This paper decomposes the external accident costs of freight trucking into two components—truck miles traveled and truck weight. The effects of miles traveled and weight on accidents are then applied to an analysis of diesel fuel taxes, which reduce truck miles traveled and increase truck weight. Exploiting a unique data set of 3.5 billion truck-level observations, I find that both measures of trucking activity increase the quantity of collisions, and truck-weight increases skew the collision distribution toward fatal outcomes. Heavier trucks do not alter the truck-only collision severity distribution, suggesting truckers do not experience truck-weight internalities. The increase in fatal collisions caused by levying a diesel tax that prices carbon emissions offsets the gains from reductions in pollution, congestion, and total collisions. The $0.37 per gallon diesel tax increase exacerbates the trucking accident externality to such an extent that it increases the external costs of trucking by $55.7 billion/year while also creating deadweight loss in the trucking industry.