Law enforcement officials would love to have a clear way to tell when a driver is too drugged to drive. But the decades of experience the country has in setting limits for alcohol have turned out to be rather useless so far because the mind-altering compound in cannabis, THC, dissolves in fat, whereas alcohol dissolves in water. …. While scientists continue to bang their heads over how to draw up a biological measurement for marijuana intoxication, legislators want a way to quickly identify and penalize people who are too high to drive. …. The shaky science around relating blood THC to driving impairment is unfair for people living in marijuana-legal states that have absolute blood THC limits for driving, says Andrea Roth, a professor of law at the University of California, Berkeley. ….
Research on THC blood levels sheds light on difficulties of testing for impaired driving
Source: National Institute on Drug Abuse (NIDA), Press Release, Science Spotlight, January 28, 2016
Effect of Blood Collection Time on Measured Δ9-Tetrahydrocannabinol Concentrations: Implications for Driving Interpretation and Drug Policy
Source: Rebecca L. Hartman, Timothy L. Brown, Gary Milavetz, Andrew Spurgin, David A. Gorelick, Gary R. Gaffney and Marilyn A. Huestis, Clinical Chemistry, Vol. 62 no. 2, February 2016