Vehicle weight is a central determinant of traffic safety, generating private benefits through occupant protection and external costs imposed on others. We estimate the causal effects of vehicle weight on accident risk, severity, and social costs using comprehensive Swedish administrative data linking police-reported crashes to labor market outcomes. To identify severity effects, we exploit within-model variation in vehicle weight, comparing otherwise similar vehicles. To estimate accident frequency, we leverage quasi-exogenous variation from leasing renewals to address selection into heavier vehicles. Heavier vehicles substantially increase harm to others in multi-vehicle collisions: a 1,000-pound increase in the striking vehicle’s weight raises the fatality probability in the struck vehicle by about 50 percent relative to the mean. Severe injuries rise similarly. We further show that more severe crashes lead to persistent reductions in victims’ earnings and employment.
