Estimating Cannabis Involvement in Fatal Crashes in Washington State Before and After the Legalization of Recreational Cannabis Consumption Using Multiple Imputation of Missing Values

Abstract

Washington State legalized recreational cannabis consumption in December 2012. We used data on all drivers involved in fatal crashes in Washington in years 2008–2019 (n=8,282) to estimate prevalence in fatal crashes of drivers with ∆9-tetrahydrocannabinol (THC, the main psychoactive compound in cannabis) in their blood before and after legalization. However, nearly half of the drivers were not tested for drugs; we used multiple imputation to estimate THC presence and concentration among them. We used logistic regression followed by marginal standardization to estimate the adjusted prevalence of THC-positive drivers after legalization relative to what would have been predicted without legalization. In the combined observed and imputed data, the proportion of drivers positive for THC was 9.3% before and 19.1% after legalization (adjusted Prevalence Ratio: 2.3, 95% Confidence Interval: 1.3, 4.1). The proportion of drivers with high THC concentrations increased substantially (adjusted Prevalence Ratio: 4.7, 95% Confidence Interval: 1.5, 15.1). Some of the increased prevalence of THC-positive drivers might have reflected cannabis use unassociated with driving; however, the increased prevalence of drivers with high THC concentrations suggests increased prevalence of driving shortly after using cannabis. Other jurisdictions should compile quantitative data on drug test results of drivers to enable surveillance and evaluation.

Societal issues and policy implications related to the use of cannabinoids, cannabis, and cannabis-based medicines for pain management

“…2 Pilots accustomed to smoking cannabis and trained in simulator tasks also have impaired function up to 24 hours after smoking. Despite these data, 1 in 5 teenagers has reported driving under influence of cannabinoids, 71 and chronic pain patients treated with medical cannabis admit driving under the influence of cannabis. 7 Due to the risk of impaired attention, memory, decision-making, and executive function with cannabis use, 19,61,80 there are also concerns about people in occupations that require unimpaired cognitive abilities, such as clinicians in acute care settings, and operators of heavy machinery.…”

Driving Performance and Cannabis Users’ Perception of Safety

“…Acute consumption of Δ9-tetrahydrocannabinol (THC) negatively affects cognitive functioning and reduces driving performance, particularly in lane position control (standard deviation of lateral position) and ability to adjust to lead car speed changes (car following). However, epidemiologic data regarding the effect of legalization on crash risk are not consisent . The varied findings partially reflect challenges in accessing robust prelegalization and postlegalization data and determining acute intoxication but also show a disconnect between impairing effects observed in controlled studies and expectations regarding crash rates.…”

High‐‘n’‐dry? A comparison of cannabis and alcohol use in drivers presenting to hospital after a vehicular collision

“…There is epidemiological evidence that drivers with THC concentration ≥ 5 ng/ml are at increased risk of crashing, although the risk is lower than for alcohol [4,5]. The prevalence of driving after using cannabis has increased in some countries [6,7], and there is concern that this trend may accelerate as more jurisdictions legalize recreational cannabis [8,9]. For example, the prevalence of injured drivers with high THC levels in Canada approximately doubled after recreational cannabis was legalized in 2018 [10].…”