UC Berkeley

Objective: To examine the independent and combined associations of reported alcohol, drug, and fatigue involvement with crash fatality among young drivers in California.

Methods: Using 2016-2024 injury crash data from the University of California, Berkeley Transportation Injury Mapping System, we performed logistic regression to examine the association between reported driver impairment and crash fatality among drivers aged 15-24. Additive interaction was assessed using binary impairment indicators and interaction terms. Relative excess risk due to interaction (RERI) and 95% confidence intervals (CIs) were estimated from adjusted odds ratios (AORs) using the delta method.

Results: Among 308,512 injury crashes, including 3,907 (1.3%) fatalities, most drivers had no reported impairment (84.8%), followed by reported alcohol-impaired (11.4%), and males comprised the majority across all impairment categories. Fatality odds were significantly increased for reported drug-impairment (adjusted OR [AOR] = 20.35, 95%CI: 18.18 to 22.77, p <.001) and alcohol-impairment (AOR = 4.65, 95%CI: 4.28 to 5.06, p <.001), with fatigue approaching significance (AOR = 1.27, 95%CI: 0.98 to 1.63, p =.07). Jointly reported alcohol and drug impairment significantly increased fatality odds (AOR = 65.72, 95%CI: 57.24 to 75.45, p <.001) and had strong evidence of synergistic interaction (RERI = 41.80, 95%CI: 32.95 to 50.65). No significant interaction was observed between reported alcohol and fatigue.

Conclusions: Reported substance-related impairment significantly increased fatal crash risk among California young drivers, with alcohol-drug co-use associated with synergistically elevated risks. Preventing polysubstance use presents an urgent opportunity for youth-focused interventions targeting risky driving behaviors among young drivers.