogram limit for THC. However, critics say heavy marijuana users can have five nanograms or more of THC in their blood for weeks at a time because the human body processes THC much more slowly than alcohol. It’s also not clear that habitual marijuana users are impaired drivers at that level.

Brian Schowalter, a Durango defense lawyer who often represents DUI defendants, said the five-nanogram limit was a political compromise. The Legislature passed the limit, and Gov. John Hickenlooper signed it into law in 2013. Schowalter also serves on the board of the National Organization for the Reform of Marijuana Laws.

“This whole five-nanogram thing, there’s no science behind that,” he said. “In my opinion, that’s an arbitrary limit.”

Schowalter said he’d rather take his chances in front of a jury with a marijuana intoxication case than an alcohol DUI. Jurors’ own experiences with alcohol and marijuana can shape their perceptions, he said.

“We know alcohol can make you reckless and make you take risks and make you aggressive,” he said. “I don’t think society has that same stereotype with marijuana.”

Durango District Attorney Todd Risberg said the five-nanogram limit creates a “permissive inference,” a legal standard whereby jurors are allowed to infer a driver may have been impaired. That’s a looser legal standard than for alcohol DUIs, whereby a driver who exceeds the 0.05 or 0.08 blood-alcohol limit is presumed to be impaired.

Risberg said the five-nanogram limit raises as many questions as it answers.

“That doesn’t mean a lot to a jury,” he said. “It doesn’t mean a whole lot to doctors who will testify at trial. And it doesn’t necessarily mean you’re impaired, which is what we’re getting at.”

A recent federal study pointed to the uncertainty of when drug use constitutes driver impairment.

“Whereas the impairment effects for various concentration levels of alcohol in the blood or breath are well-understood, there is little evidence available to link concentrations of other drugs to driver performance,” the National High Transportation Safety Administration said in a study released last month.

Some reports suggest more drivers are driving under the influence of marijuana.

In the federal study, which was based on roadside surveys of drivers nationwide, 12.6 percent of weekend nighttime drivers tested positive for THC, up by 48 percent from the same study in 2007.

“Changes in state policy on marijuana use, including medical and recreational use, may have contributed to an increase in marijuana use by drivers,” the study said. “However, the survey does not permit a state-by-state comparison.”