DEKRA simulator study: cannabis users drive faster, not more cautiously
Drivers who used cannabis felt less confident in their own driving ability but did not slow down — average speeds actually rose in a DEKRA simulator experiment.
- 41
- 80 km/h
- 1 in 10
- 17%
What Happened
DEKRA conducted the driving simulator experiment together with the Chair of Engineering Psychology at TU Dresden and the Institute for Work and Health (IAG) of the German Social Accident Insurance (DGUV). The 41 participants all held driver's licenses and had practical driving experience, with an average age of just over 24 years. They were interviewed several times and provided blood samples once before consumption and four times afterwards. Each drove a 20-minute simulator route on three separate occasions: first while sober, then half an hour after consuming cannabis, and finally two hours after consumption.
- All participants felt safe to drive
- 17% felt safe to drive
- 49% felt safe to drive
- 30 minutes after consumption, 63% of participants felt slightly impaired; two hours after, that figure was 49%.
- No one reported a more severe level of impairment than slight.
- 39% deviated significantly from the ideal lane 30 minutes after use, and 32% did so after two hours.
- Some participants kept to their lane better after cannabis use than when driving sober.
1 in 10
The simulator route was an 800-metre narrow stretch of country road with no centre marking and a speed limit of 80 km/h.
Self-assessment is often difficult because the amount of THC in something like a joint can vary greatly, so users frequently have no way of knowing how much THC they are consuming. Among participants who use cannabis more frequently, the actual blood THC level had less influence on their self-assessment than among those who use it less frequently. Overall, blood THC levels among all participants were relatively low, which Dr. Thomas Wagner links to their self-reported mild impairment, their reduced assessment of driving safety and their less optimistic confidence in their own driving abilities. The study noted that, unlike other studies, no reduction in average speed followed cannabis use.
“Given the speed limit of 80 km/h, this suggests a tendency toward a more careless approach to traffic regulations.”
“The discrepancy shown in our experiment between the self-reported impairment on the one hand and driving behavior on the other – which does not become more defensive as a result – is a further argument for not compromising on safety when setting THC limits for drivers.”
Previously from DEKRA
DEKRA regularly publishes road-safety research beyond this simulator experiment. Its Road Safety Report 2026 warned that even a quick glance at a cell phone can cause a severe crash, and that automated driving features will not fully solve the problem. Earlier in September, a joint DEKRA study of 75 real truck accidents found that driver assistance systems could have prevented or softened many crashes with pedestrians, cyclists and motorcyclists.
- DEKRA report: Even a quick glance at your cell phone can cause a severe crash
- DEKRA study: truck assistance systems could prevent or soften many crashes with pedestrians and cyclists
Background drawn from MotorClaw's earlier coverage of DEKRA's official releases.
Why this matters
The experiment found no automatic caution after cannabis use: speeds increased and about one in ten drivers at times significantly exceeded the 80 km/h limit, even though most reported only mild impairment. That mismatch between how impaired drivers feel and how they actually drive concerns everyone sharing the road. The researchers say it is a further argument for not compromising on safety when setting THC limits for drivers.
Terms in This Story
- THC
- The main psychoactive compound in cannabis; its concentration in the blood is commonly used as a measure of intoxication.
- driving simulator
- A research setup in which a person operates vehicle controls in front of a screen-based road scene, so driving behaviour can be measured without real traffic.
- psychoactive substance
- A substance that acts on the nervous system and can affect mood, perception and the performance abilities needed for driving.
Summarised from the linked release; details can be imperfect — always verify against the original source.