The purpose of a historical crash data analysis is to identify safety issues through the review of crash trends, patterns, and rates. For projects that need FHWA approval, request CRIS data through TxDOT's Traffic Safety Division (TRF) or the Design Division. For other project types, crash data can be obtained through the CRIS website. CRIS allows users to run queries on historical crash data, based on a variety of selection criteria. The CRIS query builder can be used to create a new query, load a saved query, or browse queries authored by TxDOT.

It is recommended that the most recent and complete five calendar years (January 1 through December 31) of crash data be used to conduct a standard historical crash data analysis. It is possible to use less than five years of crash data in the analysis if study area conditions experienced significant changes, resulting in the historical crash data no longer being relevant to the current conditions. For example, a traffic signal installed at an intersection five years ago would result in only four complete calendar years of crash data (beginning after the installation of the traffic signal). In this situation, only use four years of historical crash data for analysis. A minimum of three years of crash data is necessary to perform a historical crash data analysis. If a shorter study period is necessary, pre-approval by the TxDOT Project Manager who will be approving the completed analysis is recommended. The following are situations when a shorter study period may be acceptable:

More than five years of crash data can be used in analysis in certain circumstances. This would be applicable when very little or no historical crash data is available for a study area of interest or when the analysis is focused on a particular subset of crash data that occurs less frequently. For example, using more than five years historical crash data may be applicable when performing historical crash analysis in rural areas with sparce crashes, performing a detailed pedestrian and bicycle crash analysis, or preparing a commercial vehicle crash analysis. When a situation warrants the use of more than five years of crash data, eight years of historical crash data is recommended to be used in the analysis. When a longer study period is necessary, it is to be accepted by the TxDOT Project Manager who will be approving the completed analysis.

Table 6-1

shows the recommended years of data recommended for safety analysis on various project types.

When analyzing historical crash data to determine trends, patterns, and rates, many characteristics should be evaluated and the results can be displayed using a variety of methods. These methods result in the creation of crash summary tables, figures, charts, or maps. The following is a list of minimum characteristics to be included in a historical crash data analysis:

Additional analysis results or characteristics that could be evaluated include, but are not limited to the following:

There are methods that can be used to analyze historical crash data and determine crash patterns and trends. Historical crash data analysis includes calculation of crash rates and the comparison of those crash rates to statewide averages for similar facility types. A link to the statewide traffic crash rates is in

Appendix G, Section 5 – External References (Reference 11)

. Crash rates are intended to normalize the number of crashes relative to traffic or exposure. When a location or study area is determined to have a crash rate significantly higher than the statewide average (more than five crashes per million entering vehicles for intersections or 5 crashes per 100 million vehicle miles of travel for segments), further analysis may be needed. Crash rates are determined for roadway segments or intersections and can be calculated for total crashes, fatal crashes, injury crashes, and/or fatal and injury crashes.

The equation for determining a crash rate for a roadway segment is:

R = Crash rate for the road segment expressed as crashes per 100 million vehicle miles of travel (MVMT)

C= Total number of crashes in the study period

V= Daily traffic volume

L = Length of roadway segment in miles

N = Years of data The equation for determining an intersection crash rate is:

R = Crash rate for the intersection expressed as crashes per 1 million entering vehicles (MEV)

C= Total number of intersection crashes in the study period

V= Traffic volumes entering the intersection daily

N = Years of data

Roadway segment crash rates are calculated per 100 MVMT

Intersection crash rates are typically calculated per MEV. Both MVMT and MEV are traffic exposure variables. For example, if a 6-5 2024 | Traffic and Safety Analysis Procedures Manual two-mile long corridor segment has an AADT or ADT of 20,000 vehicles per day and 50 crashes from 2016 through 2020, the segment crash rate is calculated as follows:

R = 68.5 Crashes per 100 MVMT

Table 6-2

presents an

example

of a comparison of crash rates by year for road type and highway system. Crash rates higher than the statewide average indicate the need to further analyze crashes to develop countermeasures and recommend safety improvements. An additional table provided in

Appendix G, Section 4 – Example Crash Data Summary

shows crash comparisons by collision type and crash severity. This table is recommended to be used to check whether any aspect of the crash data stands out (e.g., overrepresented crash types or severity, crash rates greater than the statewide average, or patterns of fatalities and suspected serious injuries). Further analysis may be necessary if crash severities are higher than expected, the manner of collisions are different than expected, or if there are issues in unexpected locations. Further analysis can include viewing CR-3 and evaluating crash diagrams.

Figure 6-2

presents a color-coded crash heat map showing crash concentrated locations (i.e., areas where the density of crashes is high), where green represents a low crash density and red represents a high crash density. These maps are typically created using crash data and a GIS mapping software. They can be helpful in visualizing locations with a high occurrence of crashes. The CRIS query tool also provides an option to view crashes as clusters to visualize crash concentrations. The cluster view combines crashes by area and shows an overall number of crashes.