NCHRP Report 928 describes a comprehensive analysis of the factors associated with fatality rates in states, especially as they relate to the substantial drop in fatalities occurring in the years from 2008 to 2011. The analysis included predictors representing a wide variety of potentially related factors including travel, demographics, the economy, vehicle safety systems, and state spending on several categories of infrastructure and safety improvements. Understanding the broad array of factors that influence traffic safety in the United States is particularly important for state highway safety planning.

From 2008 to 2011, the United States, along with several other countries, experienced a substantial drop in annual traffic fatalities. State departments of transportation are keenly interested in capturing the key contributing factors to this decline so that the information can be used to help focus resources on effective countermeasures in future years. Safety countermeasures are implemented in a wide variety of ways—changing behavior (e.g., through driver education), changing vehicles (e.g., with Electronic Stability Control and other safety technologies), and changing the environment (e.g., improving roadways, laws, and enforcement). Safety can be influenced by factors other than safety efforts themselves, so it can be difficult to know which changes are responsible for overall reductions in fatalities.

Under NCHRP Project 17-67, the research conducted by the University of Michigan, with support from Texas A&M was asked to provide a multidisciplinary analysis of the relative influence of the types of factors that contributed to the national decline in the number of highway fatalities and rates in the United States during the years of 2008–2011. The research team used the Haddon matrix to identify many potential travel, demographic, economic, vehicle, and infrastructure influences on fatalities. Data on these factors were then collected from data sources publicly available at the state level (e.g., FHWA Highway Statistics). Annual state-level measures of these factors were compiled into a database covering the years from 2001 to 2012 and matched with fatalities from the Fatality Analysis Reporting System (FARS) database.

These data were analyzed using statistical methods to predict fatalities in states as well as changes over time. Because the statistical models closely approximated the reduction in fatalities from 2008 to 2011, the factors could then be evaluated in terms of their individual (and combined) contributions to fatalities. This report covers the scope of the problem, the data obtained to measure each factor, the statistical models, and the interpretation of results to understand how different factors play a role in total fatality counts. The knowledge gained from this process can be used to predict future fatality levels for planning at the state level and to provide insight into factors influencing these levels and actions that might reduce them.