A research study is described in which accidents experienced by tractor-semitrailers on expressway ramps were seen to depend largely upon the interaction between highway geometrics and vehicle dynamic behavior. The accident rates of tractor-semitrailers on expressway ramps in five states were scanned to select fourteen individual ramps which indicated an unusual incidence of serious accidents involving these vehicles. The geometrics of each ramp were fully defined in a computer simulation such that the dynamic behavior of example tractor-semitrailers could be examined. The paper presents the results of the combined study of accident data, simulated vehicle response, and the geometric details of ramp design. The findings of the study indicate that the maneuvering limits of certain trucks are very low, relative to those of passenger cars, such that current practice in ramp design leaves a very small margin for control of the heavy vehicles.