Examining Fatal Crash Reductions by First Harmful Events since the Introduction of the Federal Highway Safety Improvement Program

The federal Highway Safety Improvement Program (HSIP) has been associated with the reduction in fatal crashes since 2006, but the reasons for the reduction remain largely unknown. This paper examines the reduction in fatal crashes in terms of different types of first harmful events that can provide insight into crash causes and prevention strategies. In this study, fatal crashes were categorized into four types: overturn, collision with motor vehicle in transport, collision with fixed object, and collision with nonmotorist. Fixed-effects and mixed-effects Poisson models were used to estimate the magnitudes of fatal crash reduction by first harmful events for each state. Fatal crashes due to collisions with nonmotorists and motor vehicles in transport have been reduced by 10% and 5.3%, respectively, compared with the 2001 to 2005 period. Fatal crashes due to overturn and collision with a fixed object decreased in some states but remained unchanged or increased in other states. Nevertheless, the numbers of national fixed-object and overturn fatal crashes have been reduced by 3% and 0.7%, respectively, as a whole. This study also investigated possibilities that could be associated with the magnitudes of the reductions, for example, the different traffic laws among states. It was found that although different safety improvement projects were implemented to target the various types of crashes, the improvements were also likely to be beneficial to other crash types. These are referred to as spillover effects. Nationally, fatal crashes have decreased since the introduction of the HSIP partly because of the reduction in fatal crashes due to collisions with nonmotorists and motor vehicles in transport and partly because of spillover effects.