When a 23-year-old Arvada man crashed his pickup into the back of a Colorado State Patrol car in January, authorities said it was an example of what could be a disturbing trend: a rise in dangerous marijuana-impaired driving.

But now, a month later, the case has become an example of a different problem: the difficulty of tracking cases of stoned driving.

While a State Patrol spokeswoman said shortly after the collision that investigators suspect driver Keith Kilbey was impaired by marijuana when the crash happened, neither his official summons nor the public accident report mentions pot. A spokeswoman for the Adams County district attorney’s office, which is prosecuting the case, said she couldn’t comment on whether officials still contend Kilbey was stoned at the time of the crash or whether a blood test was taken. A man who answered Kilbey’s telephone declined to comment.

In addition to careless driving, Kilbey has been charged with driving “under the influence of alcohol or drugs or both,” according to his summons.

“In our system, this is handled as a DUI case,” district attorney’s spokeswoman Sue Lindsay said.

And therein lies the challenge in determining whether marijuana legalization in Colorado has led to an increase in stoned driving.

There is currently no comprehensive way to track instances of marijuana-impaired driving in Colorado. Such cases are charged in court under the same law as alcohol-impaired driving cases, meaning the two can’t be separated in judicial data. Law enforcement agencies have not historically kept separate tallies on stoned-driving cases. The State Patrol began doing so this year, but it has no numbers prior to January to compare the new results to.

“Until we have a solid amount of information, we can’t make that assessment,” said patrol Trooper Josh Lewis.

In the past, officials have pointed to the number of blood-test samples in which the state’s toxicology lab found marijuana derivatives to argue that stoned driving is increasing in Colorado. But the lab closed last year amid an integrity scandal, and the only two remaining labs in the state certified to do drug testing on blood samples say the resulting changes in their workload mean year-to-year comparisons are difficult.

“Our volume changed significantly, so it’s making my data analysis more complicated,” said Sarah Urfer, the owner of the ChemaTox Laboratory in Boulder.

The question isn’t just academic. Whether loosening marijuana laws will lead to more stoned drivers is one of the major questions in how marijuana legalization will impact Colorado. The federal government has identified stoned driving as one of the measures it will look at to determine whether legalization succeeds.

“I have not seen honest statistics,” said Dr. Robert Lantz, the director of the Rocky Mountain Instrumental Laboratories, the other lab certified to do blood-drug testing in Colorado. “So I’m waiting to see honest data.”

What numbers there are paint a blurry picture — albeit one that still shows the importance of the topic.

For instance, a study published this year by two researchers at Columbia University found evidence that stoned driving has increased across the country. Analyzing federal data, the researchers found that the percentage of drivers killed in car accidents who tested positive for marijuana nearly tripled between 1999 and 2010.

“The most likely explanation is that use of marijuana in the general driver population has been increasing, which may reflect increased use in the overall population,” said Guohua Li, one of the study’s authors.

But Li said the federal data cannot show whether the drivers were impaired by marijuana at the time of the crash; a positive test may indicate use as long as several days prior, he said. And the study also didn’t look at whether the driver testing positive for marijuana was at fault in the crash.

Li’s study didn’t include data from Colorado because the state performs postmortem drug tests on too few drivers in fatal crashes. What figures there are for Colorado show a slight increase since 2006 in fatal-crash drivers positive for marijuana.

Overall, Colorado has not seen an increase in impaired-driving prosecutions.

Since July, there have been 12,567 cases filed across Colorado where driving impaired — either by alcohol or drugs, including marijuana — was the most serious charge, according to the Colorado Judicial Branch. That puts the state on track for a little over 21,500 such cases this fiscal year, which is in line with the two previous fiscal years. The tallies don’t include filings in Denver County Court, which average around 3,000 per year.

Alcohol is involved in most DUI convictions in Colorado. According to the state Department of Transportation, marijuana was involved in only about 1,000 of the 23,500 post-conviction drug-and-alcohol evaluations performed for probation purposes.

Overall traffic fatalities are also relatively flat. There were 422 fatal crashes on Colorado roads in 2013, 12 fewer than in 2012 and 15 more than in 2011, according to CDOT.

There may soon be other figures on stoned driving in Colorado — Li said a federal study based on data from roadside checks is in the works. More details on Kilbey’s case should come next month, when he is scheduled for arraignment in Adams County Court.

And Chris Halsor of the Colorado District Attorneys’ Council said state officials are also talking about new ways to capture data on marijuana and driving, in the hopes that the issue won’t remain a mystery for much longer.

“I think for all parties involved,” Halsor said, “we want to have good numbers so we can wrap our head around if this is a problem.”

John Ingold: 303-954-1068, jingold@denverpost.com or twitter.com/john_ingold