Turner-Fairbank Highway Research Center


Data Elements

Two data element tables have been developed for all the States. Table 1 lists the crash-related variables for each State side-by-side. Table 2 lists the roadway-related variables. The purpose of developing these tables was to give the ability to the HSIS data requester to compare between States the availability of variables.

Before viewing the tables, please read the directions below for using the tables.

Using the Data Element Tables

The variables shown in the accompanying Data Element tables are obtained from four basic files available in the Highway Safety Information System (HSIS) database (i.e., accident, vehicle, occupant, and roadway). They are divided into two broad categories. The first, titled "Data Elements for Accident, Vehicle, and Occupant Files" is obtained from police accident reports and contains crash-related variables. The second, titled "Data Elements for Roadway and Roadway-Related Files" is obtained from roadway inventory files provided by each State. Note that there are also some roadway descriptors on the accident files (e.g., roadway deficiency).

The basic crash-related variables are divided into three separate subfiles for ease of computer handling: Accident File, Vehicle File, and Occupant File. Each accident in the Accident File is identified by a unique case number and has only one record (observation) per crash. The Vehicle File has one or multiple records per crash, depending on the number of vehicles involved in a crash, and each group of vehicles in a given crash are again identified by the unique case number. Similarly, the Occupant File has one or more occupants per vehicle, depending on the number of occupants in a crash. It can be linked to vehicle and accident files using the same unique case number. Note that driver data are included in the Vehicle File.

When linking these variables, the user has to specify whether the data record needed is accident-based, vehicle-based, or occupant-based. In some cases, only the Accident File variables will be needed. If an accident-based file is requested (one record per accident), but the user wants vehicle and occupant data, HSIS staff will usually send the data in three separate files and allow the researcher to perform the linkage as needed for the study being conducted. (An accident-based record that includes vehicle/occupant data has to be a fixed length — not a variable length — and would thus have many blank data items for all accidents with fewer than the maximum number of vehicles.) If the requested data is vehicle-based (one record per vehicle), then all pertinent accident variables will be linked to each vehicle in a given crash, resulting in duplicate accident variables across records. If needed, HSIS staff will work with the researcher in deciding which record type is most appropriate.

The roadway inventory variables describe a section of roadway and are available in a variety of different files for different States. Each State has a Roadlog (roadway inventory) File and a Traffic Volume File. Some have curve and grade inventory files, intersection/interchange files, or other special files. In almost all the States, the inventory data are identified by a route number and beginning and ending mileposts with additional descriptive variables attached to each segment. The researcher has to specify which specific variable is needed from a particular file, and decide if and how these data should be linked with the crash file data. One alternative is an accident-based record, in which roadway/traffic inventory data for the location of the crash are merged with the crash data into one record. A second alternative is a section-based file, where counts of crashes (either total or by type) can be added to each section-based inventory record. The latter type is usually used in studying the effects of roadway variables on crash probability. Again, if requested, HSIS staff will work with the researcher in defining the most appropriate record type. Separate crash and roadway inventory files are often sent to the researcher, who does the necessary linkage as needed. HSIS staff have developed linkage programs that are available upon request.

To allow HSIS staff to handle your request as quickly and efficiently as possible, we suggest the following steps:

  1. Define a preliminary set of variables of interest based on the Generic Listing. (Note that we cannot send complete files containing all variables due to agreements with the States. However, we do supply extract files containing subsets of variables.)
  2. Review the Discussion section and the Format section of the HSIS guidebooks for each State from which you are requesting data. This will provide very important details on both the quality of the data in the file and specific codes provided. These guidebooks can be obtained from HSIS staff.
  3. Request variables by their variable names, and not by the generic variable description.
  4. To avoid confusion and to expedite your request, list requested variables by file type. Each variable in the Generic Listing has an abbreviation for the file type in parentheses. They are:
    • A = Accident
    • V = Vehicle
    • O = Occupant
    • R = Roadlog (Roadway Inventory)
    • T = Traffic Volume
    • I = Intersection/Interchange
    • C = Curve
    • G = Grade
  5. When specifying constraints on the data, separate the constraints by the files the variables are in. For example, a constraint on ACCTYPE = "run-off-road" and WEATHER = "snow" would both come from the crash data, while a constraint on RURURB = "rural" could come from either the Accident File or Roadlog File, and a constraint on NO_LANES (number of lanes) = "2" would come from the Roadlog File.
  6. Fill out the HSIS Data Request Form and send it to the address indicated.

The above suggestions will help ensure that the researcher is familiar with the HSIS data before use and will help expedite your request when we receive it. Thank you and welcome to the HSIS system.

Data Element Tables

Table 1 — Data Elements for Accident, Vehicle, and Occupant Files

Table 2 —Data Elements for Roadway and Roadway-Related Files