National Automotive Sampling System (NASS) General Estimates System (GES)
The General Estimates System (GES) is a nationally representative sample of police-reported motor vehicle crashes of all types from minor to fatal. The system's purpose is to identify traffic safety problem areas, and to provide a basis for regulatory and consumer initiatives, and cost benefit analyses of traffic safety initiatives. Although various sources suggest that about half of motor vehicle crashes are not reported to the police, the majority of the unreported crashes involve only minor property damage and no significant personal injury. By restricting attention to police-reported crashes, the GES concentrates on those crashes of greatest concern to the highway safety community and the general public.
The police accident reports (PARs) from which NASS GES data are coded are a probability sample of police-reported crashes that occurred in the U.S. The sample frame has a multi-stage design. In the first stage, 1,195 geographic areas (primary sampling units (PSUs)) from across the U.S. were grouped into 14 categories by geographic region and type of PSU (large, suburban, and other). A sample of police jurisdictions was selected within each group. Finally, a sample of PARs was selected from each jurisdiction, stratified by type of accident. In order for a crash to be eligible for the NASS GES sample (1) a PAR must have been completed, (2) the crash must involve at least one motor vehicle travelling on a traffic way, and (3) the result must be property damage, injury, or death. Data collectors visit the selected jurisdictions weekly, sample, copy the PARs and send them to a central contractor for coding. Trained personnel code and enter the data directly into an electronic file.
The PARs are chosen from 60 areas that reflect the geography, roadway mileage, population, and traffic density of the U.S. About 400 police jurisdictions within these areas participate and 50,000 PARs from these jurisdictions are sampled annually.
National Automotive Sampling System (NASS) General Estimates System (GES) Analytical User's Manual 1988-2015. Washington, DC: U.S. Department of Transportation; National highway Traffic Safety Administration; National Center for Statistics and Analysis, August 2016.