title: The combined effects of alcohol and cannabis on driving: Impact on crash risk.
abstract: OBJECTIVE
  Driving under the influence of alcohol or cannabis alone is associated with increased crash risk. This study explores the combined influence of low levels of alcohol (BAC≤0.08) and cannabis on crash risk.
  
  
  METHODS
  Drivers aged 20 years or older who had been tested for both drugs and alcohol after involvement in a fatal crash in the United States (1991-2008) were examined using a case-control design. Cases were drivers with at least one potentially unsafe driving action (UDA) recorded in relation to the crash (e.g., weaving); controls had none recorded. We examined the prevalence of driving under the influence of alcohol, cannabis, and both agents, for drivers involved in a fatal crash. Adjusted odds ratios of committing an UDA for alcohol alone, THC alone, and their combined effect were computed via logistic regression and adjusted for a number of potential confounders.
  
  
  RESULTS
  Over the past two decades, the prevalence of THC and alcohol in car drivers involved in a fatal crash has increased approximately five-fold from below 2% in 1991 to above 10% in 2008. Each 0.01 BAC unit increased the odds of an UDA by approximately 9-11%. Drivers who were positive for THC alone had 16% increased odds of an UDA. When alcohol and THC were combined the odds of an UDA increased by approximately 8-10% for each 0.01 BAC unit increase over alcohol or THC alone.
  
  
  CONCLUSIONS
  Drivers positive for both agents had greater odds of making an error than drivers positive for either alcohol or cannabis only. Further research is needed to better examine the interaction between cannabis concentration levels, alcohol, and driving. This research would support enforcement agencies and public health educators by highlighting the combined effect of cannabis at low BAC levels.
authors.affiliation: Research Department, St. Joseph's Care Group, 580 North Algoma Street, Thunder Bay, ON, Canada P7B 5G4; Centre for Research on Safe Driving, Lakehead University, 955 Oliver Road, Thunder Bay, ON, Canada P7B 5E1; Department of Health Sciences, Lakehead University, 955 Oliver Road, Thunder Bay, ON, Canada P7B 5E1; Northern Ontario School of Medicine, Human Sciences Division, Lakehead University, 955 Oliver Road, Thunder Bay, ON, Canada P7B 5E1. Electronic address: duboiss@tbh.net.
authors.forename: Sacha
authors.initials: S
authors.lastname: Dubois
authors.affiliation: Centre for Research on Safe Driving, Lakehead University, 955 Oliver Road, Thunder Bay, ON, Canada P7B 5E1.
authors.forename: Nadia
authors.initials: N
authors.lastname: Mullen
authors.affiliation: Centre for Research on Safe Driving, Lakehead University, 955 Oliver Road, Thunder Bay, ON, Canada P7B 5E1; Northern Ontario School of Medicine, Human Sciences Division, Lakehead University, 955 Oliver Road, Thunder Bay, ON, Canada P7B 5E1.
authors.forename: Bruce
authors.initials: B
authors.lastname: Weaver
authors.affiliation: Research Department, St. Joseph's Care Group, 580 North Algoma Street, Thunder Bay, ON, Canada P7B 5G4; Centre for Research on Safe Driving, Lakehead University, 955 Oliver Road, Thunder Bay, ON, Canada P7B 5E1; Department of Health Sciences, Lakehead University, 955 Oliver Road, Thunder Bay, ON, Canada P7B 5E1; Northern Ontario School of Medicine, Human Sciences Division, Lakehead University, 955 Oliver Road, Thunder Bay, ON, Canada P7B 5E1.
authors.forename: Michel
authors.initials: M
authors.lastname: Bédard
mesh_terms: D000063:Accidents, Traffic; D000328:Adult; D000067401:Blood Alcohol Content; D016022:Case-Control Studies; D019992:Databases as Topic; D000066448:Driving Under the Influence; D013759:Dronabinol / Q000097:blood*; D005260:Female; D053593:Forensic Toxicology; D006801:Humans; D016015:Logistic Models; D008297:Male; D008875:Middle Aged; D011619:Psychotropic Drugs / Q000097:blood*; D018570:Risk Assessment; D019966:Substance-Related Disorders / Q000150:complications; D014481:United States
chemical_list: D000067401:Blood Alcohol Content; D011619:Psychotropic Drugs; D013759:Dronabinol
keywords: Alcohol; Car drivers; Crash culpability; Impairment; THC