Methamphetamine altered visual scanning during simulated highway driving without impairing the study’s vehicle-control measures. Preserved lane or steering performance therefore did not mean that every measured driving-related process was unchanged.
Hayley and colleagues studied 20 healthy adults in a randomized, double-blind, placebo-controlled crossover experiment. Participants completed four one-hour simulated drives after oral methamphetamine at 0.42 mg/kg, low-dose alcohol, their combination or placebo. Alcohol dosing targeted 0.04% BAC; that target should not be substituted for every participant’s measured exposure.
Eye tracking measured fixation rate and duration, along with stationary and transition gaze entropy. Methamphetamine increased fixation rate and duration and produced a less dispersed but more disorganized scanning pattern during highway driving, while the measured driving performance remained preserved. Alcohol alone impaired oculomotor control and driving performance. The separate treatment conditions must not be conflated into an assertion that every combination outcome reproduced either single-drug condition.
Gaze entropy summarizes the distribution and sequence of looking behavior. It is not a drug concentration, proof that a particular hazard was missed or a validated roadside diagnosis for an individual. The study’s controlled human design strengthens causal inference about measured treatment effects, but its small selected sample, simulator, dose and observation period limit generalization. It does not establish a blood or oral-fluid threshold for unsafe driving. The important forensic question is whether the opinion concerns visual exploration or vehicle control. They can diverge. An accurate reconstruction still needs actual driving evidence, timing, specimen interpretation and alternative explanations.
Compare these findings with our account of the controlled d-methamphetamine driving study. The design and measured outcome determine which conclusions can be carried into an individual case.
Original investigation: doi:10.1177/02698811241261024.