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FHWA Highway Safety Programs

2. Identifying Safety Issues

Determining what the problems are and where they are occurring will assist in making the most informed decisions regarding countermeasure selection and implementation to address intersection safety issues. When conducting a safety analysis, a minimum of 3 years of data is desired to obtain an accurate picture of the crash history within a jurisdiction, since crashes are relatively rare events and are not universally distributed across all intersections. A relatively large representative sample size for crashes in the jurisdiction will increase the chance that locations with the most severe safety issues will be identified. Due to the possibility of changes in traffic patterns and the roadway itself, data more than 5 years old are typically not desirable for assessing safety issues.

Analysis can range from simple "push pin" maps for identifying crash clusters to statistical analyses of crash rates, depending on the crash history and other data available.

There are a number of information sources used to identify crashes and risk factors at rural intersections. These include:

  • State and local crash databases;
  • Law enforcement crash reports and citations;
  • Observational information from road maintenance crews and law enforcement; and
  • Public notification of safety concerns.

In order to determine the intersections with a history of crashes (and those with a potential for future crashes), it is important to consider the types of data available and how those data can be used. In addition to the location of the crashes, the data can also provide information regarding crash causation to help identify potentially effective countermeasures.

The types of data available can range from anecdotal information, such as public input, to crash databases provided by State or local agencies. In some cases it may be beneficial to collect data from multiple sources to identify safety issues occurring at intersections.

The following discussion presents the most common information sources and recommendations for their use.

2.1. State and Local Crash Databases

Each State has a central repository for storing crash data that identifies locations with crash occurrences. Information found in a typical crash database includes: time and date of the crash, location, crash type, crash severity, and weather conditions. These data can be used to help compare a jurisdiction's intersections with others in the State. This comparison can help determine the level of need as it relates to similar locations in the region. Often States will provide or assist local agencies with their crash data analysis needs. Crash data is typically stored by the State Department of Transportation (DOT), Department of Public Safety (DPS), or Department of Revenue (DOR).

In addition, some local jurisdictions keep their own crash and roadway databases. If these exist, the information can be used as described above.

Action: Depending on your State's organizational structure, contact the county, regional, or State engineer or your State's Local Technical Assistance Program (LTAP) representative to determine if crash data within your jurisdiction is available for your use. If available, obtain 3 to 5 years of crash and roadway characteristic data. Develop a spreadsheet for intersections with a history of crashes (see Table 1). This can serve as a basic database to help identify common crash characteristics and identify appropriate countermeasures.

2.2. Law Enforcement Crash Reports and Citations

If an agency does not have access to State crash databases, law enforcement crash reports can be a valuable tool to identify the location and contributing circumstances to intersection crashes. The following variables (at a minimum) should be extracted and compiled from the crash reports:

  • Location;
  • Date and time;
  • Crash type;
  • Crash severity;
  • Weather conditions;
  • Sequence of events; and
  • Contributing circumstances.

Review of law enforcement crash reports can support decisions regarding the locations to improve and the safety treatments to select. While the information collected by law enforcement personnel may differ by jurisdiction, the basic elements should provide sufficient data to identify intersections that need improvement.

Citation records by law enforcement can provide information regarding driver behavior issues within a jurisdiction. Though not correlated directly to crash locations, citation information can indicate overall issues in the region that can potentially be addressed with enforcement and education strategies.

Police reports should be reviewed periodically in order to compile the necessary information for conducting an analysis. This information can also be stored in the spreadsheet shown in Table 1.

Action: Develop a relationship with law enforcement officials responsible for enforcement and crash investigation on their roads. This could foster cooperation in sharing crash reports and safety information and collaboration on problem intersections.

2.3. Observational Information

The crews who maintain the roads and law enforcement officers can serve as valuable resources to identify problem areas. Since they travel extensively on the local roads, they are able to continuously monitor the region for actual or potential problems (e.g., reduced sight distance due to vegetation growth, missing signs). Road maintenance crews often keep logs of their work activities connected to traffic safety issues, including sign replacements and edge drop-off repairs. These logs can assist safety practitioners in identifying recurring safety issues.

Law enforcement personnel are sometimes aware of problem areas that may not show up in the crash database or be known by public works staff, especially issues occurring at night or on weekends. This supplemental information about intersection safety can be beneficial to the safety improvement process.

Action: Develop a system for maintenance crews to report and record observed intersection safety issues and a mechanism to address them.

Set up a regular meeting with local law enforcement to discuss their observations of traffic safety issues in the local jurisdiction.

2.4. Public Notifications

Information about observed near misses or other perceived problems can support identification of intersections with safety issues and the potential for severe crashes. Occasionally, when near misses occur or an unsafe situation is observed, a citizen may notify the local government through an email, letter, telephone call, or public meeting. While this is anecdotal information, these sources can serve as important indicators that a problem may exist and would warrant further review and analysis to determine its extent.

Public notification of intersection safety issues can come from community or regional newspapers and newsletters, or correspondence from local homeowner associations, neighborhood groups, or individuals. Receiving this information could help pinpoint which intersections are candidates for review and establish links and relationships with the community to foster communication on safety related issues.

Action: Acknowledge input and, depending on the nature of the problem, establish a plan to review the identified sites. Keep a record of notifications and periodically monitor them.

2.5. Roadway Data

It is also valuable to obtain information about the roadway infrastructure. The following roadway data are often used to assist practitioners in safety analyses at intersections:

  • Roadway surface (dirt, aggregate, asphalt, concrete);
  • Lane information for approaches (number, width);
  • Median (type, width);
  • Number of intersection legs;
  • Configuration of intersection legs; and
  • Traffic control devices present (signs, pavement marking, traffic signals).

This information can be combined with crash data to help local practitioners identify appropriate locations and treatments to improve safety. For example, if a local rural intersection is experiencing a high number of right angle crashes, analysis of the inventory of roadway elements could reveal that the roadway does not have Stop signs on any approaches. An appropriate countermeasure could be to install Stop signs to provide traffic control at that location.

2.6. Exposure Data

The raw number of crashes can sometimes provide misleading information about the most appropriate locations for treatment. Introducing exposure data helps to create a more accurate comparison of locations. Exposure data provide a common metric to the crash data so intersections can be compared more appropriately.

The most common type of exposure data used at intersections is entering traffic volume. A count of the number of vehicles entering an intersection can provide information to the practitioner for comparison. For example, if two intersections have the same number of crashes but different entering traffic volumes, the location with fewer vehicles (i.e., less exposure) will have a higher crash rate, meaning that vehicles were more likely to have experienced a crash at that location. This rate reflects the fact that an increase in the number of vehicles has an effect on the expected number of crashes.

Table 1. Sample Spreadsheet to Monitor Crashes/Observations at Local Intersections
Source of Information Date (MM/DD/YYYY) Type of Information Problem Crash? Nature of Crash Time of Day (24 hr time) Weather Conditions Traffic Conditions Intersection Reviewed? Action? Date of Action
Local Newspaper 3/8/2008 Citizen Complaint Speeding N Y Pending
Local Police 4/8/2008 Police Report Crash Report Y Vehicle traveling North on Route 657 hit while making left turn onto Glade Drive 11:39 Clear Light volume N
Local Police 11/12/2009 Police Report Crash Report Y Vehicle traveling West on Glade collided with vehicle on Route 657 23:04 Rain No traffic N
Maintenance Crew 12/12/2009 Observation Missing Stop Sign N Y Replaced Stop Sign 12/19/2009
State Police 11/24/2008 Crash Report Y Vehicle traveling West on Clifton road rear-ended at intersection 19:21 Rain Light volume N