The number of drivers caught behind the wheel after using marijuana this year in Colorado is on pace to eclipse last year’s total, adding fuel to cannabis critics’ fears that the state is facing a growing problem of stoned driving.

But it is deeply in doubt whether the legislature next year will reconsider one proposal addressing the issue: creating a measurement by which drivers would be presumed too stoned to drive, which would make it easier for prosecutors to punish those who take the wheel while high.

A state Department of Public Safety working group that is studying the issue deadlocked over endorsing such an approach. One of the proposal’s legislative sponsors from last session said he doesn’t see any way it would pass after being rejected in the state Senate earlier this year.

“I think we need a presumptive limit in the state of Colorado, absolutely,” said state Rep. Mark Waller, R-Colorado Springs. “But I don’t think our current political climate is going to allow us to get there.”

The ultimate decision of whether to re-introduce a bill creating a “per se” limit of the amount of active THC drivers could have in their systems is still a ways off. THC is marijuana’s psychoactive chemical. On Wednesday, a study group will present its research to a subcommittee of the state’s Commission on Criminal and Juvenile Justice. The subcommittee will decide whether to recommend creating a limit to the full commission, which would then decide whether to ask lawmakers to carry a bill.

Significantly, the study group split sharply over whether a limit is appropriate.

“There was considerable doubt whether there is enough science to set a per se limit for all people,” said Sean McAllister, a medical-marijuana attorney who is part of the study group and who doesn’t support a limit at this time.

Figures on motorists suspected of driving stoned hint at the debate. Last year, the state health department lab screened nearly 2,600 blood samples for THC, with about 1,600 of those coming back positive. Of the positive samples, about 500 had levels higher than 5 nanograms of THC per milliliter of blood — the amount that lawmakers earlier this year proposed as the per se limit.

So far this year, the state has tested more than 2,200 samples, with about 1,000 positives and another 360 presumed positive but awaiting the required second confirmation. About 250 have registered above 5 ng/ml.

Meanwhile, medical-marijuana advocates like McAllister say some research suggests certain drivers who test above 5 ng/ml can still qualify as sober. And McAllister questioned the need for a per se limit — which eases the burden on prosecutors to prove impairment. He said prosecutors already have roughly a 90 percent success rate winning convictions when stoned-driving cases go to trial.

“The science doesn’t seem finished yet on the issue of DUI and marijuana,” McAllister said.

Other statistics cloud the issue. In 2010, 32 drivers who tested positive for marijuana were involved in fatal accidents, according to the Colorado Department of Transportation — though it is unknown whether those drivers were at fault in the accident or were stoned enough to be impaired. In 2009, when fewer drivers total tested positive for pot, 37 drivers in fatal accidents were THC-positive. In 2008, the number was 31.

Arapahoe County Sheriff Grayson Robinson, who is also part of the study group, said he believes marijuana-impaired drivers are a serious danger and that a limit is needed. But he said the details of such a limit are difficult to agree upon.

“The difficulties we’re having,” he said, “are the nuances.”

John Ingold: 303-954-1068 or jingold@denverpost.com