Highway transportation of hazardous materials (hazmat) has been an active area of research with significant efforts in the risk assessment and route selection domains. Almost all these works assume fixed (constant) truck capacity, and hence capacity-based vehicle selection has not been studied. In this paper, we make a first attempt to investigate the impact of different truck capacities on transport risk from moving flammables, such as gasoline or explosives, on a given route. We make use of three scenarios, under risk-neutral and risk-averse assumptions, to develop conditions for preferring a specific vehicle size. A problem instance based in the US is solved to gain managerial insights. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved.