2. Epidemiological studies on cannabis and crash risk

The aim is to evaluate the current evidence on the association between cannabis use and crash-risk.

Consensus cannabis expert working group.

1) What is the prevalence of cannabis use in randomly selected drivers in traffic? In most roadside surveys, THC is the second most commonly detected recreational drug, after alcohol. 2) What is the prevalence of cannabis use in crash-involved drivers? The prevalence of cannabis is generally higher in crash-involved drivers than in roadside surveys. Findings from recent studies of crash-involved drivers from different countries may vary and can be explained by cultural and legal factors that influence how often drivers in different countries use drugs or alcohol and by differences in study design. 3) What is the risk of driving after using cannabis? Cannabis is associated with a modest increase in crash risk at the population level as assessed with case-control or culpability (responsibility) studies. The increase in risk varies between studies, but the average increase is 30-40%. 4) What are the major limitations of epidemiological studies? Most studies have significant limitations that make their results difficult to interpret. The most common limitations include: failure to measure recent cannabis use or impairment, delays in obtaining blood to measure THC, high refusal rates in case control studies, the use of non-comparable controls, difficulty assigning responsibility in responsibility analyses.

Epidemiological studies suggest a modest association between cannabis use and crash risk.