Case Study:

Orange County, California

Application

Determining Walking Accessibility to Transit Stops

Pedestrian accessibility is an important factor in the determination of the alignment of a transit route and the locations of bus stops. On-board surveys in Orange County have consistently shown that 80 to 90 percent of bus riders walk to and from their bus stops.

The calculation of accessibility requires the identification of the "catchment area" around each bus stop - that is, the area from which potential riders would be willing to walk to and from the stop. GIS tools can be used to identify catchment areas in several ways. The selection of a particular approach is determined largely by the availability and accuracy of the necessary spatial data. This section presents three ways of determining catchment areas around bus stops.

Route Buffer Catchment Areas - The simplest approach to building transit catchment areas is to create a buffer around an entire route, as depicted in Figure 6. The buffer is centered on the route of interest and is defined by the maximum distance that riders find convenient to walk to and from the system - typically one-quarter mile or so. This method is commonly used to describe fixed-route service areas for federal reporting purposes. The approach implicitly assumes that all locations within the buffer are accessible to the route. However, this assumption may overestimate accessibility because the route is only accessible at bus stops. Nevertheless, this approach can be useful when bus stop locations are not available in the GIS. The approach is commonly used, for example, to examine the general accessibility of alternative future transit routes for which bus stop locations are not yet known.

Figure 6.
Catchment Area Created From a One-Quarter-Mile Buffer Surrounding a Transit Route

Stop Buffer Catchment Areas - The second approach to building catchment areas is to create a buffer around each individual stop served by a route. This approach improves the accuracy of resulting measures of accessibility to the route but requires the availability of a bus stop layer that has coordinate locations for each stop. Figure 7 illustrates the results of this approach for the same route used in Figure 6 to illustrate the use of a route-level buffer.

While the stop-level approach clearly improves the accuracy of the analysis, it implicitly assumes that all locations within each buffer are within the defined walking distance of the bus stop. The geometry and connectivity of the street system, however, may result in actual walking distances that are much further that the distance implied by the buffer. Dead-end streets, cul-de-sacs, freeways, and interchanges - all characteristics of auto-dominated development patterns - can cause circuitous walk paths that are not recognized by the buffer approach.

Figure 7.
Catchment Area Created From One-Quarter-Mile Buffers Surrounding Transit Stops

Network-Based Catchment Areas - The third approach to building catchment areas is to identify all links in the street network that can be reached from a bus stop by walking along the network for less than the specified maximum walking distance. This approach requires network analysis tools and a spatially accurate street network that identifies facilities that can be used by pedestrians - streets, roads, and pedestrian-ways in contrast to freeways and expressways.

Several methods can be used to exclude facilities not available to pedestrians. These facilities can be removed entirely from the layer that describes the streets-and-highways system. Alternatively, the not-walkable facilities can be excluded from the selected set before any network operation is performed. Finally, these facilities may be assigned artificially long distances that would prevent their inclusion in any shortest path to and from a bus stop.

The addition of pedestrian-only facilities may be important to an accurate portrayal of possible walk paths to and from bus stops. Pathways in residential areas and pedestrian connections in commercial areas can play important roles in providing access to bus stops. Depending on the location and importance of these facilities, their addition to the street network may be important to the accuracy of the results.

Most GIS software has network analysis capabilities that can be used to create network-based catchment areas. The user specifies the origin points (stop locations) and maximum walk distance. The network tools then determine the walk network around each origin point (bus stop) that lies within the user-specified distance. Figure 8a illustrates the results of this analysis for the same route shown in Figures 6 and 7. Figure 8b focuses on three of the stops on that route, contrasting the results from the buffer-based approach and the network-based approach. Because the network-based approach is able to recognize disconnects in the street network, it is able to identify the walk-accessible service area for each bus stop with much better accuracy.

Figure 8a.
One-Quarter-Mile Walk Networks Created Around Transit Stops

Figure 8b.
Comparison of One-Quarter-Mile Buffer and One-Quarter-Mile Walk Network at Transit Stops

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