DENVER (CBS4) – It’s been one of the toughest questions facing the state of Colorado — now that marijuana has been legalized, how high is too high to drive? In an effort to find out, CBS4 conducted a test with the help of a leading driving instruction company, a respected lab and marijuana businesses to provide the volunteers.

Right now a bill sits on Gov. John Hickenlooper’s desk that would designate 5 nanograms-per-milliliter of blood (ng/ml) of THC as the legal limit. This rare demonstration provides a glimpse of what it is like to drive while high from marijuana.

With help from local dispensaries, Releaf Center and Home Baked, CBS4 chose three daily marijuana users for the test: 50-year-old Ty, who describes herself as a medium to heavy smoker; 28-year-old Chris, who smokes a moderate amount every day for pain relief; and 25-year-old Lauren, who describes herself as a heavy marijuana user.

All three drivers were asked to refrain from smoking before the demonstration. When they arrived, a nurse from the Visiting Nurse Association drew blood from each driver to get a baseline THC content. According to the University of Colorado Toxicology Lab that tested the samples, Ty and Chris were both well under the 5 ng/ml limit. Lauren’s blood THC content was 11.1, more than twice the legal limit.

CBS4 asked the driver training program, MasterDrive to conduct the test on a closed course set up with cones, quick lane changes and stop signs. The course was designed as nothing more difficult than a 15-year-old driving school student would face. MasterDrive Vice President of Training Mark Stolberg accompanied each driver with a separate emergency brake on the passenger side of the car’s front seat.

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They started with practice runs, then it was time for the volunteer drivers to smoke marijuana. For the first time, just enough that they could “feel it.” With the help of the University of Colorado Toxicology Lab, their blood levels were tested again before asking them to drive the course.

After smoking just a little bit, Ty was just over twice the legal limit, 12.1 ng/ml. She says she felt under control, but had trouble maintaining the speed limit and hit several cones.

Ty says she felt more relaxed and not so uptight driving the course after smoking marijuana. But driving instructor Mark Stolberg said, “She did worse than the first time. She’s missing entire elements of the course.”

(credit: CBS)

(credit: CBS)

CBS4 cameras caught Ty repeatedly driving on the wrong side of the cones, before and after smoking. On the later runs, the instructor said she improved on the front of the course, but her speed was inconsistent and she repeated mistakes.

After Ty’s third drive, the instructor told her how she did.

“For the most part, I would say you are driving as if you are impaired,” he said.

Chris’ first smoke put him at 26.5 ng/ml, more than five times the limit, but he insisted he could drive fine.

(credit: CBS)

(credit: CBS)

Chris said he thought the new 5 ng/ml law is ridiculous.

“Only because I can be completely sober, get pulled over and have 5 nanograms in me,” Chris said.

After his first smoke, his performance is described as more conservative with more jerky steering, but he only hit a small number of cones. His speed increased as his THC levels went up throughout the tests and he struck a few more cones.

He called the course “a piece of cake.” But the driving instructor saw it differently.

“He thinks he is doing just fine, I think he’s driving like he’s impaired,” Stolberg said.

Lauren used a water pipe to ingest her marijuana. After her first smoke she was at more than three times the limit, 18.8 ng/ml.

(credit: CBS)

(credit: CBS)

Lauren didn’t think marijuana would alter her driving.

“I think I have a long way before it would be affected, I don’t think I could even smoke that much,” she said.

But early on Lauren had steering problems. Like the others, Lauren redid the course three times after smoking each time. Stolberg noted she had problems with speed and going beyond stop signs.

“I may have sped a bit,” Lauren said, “but I don’t think I knocked any cones over, or animals, or people, or anyone.”

At the end of the day no one ran off the road and the instructor did not have to grab the wheel or hit his emergency brake. Each participant had an opinion.

“To be honest, I don’t think I smoked enough here today to impair me from driving a vehicle,” Lauren said.

Chris added, he “nailed it.”

Ty says CBS4’s demonstration shows, “we can still do this, it doesn’t matter on THC levels.”

But the driving instructor saw things differently.

“Marijuana definitely inhibits your ability to drive and impairment is obvious,” Stolberg said.

Walking the line between sober and high may be difficult, but knowing the difference may be critical.

As a control, CBS4’s MasterDrive expert had a CBS4 employee do the test without any marijuana. While slow at first, the results showed that person improved with each run, while those who smoked marijuana did not see the same improvement.

– By CBS4 Investigator Rick Sallinger and producer Mark Ackerman