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“Stoned Driving”: How Do You Measure Marijuana Impairment?

There has been a lot of media attention lately to the development of breath-testing devices that claim to provide law enforcement with the ability to detect and measure the presence and amounts of marijuana in a driver suspected of so-called “stoned driving”.  The accuracy of these devices has yet to be widely-accepted in the scientific community.  See, for example, Is It Possible to Prove Driving Under the Influence of Drugs?  

Will science and profit-hungry corporations ever be able to produce a breath-analyzing device that can accurately and reliably measure the amount of marijuana in a a driver’s blood?  

Possibly, but even if such instruments could accurately and reliably measure the metabolites of marijuana in the system, the simple fact is that this would not be sufficient to determine whether it caused impairment of driving.  As a study authored by Dr. Jim Hedlund, formerly a senior official with the National Highway Traffic Safety Administration, has concluded:.

The relations between a drug’s presence in the body, its concentration, measured in blood, breath, saliva or urine, and its impairing effects are complex and not understood well. A drug may be present at low levels without any impairing effects. Some drugs or metabolites may remain in the body for days or weeks, long after any impairment has disappeared (Berning et al., 2015; GAO, 2015).

In particular, marijuana metabolites can be detected in the body for weeks after use (Berning and Smither, 2014).
On the other hand, concentrations in the body of some drugs decrease rapidly while impairing effects persist. For marijuana, THC concentrations fall to about 60% of their peak within 15 minutes after the end of smoking and to about 20% of their peak 30 minutes after the end of smoking while impairment lasts for 2 to 4 hours (Kelly-Baker, 2014; Logan, 2014).

In addition, individuals differ in how their bodies absorb and metabolize a drug. In experimental settings, wide ranges of drug concentrations produce similar levels of impairment in different individuals (Berning et al., 2015). NHTSA’s observation is generally accepted: “At the current time, specific drug concentration levels cannot be reliably equated with a specific degree of driver impairment” (Berning et al., 2015). GAO (2015) agrees: “identifying a link between impairment and drug concentrations in the body, similar to the 0.08 BAC threshold established for alcohol, is complex and, according to officials from the Society of Forensic Toxicologists, possibly infeasible.”

If you need to consult with a Los Angeles DUI attorneyOrange County DUI attorney or Riverside DUI attorney, contact The Law Offices of Taylor & Taylor at 1(844)DUI-XPRT.  With offices in Los Angeles, Orange and Riverside counties, the firm has limited its practice to DUI defense exclusively for 34 years and is consistently top-rated in surveys of Southern California attorneys and consumers.

About Lawrence Taylor

Lawrence Taylor
Lawrence Taylor is one of the most respected DUI defense attorneys in the country. With over 43 years experience in DUI defense, he has lectured to attorneys at over 200 seminars in 41 states. An original founder and former Dean of the National College for DUI Defense, Mr. Taylor's book "Drunk Driving Defense" has been the best-selling textbook on the subject for 31 years and is now in its 7th edition. He is today one of only 5 DUI attorneys in California who is Board-certified as a DUI defense specialist. A former Marine and graduate of the University of California at Berkeley (1966) and the UCLA School of Law (1969), Lawrence Eric Taylor served as deputy public defender and deputy district attorney in Los Angeles before entering private practice. He was the trial judge's legal advisor in People vs Charles Manson, was Supreme Court counsel in the Onion Field murder case and was retained by the Attorney General of Montana as an independent Special Prosecutor to conduct a one-year grand jury probe of governmental corruption. Turning to teaching, Mr. Taylor served on the faculty of Gonzaga University School of Law, where he was voted Professor of the Year, was invited to be Visiting Professor at Pepperdine University Law School, and was finally appointed Fulbright Professor of Law at Osaka University in Japan. Mr. Taylor continues to limit the practice of his 5-attorney Southern California law firm to DUI defense exclusively. With offices in Long Beach, Irvine, Beverly Hills, Pasadena, Riverside and Carlsbad, Mr. Taylor and his firm of DUI defense attorneys may be reached through their website at www.duicentral.com or by telephone at (800) 777-3349.

If you would like to contact the author, please visit: http://www.duicentral.com/


One comment

  1. Thanks Lawrence, interesting read. We’ve got random drug tests happening here in Queensland with instruments trying to detect THC in saliva. However, if it’s not conclusive and even if a blood test is done claims have been reported like that at CBS News that “frequent users of the drug can exhibit persistent levels of the drug long after use, while THC levels can decline more rapidly among occasional users.”

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