Differences in secondary-task engagement and vehicle control among occasional and daily cannabis users

Many cannabis users self‑report no impact on driving, despite research showing degraded lane keeping-particularly among occasional users. Drivers also commonly engage in tasks while driving that take the drivers' eyes and attention off the roadway. The safety impact of driving after cannabis use while engaging in secondary tasks is not well understood. We examined differences in secondary‑task engagement after cannabis use and the associated impacts on driving performance.

Driving performance on a simulator was collected from occasional flower, daily concentrate and daily flower users of cannabis across three time-points (baseline, 30-minutes post-dose, and 90-minutes post-dose), as well as from non-users at the same relative time-points. Participants completed text‑reading; media‑selection; heating, ventilation, and air conditioning (HVAC) control adjustments; and phone tasks of varying difficulty using a touchscreen infotainment system. For a subset of the participants, visual-engagement data was collected using a driver monitoring system.

Task‑engagement durations did not differ meaningfully between groups. Non‑users initiated tasks more quickly as the session progressed, whereas occasional‑flower users delayed initiation after cannabis use. For vehicle control during the task, occasional flower users showed the largest speed reduction across all tasks (∼1 mph). After cannabis use, all groups reduced speed more on the easy tasks; however, only the occasional‑flower group showed an additional reduction on the difficult tasks. Occasional‑flower users showed less post‑task recovery of speed relative to non‑users and daily‑flower users. After use, occasional-flower users had fewer fixations and reduced horizontal scanning on difficult tasks; other groups showed stable fixation counts and increased scanning with difficulty.

Systematic differences between non‑users and cannabis users were evident even in the absence of acute use. Greater speed reductions among occasional users suggest compensatory strategies, whereas daily users engaging in distraction while response capacity is degraded may face elevated risk.