that more drivers are operating vehicles under the influence of marijuana
and other controlled substances are unsupported by federal data, according to the conclusions of a November 2014 paper
published by the National Highway Traffic Safety Administration’s (NHTSA) Office of Behavioral Safety Research.
According to the paper, which analyzes the total number of drivers involved in fatal crashes from 2008 to 2012, inherent limitations involved in data collection prevent researchers from drawing any definitive conclusions in regard to whether the percentage of drugged driving incidents has increased, decreased, or remained stable during this time period.
In particular, the authors acknowledge:
The majority of drivers involved in fatal crashes during this time were not tested for drugs (only 41% and 40% of drivers were tested in 2008 and 2012, respectively);
Drug positive drivers are not necessarily drug impaired since, “in some cases, drug presence can be detected for a period of days or weeks after ingestion.”
At present, there is no consistent set of policies or procedures for drug testing from state to state; and
Decreases in drug testing costs may have led to an increase in the number of drivers tested, as well as the range of drug types tested.
NHTSA concludes, “These facts… demonstrate that we cannot infer whether drugged driving has increased; similarly we cannot know the extent to which drugged driving differs across States.”
While the ingestion of cannabis can influence
psychomotor performance, particularly in more naïve users, its role in fatal crashes appears comparatively low
to that of other controlled substances, particularly alcohol. According to a 2012 review paper
of 66 studies assessing drug positive drivers and crash risk, marijuana-positive drivers possessed an odds-adjusted risk of traffic injury of 1.10 and an odds-adjusted risk of fatal accident of 1.26. This risk level was among the lowest of any drugs assessed by the study's author and it was comparable to the odds ratio associated with penicillin (OR=1.12), anti-histamines (OR=1.12), and antidepressants (OR=1.35). By contrast, a 2013 study
published in the journal Injury Prevention reported that drivers with a BAC of 0.01 percent are "46 percent more likely (OR = 1.46) to be officially blamed for a crash than are the sober drivers they collide with."