Edible THC Increased Lane Weaving Without Increasing Average Speed
Edible THC increased lane-position variability in a randomized simulator trial even though average and maximum speed did not show the same acute changes. The distinction matters because a driver can maintain speed while showing poorer lane control.
Different doses in the same adults
Forty adults were included in the crossover analysis comparing placebo with 2, 10 and 20 mg edible THC. The primary outcome was standard deviation of lateral position, a measure of weaving within a lane.
Whole-blood cannabinoids and subjective ratings were measured alongside driving outcomes. These were simulator results under study conditions, not observed crashes or an individual roadside diagnostic rule.
One driving measure could not summarize the others
Relative to placebo, lane-position variability increased by approximately 2.0 cm after 10 mg and 3.3 cm after 20 mg THC. The 2 mg contrast for this primary endpoint was not significant.
Speed variability increased at the highest dose, although mean and maximum speed did not show corresponding acute treatment effects. Reaction-time findings followed another pattern, including effects at selected doses. A null average-speed result therefore did not negate other performance changes.
Concentrations did not define a clearance threshold
Reported whole-blood THC peaks were approximately 1.6 ng/mL after 10 mg and 3.4 ng/mL after 20 mg. These low group concentrations coexisted with significant lane-control findings, but they should not be recast as universal impairment cutoffs.
The driving scenario was relatively straightforward and did not represent every road environment. Group averages cannot determine whether a particular person was impaired at an earlier event. A complete interpretation retains the dose, route, timing, endpoint and uncertainty rather than treating either normal speed or a low THC number as decisive.
Related: Interpreting cannabinoid concentrations and functional effects.
Primary research
Dose-Dependent Effects of Cannabis Edibles on Simulated Driving Performance: A Randomized Clinical Trial. JAMA Network Open (2026).
