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Motor Vehicle Accidents (MVAs) are a significant cause of injury and mortality worldwide (Kelly, Darke, and Ross 2004) and are the leading cause of death among people age 16 to 20 in Canada and other industrialized nations (MacDonald, Yanchar, and Hebert 2007). Driving under the influence of alcohol (DUIA) and driving under the influence of cannabis (DUIC) are major public health concerns in Canada and elsewhere, as they are pre-eminent risk factors for MVAs.
In Canada, the incidence of DUIA has declined consistently since the 1980s (Mayhew, Brown, and Simpson 2005). However, the prevalence of DUIA among drivers 18 years and older has increased from 14.7% in 2005 to 19.0% in 2009 (Vanlaar, Marcoux, and Robertson 2009). Likewise, the prevalence of DUIC is also on the rise in Canada, with the 2004 Canadian Addiction Survey (CAS) reporting that the percentage of Canadian drivers who drove within two hours after using cannabis in the previous year had doubled from 2.1% to 4.8% between 1988 and 2004 (Adlaf, Begin, and Sawka 2005). Similarly, data from the Road Safety Monitor survey indicated that the prevalence among respondents of driving within two hours of cannabis use increased from 1.5% in 2002 to 2.4% in 2005 (Simpson, Singhal, Vanlaar, and Mayhew 2006). The prevalence of DUIC is particularly high among young people. In two national, general-population surveys conducted in the late 1980s (Jonah 1990; Eliany, Giesbrecht, Nelson, Wellman, and Wortley 1990), DUIC was most prevalent among 18 to 24 year olds (5.4% and 6.1% respectively). The 2004 CAS indicated an average age of DUIC drivers of 28.7 years (Adlaf et al. 2005).
It is well documented that both alcohol and cannabis affect numerous driving-related skills--including reaction time and cognitive skills such as tracking, attentiveness, and psychomotor coordination--and impair driving ability in a dose-dependent manner (Ogden and Moskowitz 2004; Ramaekers, Berghaus, van Laar, and Drummer 2004; Sewell, Poling, and Sofuoglu 2009). However, whereas impairment from alcohol has been demonstrated at very low levels of blood alcohol concentration [BAC] (Wals and Mann 1999), some drivers have been reported to compensate for psychomotor deficiencies from mild cannabis-related impairment (Sewell et al. 2009).
Alcohol- and cannabis-related impairment significantly increases one's chances of being involved in a MVA. Research indicates that having either or both alcohol and active Tetrahydrocannabinol (THC) in one's blood (Ogden and Moskowitz 2004; Ramaekers et al. 2004) is associated with an increase in one's collision culpability. The fact that THC metabolites remain present and are detected in blood and other bodily fluids far beyond the time of actual impairment has long restricted the quality of evidence on the link between cannabis use and accident involvement. While the existing research evidence is somewhat equivocal, several recent and high-quality case control studies have shown that individuals who drive under the influence of cannabis or alcohol increase their odds of having a serious or fatal accident by 2.3-2.5 times and 3.8-4.6 times, respectively (Gerberich, Sidney, Braun, Tekawa, Tolan, and Quesenberry 2003; Mura, Kintz, Ludes, Gaulier, Marquet, Martin-Dupont, Vincent, Kaddour, Goulle, Nouveau, Moulsma, Tilhet-Coart, and Pourrat 2003). The combination of the two substances in the context of driving has additive or even multiplicative effects, increasing the odds of MVA involvement even further (Ramaekers et al. 2004). The precise number of cannabis-related traffic fatalities in Canada is unknown; however, the number of people killed in an MVA involving DUIA has steadily decreased from 1,296 fatalities in 1995 to 863 fatalities in 2007 (Vanlaar et al. 2009). Generally, individuals with high-frequency alcohol or cannabis use patterns are significantly more likely to be …