Guidebook on Identification of High Pedestrian Crash Locations

CHAPTER 5. CONDUCT ASSESSMENT

OVERVIEW

Several tools are available to assist in conducting the assessment. Most of the tools have data requirements in addition to crash data and may require additional training before an assessment can be conducted.

AVAILABLE TOOLS

Several tools are available to manage the assessment. These tools can implement a range of performance measures and incorporate different screening techniques to identify locations of concern. Available tools include the following:

• GISs are digital systems intended to code, display, and analyze data in relation to their geolocation (i.e., data with geodetic system of coordinates). GISs manipulate layers containing different types of data. For the purpose of safety analyses, GIS tools allow displaying and analyzing a layer with crash data on a map. Depending on the purposes, crashes can be analyzed in terms of their relation to each other, or in relation to other layers containing different types of data, for example, road infrastructure data, census tract data, and land use data.
• United States Road Assessment Program has a risk-mapping protocol to demonstrate which roads have the highest and lowest risk of fatal and serious injury crashes.⁽²³⁾
• Safety Analyst is a set of software tools used for highway safety management.⁽²⁴⁾ The software automates the six main steps of the highway safety management process, which has as the first step network screening. The other five steps are diagnosis, countermeasure selection, economic appraisal, priority ranking, and countermeasure evaluation. As noted on the related website, “Safety Analyst can be used to proactively determine which sites have the highest potential for safety improvement, as opposed to traditional (reactive) safety assessments done conventionally,” and it implements the procedures from part B (Roadway Safety Management Process) of the HSM.^(24,2)
• NCHRP Report 803, Pedestrian and Bicycle Transportation Along Existing Roads—ActiveTrans Priority Tool Guidebook, is a guidebook that presents the ActiveTrans Priority Tool (APT), a step-by-step methodology for prioritizing pedestrian and bicycle improvements along existing roads.⁽²⁵⁾ The APT is intended to be used by planners and other agency staff charged with managing a pedestrian or bicycle prioritization effort. It is designed to encourage practitioners to prioritize pedestrian and bicycle improvement locations by establishing a clear prioritization process that is responsive to agency/community values, and it is flexible, transparent, and receptive to the unique needs of pedestrians and bicyclists. The methodology combines the crash data with other data such as demand, connectivity, or equity using weights determined by the user.

Tools that are available for conducting a proactive approach include the following:

• The HSM provides information on available crash prediction models.⁽²⁾ Crash prediction models can be used to determine the expected crash frequency at a location. This information can be used to prioritize the locations that may potentially need treatments. The predicted crash frequency can also be combined with existing crash data to identify expected crash frequency for a location.
• A current NCHRP project (NCHRP 17-73) is developing a process for conducting systemic safety analyses for pedestrians using analytical techniques to identify pedestrian activities (including behavior), roadway features, and other contextual risk factors (e.g., land use) that are associated with pedestrian crashes.⁽⁴⁾

While several performance measures along with different screening techniques can be used to identify locations of concern, it appears that the tool being used by most agencies is GIS. GIS provides the opportunity to manage the data in an efficient and productive manner and then visually see the results. Figure 13 displays various conceptual elements of a GIS application. The main components for GIS are a core to host, manipulate, and analyze data (shown in red or as either rectangles or a cylinder); inputs, consisting of data and processing packages (shown in green or as either cards or a barrel); and outputs, consisting of data and information (shown in yellow or as either rounded cards or a rounded parallelogram).

Supplemental Material C in chapter 7 presents several examples of the use of GIS in displaying crash data.

©Texas A&M Transportation Institute.
Figure 13. Flowchart. Conceptual elements of a GIS application.

ASSIGNING CRASHES TO NETWORK ELEMENTS

Crash Concentration

With the availability of geographic coordinates (such as latitude and longitude), crashes can be spatially assigned to intersections or segments. Having latitude and longitude available for crashes may boost the sense of confidence of knowing the location of the crash more precisely. However, research with geolocated crash data has shown that the exact location of a crash is not always coded accurately.⁽²⁶⁾ With that in mind, the following section presents various alternatives to aggregate crashes by their geolocations. In one simple step, the tool “Collect Events” in ArcGIS aggregates points that are perfectly overlapping. The resulting layer of points can then be displayed showing the aggregated crashes, as figure 14 illustrates. The map in figure 14 displays the color and size of the dots proportionally to the crash frequency represented.

Screen capture ©Texas A&M Transportation Institute using ArcGIS software by ESRI. ArcGIS Desktop: Release 10.4.1. Service Layer Credits: Esri, HERE, DeLorme, USGS, Intermap, INCREMENT P, NRCan, METI, NGCC, ©OpenStreetMap. Crash data provided by the Texas Department of Transportation.
Figure 14. Graphic. Aggregation of pedestrian crashes.

Intersection Buffer

A basic analysis of intersections should identify crashes that occur in the intersection, along with crashes on approaches that are considered intersection-related crashes for further analysis. If an inventory of intersections is available, the main task is identifying a buffer around the intersections and selecting those crashes within that buffer for further analysis. It is common to set buffers of a specific distance (e.g., 250–300 ft) around an intersection, classify the crashes within as “intersection related,” and use only those crashes in the analysis (see figure 15). However, using this method still carries the risk of capturing nonintersection-related crashes or leaving intersection-related crashes out. Therefore, the research team recommends that agencies use the variable that indicates whether the crash is an intersection crash or not, when such information is available.

Research has shown that an average distance of 300 ft around the intersection is appropriate for developing SPFs, or crash frequency prediction equations.⁽²⁶⁾ If no intersection inventory is available, a layer of intersections can be generated based on the points of intersection between georeferenced road segments, and these intersection locations can then be overlapped with layers of crashes of interest.

Screen capture ©Texas A&M Transportation Institute using ArcGIS software by ESRI. ArcGIS Desktop: Release 10.4.1. Service Layer Credits: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN. Crash data provided by the Texas Department of Transportation.
Figure 15. Graphic. Example of buffers around intersections.

CRASHES TO EXPOSURE

The relationship between pedestrian and bicyclist crashes and exposure measures is still the subject of active research.⁽⁵⁾ Several exposure measures have been considered, including population, number of trips, and number of pedestrians (crossing or walking along street). In contrast to AADT for other types of crashes, there is no consensus on what measure or combinations of measures of exposure are best to incorporate in these types of analyses, mostly because of availability and strength of correlation.