In dealing with a case of driving under the influence of alcohol, the primary evidence is the driver’s blood-alcohol level, determined by blood or breath testing. If it is .08% or higher, the driver is presumed to be under the influence to the degree that he cannot safely operate a motor vehicle. With the passage of so-called "per se" laws in all states, a second crime can also be charged — the crime of simply having a blood-alcohol level of 08% or higher.
Although the breath and blood analysis procedures may be unreliable and inaccurate, the scientific principles underlying them have been shown. Blood-alcohol concentrations can be measured, if not always accurately,and the majority of people will be "under the influence" with .08% or higher of alcohol in their blood. This, of course, varies greatly with individual metabolism, tolerance, etc.
But what about driving under the influence of drugs or marijuana? How do you measure the amounts in the blood? At what levels is a driver "under the influence"?
Answer: No one knows.
The following are excerpts from a news release issued one week ago by the Governors Highway Safety Association. Entitled "Drug Impaired Driving: A Guide for What States Can Do", the press release presents scientific conclusions from a study authored by Dr. Jim Hedlund, formerly a senior official with the National Highway Traffic Safety Administration.
New Report Urges National, State Action on Drugged Driving
Washington, DC. Sept. 30 – This report summarizes the current state of knowledge on drug impaired driving, including what little is known about the costs
and effectiveness of these actions, and identifies actions states can take to reduce drug-impaired driving…
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;
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.”
Alcohol is far simpler because it is quickly absorbed into the body and impairment is directly related to BAC.
The only generally accepted conclusion regarding drug levels and impairment is that impairment usually increases as a drug’s
In other words, law enforcement has no accurate way to prove that a suspect is impaired by drugs or marijuana. The only "evidence" of driving under the influence of drugs: the police officer’s subjective and less-than-expert opinion.
(Thanks to DUI defense attorney George Bianchi of Seattle, Washington.)