Following Distance Complicated THC Reaction-Time Findings on a Real Highway
A longer reaction time during car following can partly reflect a longer following distance. In an on-road cannabis experiment, raw reaction-time changes and headway moved together. Statistical adjustment for headway reduced the apparent THC effect and left no significant adjusted reaction-time increase. That finding changes the interpretation of the task without proving that THC had no driving effects.
The experiment followed actual moving vehicles
The normal-highway study in Robbe and O’Hanlon’s 1993 report initially enrolled sixteen cannabis-experienced adults. One was excluded after cannabinoid analyses did not show evidence of inhalation, leaving fifteen for the driving analyses. An additional participant lacked complete plasma sampling but remained in the behavioral analyses.
Participants received placebo and THC at three target levels: 100, 200 and 300 micrograms per kilogram. Active and placebo treatments were counterbalanced within each level, but the levels themselves progressed from lower to higher doses for safety. Thus dose level and stage of the study were not independently randomized.
Reaction time was not an emergency-braking test
Drivers followed a lead vehicle whose speed repeatedly decreased and increased. The study estimated their response delay from the relationship between the two vehicles’ recorded speeds. This was an average response to repeated speed maneuvers, not a direct measurement of braking latency to a sudden collision threat.
Although drivers were instructed to maintain a specified separation, their actual headway varied. Mean increases over the corresponding placebo condition were approximately eight, six and two meters across the ascending THC levels. The corresponding raw reaction-time increases were approximately 0.55, 0.41 and 0.19 seconds.
Headway changed the statistical question
Across conditions, reaction time was strongly related to mean separation distance, with a reported correlation of 0.76. At a greater distance, changes in the lead vehicle’s speed can be harder to perceive. The investigators therefore used headway as a covariate to estimate reaction-time differences at a common distance.
The adjusted increases were smaller, and none was statistically significant. This is a conditional comparison: what the measured response delay would look like after accounting statistically for headway. It is not proof that changing distance was the only cause, that every participant was unaffected, or that adjustment established equivalent performance.
A compensatory behavior may be part of the drug response
The authors considered whether participants initially left more space out of caution, becoming more confident as the study progressed. They also acknowledged another explanation involving dose-related caution. Because doses always ascended, the design could not cleanly distinguish those possibilities.
That matters when deciding whether headway should be treated simply as unwanted noise. If a drug changes the distance a driver chooses, accounting for that distance changes the effect being estimated. The raw result and adjusted result answer related but different questions.
Other driving findings remained relevant
In the separate lane-tracking portion of the same experiment, the two higher THC levels significantly increased lateral-position variability relative to placebo; the lowest did not. A null adjusted result for car-following response delay therefore cannot be generalized to every measured driving function.
The report also identified limitations of the car-following task: predictable maneuvers, imperfect headway measurement and inconsistent reliability across repeated tests. Cannabinoid measurements were in plasma, not whole blood. The small selected sample, supervised conditions and specific smoking regimen do not establish an individual driving-clearance rule.
Related: questions to ask before relying on cannabis tolerance testimony.
Primary research
Robbe HWJ, O’Hanlon JF. Marijuana and Actual Driving Performance. National Highway Traffic Safety Administration, November 1993. Report DOT HS 808 078. doi:10.21949/1525346.
