Why Cannabis Driving Reviews Can Find Lane-Weaving Effects but No Average-Speed Effect
Cannabis can affect lane control without producing a statistically significant change in average speed. A 2021 review of human experimental research found stronger pooled evidence for lane weaving and reaction-time impairment than for speed or following distance. Those outcomes describe different driving demands.
Lane position and speed answer different questions
Lane-position variability measures how consistently a vehicle stays in its lane. Average speed measures how fast it travels over the assessed interval. A driver can maintain a similar average speed while showing poorer lateral control or slower responses. Combining those observations into a single statement that driving was unchanged discards the positive findings. Conversely, a lane-control effect does not establish that every driving measure deteriorated.
The review found disagreement about compensatory slowing
Sevigny described a meta-analysis that detected significant effects on lane-position measures and reaction time, but not speed, speed variability or car-following headway. Other syntheses discussed in the review concluded that drivers reduced average speed after THC. The review treated compensation as plausible while acknowledging that the pooled statistical conclusions differed. It did not establish that slower travel fully offset other deficits.
A null pooled speed result has limits
A nonsignificant result is not proof that speed can never change after cannabis. Study tasks, exposure conditions and the precision of an estimate matter. The review identified dose, delivery method, time since use and tolerance as moderators. It also distinguished driving-related cognitive tests from simulator and on-road performance. A finding from one endpoint should retain its task, timing and study context.
Interpreting the evidence
For an impairment opinion, identify the actual endpoint rather than referring to an undifferentiated driving score. This review supports a pattern of selective acute impairment; it does not provide an individual driving-clearance time or a universal THC concentration threshold.
Related reading: validation of cannabis driving tests.
Research reference
Eric L. Sevigny (2021). Cannabis and Driving Ability. Current opinion in psychology. doi:10.1016/j.copsyc.2021.03.003. Evidence basis: review/commentary abstract and selected explanatory passages.
