Drug-impaired driving continues to pose a significant challenge for traffic safety professionals, law enforcement officers, and forensic toxicologists. While alcohol impairment can often be evaluated using established blood alcohol concentration thresholds, interpreting drug test results is much more complex. A recent study conducted in Harris County, Texas, examined whether drug concentrations observed in impaired drivers overlap with concentrations commonly found in fatal overdose cases.
The findings revealed that some drivers were operating vehicles with drug concentrations that rivaled or exceeded levels seen in postmortem investigations, blurring the distinction between therapeutic, recreational, toxic, and potentially lethal concentrations.
Study Purpose
Researchers compared blood drug concentrations from impaired driving (DWI/DUID) cases investigated between 2014 and 2020 with concentrations measured in postmortem toxicology cases from the same time period in Harris County. The goal was to better understand how drug levels detected in living drivers compare to those found in overdose and death investigations.
The study focused on six commonly encountered drugs:
- Fentanyl
- Oxycodone
- Hydrocodone
- Clonazepam
- Methamphetamine
- Delta-9-tetrahydrocannabinol (THC)
Key Findings
Researchers identified eight impaired driving cases involving exceptionally high drug concentrations. In several instances, the concentrations exceeded those found in the majority of postmortem cases.
Some of the most notable findings included:
- Fentanyl: 220 ng/mL
- Oxycodone: 680 ng/mL
- Hydrocodone: 310 ng/mL and 490 ng/mL
- Clonazepam: 330 ng/mL
- THC: 160 ng/mL
- Methamphetamine: 3,500 ng/mL and 7,100 ng/mL
The 7,100 ng/mL methamphetamine concentration was higher than 98% of postmortem methamphetamine cases examined by the Harris County Institute of Forensic Sciences. Similarly, the oxycodone and hydrocodone concentrations exceeded more than 90% of postmortem findings for those drugs.
Noteworthy Driver Cases
Many of the drivers with extremely high drug concentrations exhibited obvious signs of impairment, including:
- Crashes and traffic violations
- Failure to maintain lanes
- Passing out behind the wheel
- Slurred speech
- Poor balance and coordination
- Inability to complete field sobriety tests
- Confusion and altered consciousness
For example, one driver involved in a hit-and-run collision had a fentanyl concentration of 220 ng/mL. Another driver with methamphetamine levels of 7,100 ng/mL ran multiple stop signs, attempted to evade law enforcement, and required medical treatment after being stopped.
Why These Findings Matter
The study highlights a critical issue in impaired driving investigations: drug concentration alone does not determine impairment. Unlike alcohol, many drugs do not have a clear relationship between blood concentration and driving performance. Two individuals with the same drug concentration may exhibit vastly different levels of impairment due to factors such as:
- Drug tolerance
- Frequency of use
- Route of administration
- Metabolism
- Health conditions
- Interactions with other drugs
Researchers suggest that some of the drivers in the study may have developed substantial tolerance, allowing them to remain conscious despite drug concentrations that could be fatal to other individuals.
Implications for Drug-Impaired Driving Research
This study reinforces the importance of using multiple sources of evidence when evaluating drug-impaired driving cases. Toxicology results should be considered alongside:
- Driving behavior
- Officer observations
- Standardized Field Sobriety Test results
- Drug Recognition Expert (DRE) evaluations
- Medical and drug-use history
The findings also demonstrate the need for continued research on the relationship between blood drug concentrations and impairment. As drug use patterns evolve and emerging substances become more common, understanding how these drugs affect driving ability remains essential for improving roadway safety and enforcement practices.
Takeaway for Traffic Safety Professionals
The Harris County study provides a powerful reminder that extremely high drug concentrations can occur in active drivers, not just in fatal overdose cases. While toxicology results are valuable tools for investigating impaired driving, they should never be interpreted in isolation. Effective assessment of drug impairment requires a comprehensive review of toxicological evidence, observed behavior, and the circumstances surrounding the driving event.
Bottom line: Some impaired drivers may have drug concentrations comparable to or exceeding those found in overdose deaths, making drug-impaired driving investigations far more complex than simply comparing a laboratory result to a predetermined threshold.
Source:
Dempsey, S. K., Gray, T. R., Kelly, A. T., Stout, P., & Lee, D. (2022). Comparison of High Drug Concentrations in Impaired Driving and Postmortem Casework in Harris County, TX. Journal of Analytical Toxicology, 46(8), 817–824. https://doi.org/10.1093/jat/bkac032
