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.”
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