Alcohol-related driving impairment can appear in one measure while another remains unchanged. In a randomized crossover experiment involving 24 drivers, total recorded driving errors distinguished placebo, nominal 0.05% and nominal 0.08% alcohol conditions. Individual measures—including lane weaving—did not consistently distinguish the two alcohol conditions. The finding supports examining a performance profile rather than treating one normal result as proof of unimpaired driving.
The 2015 study by Kenntner-Mabiala and colleagues was designed to calibrate a simulator for future research on medicinal drugs. Its particular contribution is methodological: the road scenario and the chosen outcome influenced whether an alcohol effect was detected.
What the 24 drivers actually did
Thirteen men and eleven women completed three sessions in a randomized, double-blind, placebo-controlled crossover design. Their mean age was 30 years, with a range of 23–53. Participants were healthy light-to-moderate drinkers who had completed extensive simulator familiarization. None dropped out.
The oral beverage contained 37.5% vodka mixed with a caffeine-free soft drink, with the alcohol quantity individualized using estimated body water and a meal correction. The 400 mL beverage was divided into four 100 mL portions, each consumed over five minutes. A ten-minute familiarization drive occurred between the second and third portions. This was an individualized dosing protocol, not one fixed grams-per-kilogram dose for every participant.
Testing began 45 minutes after drinking started. The approximately 80-kilometer, one-hour course included rural, highway and urban scenarios, plus a monotonous simulated nighttime section. Parallel course versions reduced repetition effects. Trained investigators observed driving from another room and registered errors while blinded to treatment.
The reported “BAC” was monitored by breath testing
Alcohol was measured six times with a Draeger breathalyzer. Although the paper labels its conditions and results as BAC percentages, it does not describe laboratory measurement of collected blood ethanol. These values should therefore be understood as the study’s breath-derived BAC reporting, not independently measured venous whole-blood concentrations.
In the nominal 0.05% condition, the reported mean fell from 0.058% at the start of driving to 0.044% at the end. In the nominal 0.08% condition, the corresponding means were 0.079% and 0.077%. The conditions were not perfectly constant concentration clamps. Neither the nominal targets nor these group means establish what every driver’s concentration was throughout the course.
Total errors separated conditions that individual tests did not
Mean total errors increased from 50.2 under placebo to 71.6 in the lower alcohol condition and 86.2 in the higher condition; standard deviations were 21.4, 37.5 and 50.3, respectively. All three pairwise comparisons were significant. The large standard deviations show substantial variation around the group averages.
The categories included lateral-control errors, longitudinal-control errors, cognitively based tactical errors, and critical situations or collisions. The aggregate count captured different kinds of deterioration: one driver might become inappropriately slow while another might speed or tailgate. A summary based only on average speed could conceal those different mistakes.
Lane weaving depended on the road
Standard deviation of lateral position, or SDLP, quantifies variation in lane position. In the monotonous vigilance section, mean SDLP increased from 0.20 meters under placebo to 0.23 and 0.25 meters in the alcohol conditions. Alcohol-placebo differences were significant, but the two alcohol conditions were not significantly different from each other.
On the difficult winding road, SDLP did not show a significant overall alcohol effect (p = 0.220). Lane departures on that same road did: mean departures increased from 2.9 under placebo to 6.4 and 6.7. Conversely, the simple route produced no lane departures in any condition. The absence of departures there did not contradict the alcohol-related changes detected elsewhere.
That contrast is central to interpreting a performance test. An endpoint can be insensitive because the task produces too few errors, because performance is already constrained by difficulty, or because a different measure better captures the relevant failure. The authors discussed possible ceiling effects; the experiment did not establish one universal explanation for every null result.
The null findings constrain the conclusion
Accepted gaps at intersections and when passing obstacles did not significantly change. Neither did the percentage of time spent in the highway’s left lane. Critical situations and collisions also did not increase significantly (p = 0.958), despite an increase in total errors. A collision-rate claim cannot be substituted for the measured error-count result.
Investigators’ overall fitness ratings significantly differed between placebo and the higher alcohol condition, while the lower-condition comparison was only a trend (p = 0.061). Participants rated their own performance worse after alcohol, but their ratings did not distinguish the two alcohol conditions. These results do not support the claim that every drinker is unaware of any deterioration.
What an expert can infer from this calibration
A comparison with alcohol can help interpret a drug study only when the task, measurement and experimental conditions justify that comparison. This experiment did not validate a formula converting any medication’s effect—or a particular driver’s behavior—into an equivalent blood alcohol concentration. Numerical results do not transfer automatically among simulators or from a simulator to a real collision.
The sample was small, selected for good simulator tolerance and extensively trained. Annual driving mileage varied widely. Caffeine and nicotine were not restricted, and participants were not screened for psychoactive drugs. Randomization and within-person comparisons reduce some confounding but do not establish that every possible influence was absent.
The defensible application is to identify which functional ability was tested, which measures changed, which did not, and how well the setting resembles the disputed event. It complements our analysis of alcohol-sensitive MedDrive scores and road validation: sensitivity to an experimental exposure is different from validated classification of an individual driver.
Primary scientific source
Kenntner-Mabiala R, Kaussner Y, Jagiellowicz-Kaufmann M, Hoffmann S, Krüger HP. Driving Performance Under Alcohol in Simulated Representative Driving Tasks: An Alcohol Calibration Study for Impairments Related to Medicinal Drugs. Journal of Clinical Psychopharmacology. 2015;35:134–142. doi:10.1097/JCP.0000000000000285.
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