Even the government acknowledges the difficulty of proving impairment in a DUID case

Even the government continues to acknowledge the difficulty of proving impairment in a DUID case.

Just a day ago….

NHTSA’s Understanding the Limitations of Drug Test Information, Reporting, and Testing Practices in Fatal Crashes by Amy Berning & Dereece D. Smither was released.
DOT HS 812 072

This recent publication reads as follows:

In addition, while the impairing effects of alcohol are well- understood, there is limited research and data on the crash risk of specific drugs, impairment, and how drugs affect driving- related skills. Current knowledge about the effects of drugs other than alcohol on driving performance is insufficient to make judgments about connections between drug use, driving performance, and crash risk (Compton, Vegega, & Smither, 2009).

The authors in referencing the Compton, Vegega, & Smither 2009 study are pointing to another NHTSA publication (non-peer review) “Drug-Impaired Driving: Understanding the Problem & Ways to Reduce it” by Compton, R., Vegega, M., and Smither, D. That was a report to Congress and is part of the Congressional record.

R. Compton is Dr. Richard P. Compton, MD. He is a psychiatrist and psychologist by training. He currently is the Director Office of Behavioral Safety Research at US DOT / National Highway Traffic Safety Administration.

Yes, he published the LAPD DRE study [formally known as Field Evaluation of the Los Angeles Police Department (DOT HS 807 012) in December 1986]. The LAPD DRE study has been used by many, who have not looked fully at the data and the writing, as validation of the DRE process and the ability to validly opine impairment based upon the DRE protocol. [As an aside a fair reading of the paper and its data does not support such a broad conclusion.]

Dr. Compton has also published other studies that consistently state the scientific truth that DUID impairment calls are very difficult. Including:

  • Compton, R.P. (1988). Use of controlled substances and highway safety: A report to Congress (DOT HS 807 261). Washington, DC: National Highway Traffic Safety Administration.
  • Compton, R. P., Preusser, D. G., Ulmer, R. G., & Preusser, C. W. (1997). Impact of drug evaluation and classification on impaired driving enforcement. In the Proceedings of the 14th International Conference on Alcohol, Drugs and Traffic Safety, Annecy, France.
  • Shinar, D., & Compton, R. P. (2002, August). Detecting and identifying drug impaired drivers based on observable signs and symptoms. In the Proceedings of the 16th International Conference on Alcohol, Drugs and Traffic Safety, Montreal, Canada.

There are lots of quotes in the “Drug-Impaired Driving: Understanding the Problem & Ways to Reduce it” paper that continue to point out how difficult opining impairment in DUID cases truly are. For all involved in DUID, it is a paper that is well worth the time invested in reading it. Here are some of a few quotes from it.

Since the effects of alcohol on driving performance are relatively well understood, it is useful to review and contrast what is known about alcohol with what is known and not known about other drugs.


Unfortunately, the behavioral effects of other drugs are not as well understood as the behavioral effects of alcohol. Certain generalizations can be made: high doses generally have a larger effect than small doses; well-learned tasks are less affected than novel tasks; and certain variables, such as prior exposure to a drug, can either reduce or accentuate expected effects, depending on circumstances. The ability to predict an individual’s performance at a specific dosage of drugs other than alcohol is limited.

Most psychoactive drugs are chemically complex molecules whose absorption, action, and elimination from the body are difficult to predict. Further, there are considerable differences between individuals with regard to the rates with which these processes occur. Alcohol, in comparison, is more predictable. A strong relationship between BAC level and impairment has been established, as has the correlation between BAC level and crash risk.

Factors that make similar prediction difficult for most other psychoactive drugs include:

  • The large number of different drugs that would need to be tested (extensive testing of alcohol has been undertaken over many decades; whereas relatively little similar testing has occurred for most other drugs);
  • Poor correlation between the effects on psychomotor, behavioral, and/or executive functions, and blood or plasma levels (peak psychomotor, behavioral, and executive function effects do not necessarily correspond to peak blood levels; detectable blood levels may persist beyond the impairing effects or the impairing effects may be measurable when the drug cannot be detected in the blood);
  • Sensitivity and tolerance (accentuation and diminution of the impairing effects with repeated exposure);
  • Individual differences in absorption, distribution, action, and metabolism (some individuals will show evidence of impairment at drug concentrations that are not associated with impairment in others; wide ranges of drug concentrations in different individuals have been associated with equivalent levels of impairment);
  • Accumulation (blood levels of some drugs or their metabolites may accumulate with repeated administrations if the time-course of elimination is insufficient); and
  • Acute versus chronic administration (it is not unusual to observe much larger impairment during initial administrations of drugs than is observed when the drug is administered over a long period of time).

The result of these factors is that, at the current time, specific drug concentration levels cannot be reliably equated with effects on driver performance.


Congress requested that an assessment of methodologies and technologies for measuring driver impairment resulting from use of the most common illicit drugs (including the use of such drugs in combination with alcohol) be conducted. The measurement of driver impairment is challenging since driver performance is a product of manual, cognitive, and perceptive skills and the range of performance reflected in the normal driver population is large.

Current knowledge about the effects of drugs other than alcohol is insufficient to allow the identification of dosage limits that are related to elevated crash risk.


The development of a method of measuring driver impairment due to the use of drugs would greatly enhance the ability of law enforcement to investigate suspected drug-impaired driving cases. However, there is currently no accurate and reliable way to measure the level or degree of driving impairment associated with the use of drugs.

All of this supports our blog posts from long ago in our six part series of “Pharmacology for Lawyers”:

Part 1. Introduction
Part 2. Pharmacokinetics
Part 3. Pharmacodynamics
Part 4. Bioavailabilty
Part 5. “Free versus Bound Drug”
Part 6. Elucidating Pharmacodynamic Effect from an Analytical Chemistry Result

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