Why Did We Bring This Story To You?

Since the November election, when Colorado voters overwhelmingly approved Amendment 64, we’ve been taking an in-depth look at how the legalization of recreational marijuana will affect Coloradans. Perhaps the most hotly contested issue has centered around marijuana and driving.

We can all agree on the need to minimize the number of intoxicated motorists on our roads. But, setting a definition for marijuana intoxication has proven difficult. Marijuana is very different from alcohol. Currently, there is no easy roadside test like a breathalyzer that can provide a police officer immediate, accurate results. The Colorado legislature has created a 5 nanogram-per-milliliter (ng/ml) limit for THC, which would be measured by a blood test if an officer suspects a driver is impaired by marijuana. The 5 ng/ml limit would be used similarly to a .08 blood alcohol level to determine the point at which marijuana is considered to have a negative effect on your control behind the wheel. Although it differs from person to person, most casual drinkers have some idea where their .08 lies (probably more than a glass of wine at dinner). However, because the 5 ng/ml standard is new, most marijuana users have no idea what that number means to them.

So that’s what we’ve set out to test — to find out what the 5 ng/ml limit means in the real world and how marijuana effects motorists. We realize our demonstration does not meet the criteria for a scientific study. But, we worked carefully to use as scientific a process as possible, given our obvious limitations in sample size and scope.

THE STORY: Too High To Drive? CBS4 Puts Stoned Drivers To The Test

Who Was Part Of Our Test?

We worked with members of the medical marijuana community (Releaf Center and Home Baked) to screen and choose three volunteer participants for our demonstration. In our screening process, we strived for age and gender diversity, and chose people who said they were in good health and claimed to be good drivers. Going into the test, each participant said they regularly drove under the influence of marijuana. Each also said they expected to drive better after smoking.

We also found it important that our participants were regular users. We may have gotten very different results if we had chosen novices, but we thought it was important for safety and legal reasons to use experienced, daily users.

The only stipulation for our participants: we asked them to refrain from using marijuana the morning of the demonstration.

What Was Our Process?

We provided transportation for our three participants to Master Drive’s training facility in Arapahoe County. On arrival, each participant was given a blood test to determine a baseline THC content. Then, each driver was given a baseline driving test. For the first lap, the participants were passengers as the driving instructor took them through the course. Then each participant was given the opportunity to try the course, presumably in a sober state.

Then, each driver individually went to an outdoor area where they were asked to smoke just enough marijuana so they could ‘feel something’. Presumably we were asking each participant to try to find his/her own 5ng/ml and stop at that point. (Each smoked different strains of mid-to-high potency sativa blends, with different ingestion methods, e.g. glass pipe, joint and water bong.)

After smoking, the participant waited 15 minutes, and then had a blood test to obtain a new THC level. At that point, the participant was asked to drive two laps, with a driving instructor equipped with an emergency brake in the passenger seat.

We repeated three cycles of smoke, 15-minute wait, blood test and drive.

Later, we added an element of control to the test. We asked a CBS4 employee to take a THC blood test and then drive the course, multiple times.

How Did The Drivers Do?

It was difficult from our vantage points to assess with the naked eye how each participant did as they completed a lap. It was clear that the driving instructor never had to use his emergency brake or grab the steering wheel. The driving instructor scored each lap, and was critical of the participants’ driving skills throughout the process. Deploying ten different cameras including an aerial drone, and three GoPro cameras attached to the interior and exterior of the car, we attempted to capture every angle of the drives, giving empirical evidence to prove or disprove the instructor’s assertions.

LINK: MasterDrive Score Sheets

How Was Lab Testing Done?

A nurse from the Visiting Nurse Association drew our blood samples on-site. The tests were sent to the University of Colorado Toxicology lab for analysis. CU Toxicology used the actual language in the Marijuana DUI bill passed by the Colorado legislature to create a specific blood test for our demonstration. The goal was to replicate the same test the State Lab would use on people pulled over under the new law. We chose not to test for any other substances other than THC.

What Did We Learn?

(credit: CU Toxicology)

(credit: CU Toxicology)

(credit: CU Toxicology)

(credit: CU Toxicology)

Each participant stated they expected to be over 5 ng/ml on their baseline test (without smoking that day, due to residual marijuana usage). Two were surprised to learn their baseline THC levels were far below the 5 ng/ml legal limit (1.4 ng/ml and 1.8 ng/ml). The third participant tested at 11.2 ng/ml, more than twice the legal limit even after refraining from smoking for 10 hours. That participant admitted to eating marijuana edibles, and using concentrated hash the night before.

After the initial useage – which was supposed to be just enough to “feel something” — all three of the subjects were two, three, and five times over the 5 ng/ml legal limit (12.1 ng/ml, 18.8 ng/ml, 26.5 ng/ml).

(credit: CU Toxicology)

(credit: CU Toxicology)

During the third round of smoking, two participants said they were ‘as high as they get’ — so they smoked only a small amount of marijuana and stopped. Their THC levels dropped dramatically in the third blood test, perhaps showing how quickly marijuana can leave a person’s system. We consistently tested THC levels 15 minutes after smoking, which may not reflect real-world traffic stops where blood THC testing would presumably take longer to administer.

Introducing a control element, a CBS4 employee also took a THC blood test and multiple driving tests. She also had problems navigating the course on her initial runs, but quickly mastered the course with practice. The other participants did not show a similar improvement that would come from learning the course after driving it numerous times.

– Written by CBS4 producer Mark Ackerman


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