Association of Driving With Blood Delta-9-Tetrahydrocannabinol: A Systematic Review

IMPORTANCE: Driving under the influence of cannabis increases the risk of motor vehicle collisions. In some jurisdictions, deterrence rests on the ability to detect delta-9-tetrahydrocannabinol (THC) in blood. Recent evidence suggests that there may be a nuanced relationship of blood THC to driving. OBJECTIVE: The purpose of this systematic review was to summarize all published papers investigating the presence of a linear relationship between blood THC and driving, primarily measured by simulated driving in the lab. OUTCOMES: The main outcomes assessed included "weaving"/lateral control (eg, standard deviation of lateral position), speed, car following (following distance; coherence), reaction time, and overall driving performance. RESULTS: Of the 4845 records from the literature search, only 12 met the inclusion criteria. Ten of these reported no significant linear correlations between blood THC and measures of driving (8 out of 9 for "weaving"/lateral control, 4 out of 5 for speed, 2 of 3 for car following tasks (coherence/headway maintenance task), 1/1 for reaction time, 3/3 for overall driving performance). The studies that did find an association between driving and blood THC employed complex driving situations. CONCLUSIONS: This synthesis has important implications for road safety given driving situations can be complex due to challenging road situations and increases in potency of cannabis over the past years. Current methods of detection of impairment may be suited to some types of situations but more large-scale studies on the relationship of blood THC and driving are needed that systematically vary driving complexity and cannabis potency.