New bill says a jury can infer that a person is under t... New bill says a jury can infer that a person is under the influence of marijuana when their nanogram level is over 5 nanograms of delta-9 THC (flickr, Kesneme)
After years of debate and six stalled bills the Colorado legislature have finally sent a 'Too High To Drive' bill to the governor's desk. House Bill 1325 sets the limit at five nanograms per milliliter of Tetrahydrocannabinol -- the psychoactive compound found in marijuana -- in the blood stream. But what does that really mean?

The answer is not very clear.

The bill has been contested because it's not that easy to judge when someone has had too much marijuana. The drug does not affect drivers the way alcohol does. While five nanograms might be too much for an occasional user, a cancer patient who uses frequently may be able to operate a vehicle fine. Just as absorption rates vary from person to person, levels of THC presence varies in marijuana from strain to strain, so there would be no solid indicator for users to know when they've used too much to drive.

While drunk driving impairment tests such as the breathalyzer or roadside field sobriety tests are well understood and regularly administered, judging how much marijuana someone has smoked and how impaired their driving in the same way is problematic.

"Cannabinoids employ counterclockwise hysteresis, meaning the peak effects of the drug lag well behind peak drug blood levels" said Paul Armentano, Deputy Director of NORML, told AOL Autos. Meaning the highest concentration of THC in the bloodstream is found immediately after a user smokes, but the highest level of impairment comes 20-40 minutes after using when THC blood levels have dropped.

"This would be the equivalent of lining up three tequila shots, and after the first shot a person would have their BAC be .15 then after 3 shots it would be 0.02." Armentano said.

The National Highway Traffic Safety Association also considers blood-testing a poor judge of driving impairment, saying on their Drugs and Human Performance Fact Sheet "It is difficult to establish a relationship between a person's THC blood or plasma concentration and performance impairing effects. ... It is inadvisable to try and predict effects based on blood THC concentrations alone."

The bill now goes to the Governor's desk for ratification.

The Associated Press contributed to this report.


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