Author: Cinthya Fillips, Director, Center for Alcohol and Drug Education Studies, Texas A&M Transportation Institute
Drug-impaired driving is escalating on our roadways. To combat this, the U.S. Department of Transportation’s National Highway Traffic Safety Administration released the “Drugs and Human Performance Fact Sheets: 2024” in December 2024. This update zeroes in on how psychoactive substances, especially drug combinations, impair driving. It’s a critical resource for drug recognition experts (DREs), law enforcement, attorneys, judges, and toxicologists, blending lab studies, simulations, and case reports.
What’s Inside?
Couper and colleagues detail the effects of major drug categories—ethanol, stimulants (e.g., cocaine, methamphetamine), benzodiazepines (e.g., alprazolam), cannabinoids (e.g., THC), opioids (e.g., fentanyl), and more—based on the latest lab studies, driving simulations, and case reports. It covers pharmacokinetics, impairment signs, and Drug Evaluation and Classification (DEC) profiles, plus drug combinations like ethanol with cannabis or opioids with depressants.
Key Effects on Driving
- Ethanol: Sedation and slowed reflexes kick in at BACs like 0.08 g/dL, worsening with other drugs.
- Stimulants: Boost alertness but risk agitation and crashes post-peak (e.g., methamphetamine at 178 ng/mL).
- Benzodiazepines: Cause sedation and ataxia, impairing coordination (e.g., alprazolam at 42 ng/mL).
- Cannabinoids: THC (8.2-13.1 ng/mL) increases lane weaving, akin to BAC 0.05-0.08 g/210L.
- Opioids: Sedation and slow reactions (e.g., fentanyl at 208 ng/mL) pose risks, though tolerance varies.
- Z-Drugs (e.g., Zolpidem): Linked to amnesia and sleep-driving at 80-1,400 ng/mL.
- Spotlight on Drug Combinations
The report dives deep into polydrug effects, a growing challenge in impaired driving cases. Here’s what it reveals:
- Ethanol and Cannabis: A common duo in arrests, this mix additively ramps up cognitive and motor deficits. Ethanol boosts THC absorption, hitting 47.9 ng/mL with alcohol versus 38.2 ng/mL alone, delaying peak alcohol levels and increasing crash risk.
- Opioid and Stimulant (e.g., Heroin and Cocaine): Known as a “speedball,” this combo heightens euphoria while cutting sedation. Cocaine’s cardiovascular spike meets morphine’s opioid high, with mixed effects persisting longer than solo.
- Opioid and Depressant (e.g., Opioids and Benzodiazepines): Synergy here is deadly—sedation and respiratory depression soar. Adding diazepam (40 mg) to buprenorphine (11.1 mg) boosts sedation and reaction time deficits, quadrupling overdose risk.
- Stimulant and Depressant (e.g., Methamphetamine and Alprazolam): Benzodiazepines temper stimulant agitation, but real-world driving impairment jumps—84% of combined cases showed impairment versus 64% for amphetamines alone.
These findings highlight variability—dose, tolerance, and polydrug use complicate detection. The report ties blood/oral fluid levels to observable signs, enhancing DRE assessments.
Next Steps
Professionals should leverage this data to refine training, update testing protocols (e.g., oral fluid use), and strengthen legal arguments. It’s a call to adapt to emerging drugs like synthetic opioids and evolving use patterns. Pairing this with real-time case data could sharpen enforcement and prosecution strategies.
Get the Document
Download the full report from the National Transportation Library’s Repository & Open Science Access Portal at Drugs and Human Performance Fact Sheets: 2024. It’s packed with references to dig deeper.
Why Read It?
These fact sheets bridge science and practice. Dive in to stay ahead in the fight against impaired driving.
