Among 609 traffic crash victims in Porto Alegre, Brazil, 13.3% of drivers tested positive for cannabis, with alcohol abuse or dependence increasing the odds of any substance-related crash 5.2-fold.
Read this if you are interested in the relationship between cannabis use and traffic safety.
13.3% of crash-involved drivers tested positive for cannabis
What the researchers found
Emergency room data from two Porto Alegre hospitals over 45 days identified 609 traffic crash victims. Key findings:
Drivers were mostly male (p<0.001) with higher rates of binge drinking (p=0.003) and marijuana use (p=0.005) compared to pedestrians and passengers. Cannabis prevalence was 13.3% among drivers.
Positive blood alcohol was found in 7.8% of drivers and 9.2% of pedestrians. The variables most associated with alcohol-related crashes were binge drinking in the prior year (OR 2.4) and coming from a party or bar (OR 8.7).
Alcohol abuse or dependence increased the odds of an additional substance-related crash by 5.2 times, indicating that alcohol problems substantially increased the risk of polysubstance-impaired driving.
Why it matters
The 13.3% cannabis prevalence among crash-involved drivers in Brazil highlighted cannabis as a significant traffic safety concern beyond alcohol alone.
The numbers in context
609 victims, 72% male, median age 29. Cannabis positive: 13.3% of drivers. BAC positive: 7.8% of drivers. Binge drinking: OR 2.4. Coming from party/bar: OR 8.7. Alcohol dependence increased other substance-related crash risk 5.2-fold.
How the study worked
Cross-sectional study with consecutive sampling of non-fatal traffic crash victims at two emergency rooms over 45 days. Structured interview, breathalyzer, and salivary drug testing. Multinomial logistic regression for associated factors.
What this study cannot tell us
Cross-sectional emergency room sample (no comparison to non-crash drivers). Salivary testing may detect cannabis from days prior. Cannot determine whether cannabis caused or contributed to the crash.
How to read the evidence
Cross-sectional emergency room study with objective drug testing but no comparison to non-crash drivers.
When this study was published
Published in 2011. Traffic safety research on cannabis has expanded globally.
The bigger picture
This Brazilian data contributed to global evidence that cannabis-impaired driving was a significant public health issue requiring specific policy attention alongside alcohol-impaired driving.
Questions still open
- What proportion of cannabis-positive drivers were actually impaired at the time of the crash? How should Brazilian traffic safety policy address cannabis alongside alcohol?
Common questions
How common is cannabis-impaired driving?
Is combining alcohol and drugs worse for driving safety?
Read the original research
Factors associated with alcohol and drug use among traffic crash victims in southern Brazil.
Accident; analysis and prevention, 43(4), 1408-13
Citation
De Boni, Raquel; Bozzetti, Mary Clarisse; Hilgert, Juliana; Sousa, Tanara; Von Diemen, Lisia; Benzano, Daniela; Menegon, Guilherme; Holmer, Barbara; Duarte, Paulina do Carmo Arruda Vieira; Pechansky, Flavio. (2011). Factors associated with alcohol and drug use among traffic crash victims in southern Brazil.. Accident; analysis and prevention, 43(4), 1408-13. https://doi.org/10.1016/j.aap.2011.02.016