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WEYMOUTH, Mass. — A series of driving tests involving volunteers under the influence of marijuana revealed a wide range of effects on drivers, based on their tolerance for the drug and other factors. FOX25 Investigates organized the controlled tests on a closed course, under the supervision of a doctor, law enforcement, and professional driving instructors with In Control Crash Prevention.

While some of the drivers experienced relatively minor problems after smoking increasing amounts of marijuana, at least one driver was involved in the equivalent of a potentially deadly crash, according to professional observers.

The Test

Before smoking marijuana, volunteers took several practice runs on the course in Weymouth which included a five cone slalom, a narrow turn around lane, a four-way stop, a high speed straightaway with an emergency stop and lane change, as well as a simulated garage in which drivers had park while going in reverse.

The Volunteers

FOX25 Investigates selected four volunteers who smoke marijuana recreationally and for medicinal purposes. The volunteers submitted baseline urine samples as well as additional samples after smoking and driving.

* QuentinA 36 year-old male who described himself as a moderate user of marijuana. Lab results indicated he had 94 nanograms of THC (the active ingredient in marijuana) per milliliter (94ng/mL) in his system before starting the driving test. LabUSA, Inc, which tested the samples, considers any levels above 50ng/mL a “positive” test. However, instructors noted no signs of impairment during Quentin’s first runs on the driving course.

“I don’t feel like (marijuana) impairs me at all,” he told FOX25’s Eric Rasmussen before starting the test.

* Carol: A 56 year-old female who said she smokes small amounts of medical marijuana, daily, to combat the symptoms of an auto-immune disorder.

“It’s been the only thing that has helped me… I’ve had four different kinds of chemo, I’ve been on every immunosuppressant they make and nothing even touched it and this keeps me from flaring up really bad,” she said before the test.

Lab results indicated she had 94ng/mL of THC in her system before starting the driving test. In Control driving instructor, Kevin Stromski, noted that Carol had only minimal trouble with the course prior to smoking any marijuana that day. “By the third run, she was pretty smooth,” said Stromski. “Slower than the other three, but she got it.”

* Corinne: A 24 year-old female who described herself as a daily recreational user. Lab results indicated she had 100ng/mL of THC in her system before starting the driving test. Corinne told FOX25 she did not consider herself a “good driver,” but the instructor noted no obvious signs of impairment during her initial runs through the course.

* Don: A 65 year-old male who said he’s a moderate user of medicinal marijuana for pain. Lab results indicated he had 104ng/mL of THC in his system before starting the driving test. However, the driving instructor said Don excelled in the first runs of the driving test.

“That’s why I’m here,” said Don. “I wanted to see how it affects my ability to drive.”

Round One

Each volunteer smoked three-quarters of a gram of the same marijuana, “Critical Cheese,” donated by a local grower. Testing of the marijuana indicated a THC content of 22.74 percent.

Quentin navigated the course without any significant issues, although the instructor noted he was going “a little slower.” Don had similar results, but he knocked down one cone in the slalom.

Carol did not hit any cones, but she did back into the rear barrier in the parking test and the instructor observed other changes. “She seemed a little bit more confused,” said Stromski. “Not terrible, but definitely more affected.”

Corinne also avoided hitting any cones on her first run, but failed to stop after the high speed emergency lane change. “It tells us she wasn’t thinking clearly and any time you aren’t thinking clearly behind the wheel, that has to be unsafe,” said Stromski.

SCROLL DOWN to watch all four drivers take the test.

Round Two

All four volunteers began to show some signs of impairment after smoking an additional three-quarters of a gram, raising their intake of THC to more than 300mg, but the effects of the marijuana varied greatly.

While Don and Quentin appeared to have only minor issues on the course, the same could not be said for Carol.

At the end of the high speed straightaway and emergency lane change, Carol crashed through several cones. “I thought I was fine,” she said. “I’ve never done that. That’s crazy.”

Middleboro Police Sergeant and Drug Recognition Expert (DRE) Deborah Batista evaluated each of the volunteers and observed them navigate the course. “The way she took the slalom, where she was making such the wide turns, she doesn’t have good control of the car,” said Batista. “So, at a higher speed, it’s just going to come a little bit quicker.”

Corinne also hit a series of smaller cones on the side of the emergency stop and lane change. “I feel pretty ‘loosey goosey.’ I feel pretty silly,” she said after the second round of smoking and driving.

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Round Three

By the time volunteers had smoked marijuana for a third time over about two hours, all of them showed more signs of impairment, but the impact on their driving skills varied from person to person.

Quentin and Corinne sped up through some parts of the driving course while Carol and Don slowed down significantly.

“If you take Corinne versus Carol — two polar opposites of the effects of marijuana,” said Stromski.

“I would never drive this high and I don’t get this high,” said Don, who forgot to put on his seat belt before beginning round three.

Despite hitting a cone, Quentin was less willing to acknowledge his driving skills might have been affected. “I don’t feel dizzy,” he said. “I don’t feel like my judgment is hindered in any certain type of a way.”

“He was easily the most confident driver going into it and clearly still is the most confident, but I think it might almost be false confidence,” said Stromski.

Evaluating the Results

Dr. Jordan Tishler of Inhale MD in Cambridge, who prescribes medical marijuana, also observed the driving test.

“What we saw today was that cannabis does affect your driving. That it is dose-dependent, meaning the more they used, the less competent they were,” said Tishler.

Tishler said each volunteer’s level of marijuana tolerance coming into the test affected their ability to navigate the course.

“I think it’s also important, in light of the new recreational laws, that we see that there are potential issues here. Also, that the sky isn’t falling and not everybody crashed and burned,” said Tishler. “We’ve got to have an intelligent, science-based conversation about how we have law enforcement interact with people going forward.”

Sgt. Batista also evaluated each volunteer with an extensive series of tests used by Drug Recognition Experts. At the end of the three rounds of smoking and driving, Batista said all four volunteers showed signs of impairment, but pointed out that conclusion is reached by evaluating a wide array of factors and not a single test.

“What we find is with younger people, they feel that smoking marijuana will enhance their driving abilities,” said Batista. “And, as we saw today, that is not necessarily the truth.”

Both Batista and Tishler agreed that establishing a “legal limit” for drivers in Massachusetts will be difficult. Lab results showed each volunteer’s THC levels fluctuated only slightly, but Tishler says that highlights the challenges of relying on blood or urine samples to determine whether a driver is intoxicated at the moment they’re pulled over or arrested.

“Even with blood levels, testing for cannabis metabolites… they’re just not the appropriate test,” said Tishler. “(The volunteers) got good and stoned, but none had levels change in a significant way.”