If a car, already on the road, is replaced by another one, more expensive to collide with, a negative externality spills over to other drivers. This paper studies such externalities, relating them to insurance and incentives. It formalizes links from liability rules to choice of car. By assumption, insurance is cooperative but car acquisition is non‐cooperative. Construing drivers' interaction as a large game, the paper considers how Nash equilibrium ‐ and its efficiency or fairness ‐ is shaped by the underlying liability regime. This article is protected by copyright. All rights reserved.