With two cases pending in Boulder County alleging that drivers in vehicular fatalities were under the influence of marijuana, prosecutors are faced with proving beyond a reasonable doubt a connection between “bad driving” and THC blood levels.

Impaired driving already is Boulder County District Attorney Stan Garnett’s primary public safety concern and he said that now the legalization of medical and recreational marijuana has posed complexities in evaluating recent traffic violations.

“Would this crash have happened if marijuana weren’t a factor?” Garnett said, noting a question that often crosses his mind when such cases appear on his desk.

In Boulder County, there were 2,143 DUI cases in 2014, there were 1,887 in 2015 and there are 1,242 so far this year, and although statistics are not categorized by alcohol or drug, prosecutors estimate up to 30 percent of their impaired driving case load involves marijuana.

The drivers in two fatal crashes — the first in Boulder that killed two people at a stoplight and the second in Longmont that killed an 8-year-old girl on her bike — are being charged with vehicular homicide coupled with driving under the influence of marijuana theories.

An arrest affidavit released Friday reveals police suspect that Kyle Couch, 20, of Longmont, was driving his Ford F-250 pickup with at least 1.5 nanograms of THC in his blood, indicated by a sample drawn more than two hours after the collision.

He is expected to be charged with vehicular homicide and other crimes August 12.

The amount of THC present in the blood of Quinn Hefferan, 17, of Boulder, who is being prosecuted as a juvenile, have not yet been released. But he told police he had smoked marijuana hours before the May 7 crash.

Having five or more nanograms of THC present in blood samples at this point does not provide as clear cut an indication as the illegal 0.08 DUI per se blood alcohol limit, as the central nervous system depressant has been studied for decades, officials have said.

Defense attorney: It’s relative

The closest Colorado law comes to a marijuana limit is termed a “permissible inference” and it’s up for debate among prosecutors and defense attorneys as well as marijuana advocates.

Boulder defense attorney Jason Savela, of The Savela Law Firm, said he does not want anybody under the influence of drugs or alcohol on the roads, but he also does not want innocent people convicted.

He said he believes the current measurement scale is flawed if regular users above a certain baseline, and otherwise highly functional, are being considered impaired.

“What might intoxicate you on marijuana might not intoxicate me on marijuana,” Savela said.

But just how much does marijuana impair a person’s ability to drive?

A study commissioned by the National Highway Traffic Safety Administration and released in 2015 found there was no “significant increased risk of crash involvement” due to marijuana use when the study factored in other risk factors for the drivers involved, including age and gender.

“The findings are that if marijuana has an impact on someone’s impairment, it is a short-lived impairment or a nominal impairment, especially compared to alcohol,” said Paul Armentano, deputy director of the National Organization for the Reform of Marijuana Laws. “It was almost statistically insignificant.”

Armentano said studies also have shown that drivers who are high on marijuana tend to engage in compensatory driving behaviors such as making fewer lane changes, reducing their speed and increasing the distance between themselves and other cars. He said it plots lowest on the spectrum of impaired driving.

Savela said it depends on the tolerance level, but studies have shown marijuana smokers drive better than the aggressive and overconfident alcohol drinkers and drivers.

“They tend to drive slower and more cautiously,” Savela said. “The one place when they are worse than everybody is with an emergency situation and that’s a proverbial ‘kid running out in the road.'”

Urban myths employed?

Boulder County Deputy District Attorney Ryan Day said while the drug might not compare to alcohol, a major concern is its effect on short-term memory loss — a hazard when behind the wheel.

He said a driver might observe a stop sign or a crossing cyclist, to which they need to react, and then forget to react — factors in Boulder County’s two recent fatal crashes.

According to the Colorado Department of Transportation, the number of drivers in fatal crashes who tested positive for cannabis has spiked over the past decade, with 40 reported in 2003, climbing to 84 in 2014.

However, the highest number of fatality crashes involving a “drugged driver” was 146 in 2003, with the second highest being 139 in 2014.

Any increase in marijuana-related traffic violations including DUIs is likely because police are increasingly aware of marijuana’s presence, officials said, not always because more people are consuming marijuana and then driving.

Longmont traffic Detective Mike Stogsdill said traffic stop procedures involve questioning drivers, asking them to submit to various roadside maneuvers, such as the fabled “one leg stand” and “walk and turn.”

“We don’t look specifically for a type of drug,” Stogsdill said about traffic stops. “We do the same series of tests for each individual.”

Now with marijuana’s prevalence, he said officers are checking for lack of convergence, or when drivers’ eyes are unable to cross, which is one indication of cannabis impairment.

Savela said he believes officers are relying on urban myths of intoxication, such as the belief that marijuana users’ tongue turns green, rather than scientifically proven indications.

“If you’re stumbling and can’t walk, that’s a good one,” Savela said.

In the event that there are no obvious signs of alcohol impairment or intoxication, officers expand their inquiry to include blood samples and subsequent consultation with drug recognition experts.

Once a driver is officially arrested, toxicology becomes an option.

Day, who has prosecuted DUI cases for 14 months in Boulder County, said a misconception is that marijuana stays in the blood longer than alcohol, and that therefore its mere presence might not necessarily signal impairment.

He said contrary to common thought, toxicology tests separate the inactive THC stored in fat cells and active THC with the intoxicating effect.

“Many people choose to smoke marijuana because it’s intoxicating and it makes them feel good,” Day said. “The same component of marijuana that gives them that feeling, impairs them.”

Forensic toxicologists have even found that blood tests can establish a timeline of drug use.

DA advocates reclassification of pot

Another issue at hand, according to Armentano of NORML, is that a person’s THC level is highest immediately after using the drug.

“What we tend to find is that the greatest difference in performance from baseline tends to occur about 20 to 40 to 60 minutes after inhaling, but during that time their levels are falling,” Armentano said. “The maximum drug effects lag behind the maximum blood levels. That’s why we can’t correlate high THC levels with impairment of performance.”

Even so, Day said, there is an emphasis to test a driver’s blood as soon as possible following a traffic violation.

A fuller understanding of marijuana’s effects on a user’s ability to drive is several breakthroughs away before it’s as developed as alcohol’s affect on judgment and motor skills, experts said.

Thomas Raynes, Colorado District Attorney’s Council executive director, said the five nanogram indicator came from negotiations a few years ago in a DUI task force through the state’s Commission on Criminal and Juvenile Justice.

“Part of the confusion I think sometimes for public and juries is, ‘what’s this five nanograms thing?” Raynes said. “It’s just one more piece of evidence. We need a lot more than the five nanograms. We need to be able to show more conduct.”

There was a time when the legal alcohol limit was 0.15. He said the same evolution over time will take place with marijuana laws, he said.

Testing isn’t perfect, Raynes conceded, and won’t improve overnight.

At the federal level, Garnett said he wants to see the Drug Enforcement Administration delist marijuana as a Schedule 1 drug and reclassify it as a Schedule 2 drug; that way, researchers can receive federal money to establish more effective ways to prove marijuana’s crash risk.

Garnett said a marijuana law should eventually address what the alcohol limit already does, in which it “doesn’t matter if you can do it safely — it’s just illegal to do it.”

Amelia Arvesen: 303-684-5212, arvesena@times-call.com or twitter.com/ameliaarvesen