Patterns of Injury Severity and Its Likelihood in Two-vehicle Crashes

Understanding injury severity patterns in roadway crashes is important not only from the view point of treating crash victims, but also for directing the crash avoidance efforts of traffic safety agencies and motor vehicle manufacturers. The factor that is most discussed in this context is vehicle incompatibility. However, there are other vehicle-, occupant-, and roadway-related factors, too, that play roles in injury severity. In order to investigate these factors in relation to the injury severity, this paper considers a two-vehicle crash as a ‘system’ with its elements: vehicles, drivers, and roadway. Some of the possible inputs (contributing factors) to this system are considered with a focus on injury severity of the driver as an outcome. The differences in weights, heights, and shapes, etc. of the crash-involved vehicles, vehicle speed, drivers’ ages and genders are the factors in question. Data mining the crash databases compiled by the National Highway Traffic Safety Administration (NHTSA) makes many important revelations. The association between the subject variables and driver injury severity is studied through contingency analysis. Configuration frequency analysis helps to identify patterns of injury severity. The main objective of the study is achieved by building a logit model that can be used to predict the likelihood of injury severity from a given set of vehicle-, driver-, and roadway-related crash characteristics. INTRODUCTION AND RATIONALE Reducing crashes on the roadways is of paramount importance, as is the reduction in crash injury severity. Many studies have been conducted on injury severity [e.g., 5,9]. Most of them are based on controlled experimentation and look at the phenomenon purely from an engineering or medical point of view. One of the reflections of these studies is that vehicle incompatibility contributes to injury severity in a crash [6]. In other words, the larger differences in the sizes and weights of the crashinvolved vehicles are likely to result in more serious injuries to occupants of the smaller vehicle. Injury and its severity is the resultant effect of collision between two bodies (vehicles). Therefore, in order to study the magnitude of this effect, crash phenomenon must be looked at as an impact, i.e., the effect of transfer of energy from one vehicle to the other [7]. This results in change of relative velocity. As kinetic energy [=1⁄2(Mass*Speed)] increases with square of velocity, the energy of motion of the striking vehicle dissipated to the other vehicle increases not only with the increasing mass but also with the increasing velocity. The vehicle occupants acquire the same velocity as the vehicles they are riding. Thus, while the colliding vehicles undergo changes due to conservation of momentum generated by the impact, their occupants, too. Also, the roadway conditions, to a large extent, govern the last moment changes, such as velocity change, etc. This makes it imperative in an injury severity related study to consider the vehicle, occupants, and roadway together. Following this rationale, a crash is considered as a system with interacting elements: Vehicles, Occupants, and Roadway. An object with small inertial mass changes its motion more readily than an object with large inertial mass. This argument applies to both the vehicles in crash and their occupants. This shows that for injury severity, if vehicle mass is a contributing factor, so is the occupant body mass. In fact, each element of the crash system has certain associated characteristics that contribute to the outcome (injury severity) of this system. Using multivariate statistical methods, this paper explores factors that contribute to injury severity. Injury severity patterns are identified as well