Case Study:

Orange County, California

Application

Measuring Regional Accessibility

The methods described above assist in the measurement of accessibility by identifying the street network within walking distance of a transit stop and the total population and employment accessible to a stop. Accessibility data for individual stops can be further compiled to determine accessibility measures for individual routes, for the entire transit system, or at a traffic analysis zone (TAZ) level. Potential route, system, and TAZ-level measures might include:

• Total population or employment within walking distance of a transit route or the transit system;

• Percent of regional population or employment within walking distance of transit;

• Percent of population with transit access by TAZ;

• Percent of streets within a one-quarter-mile radius of transit stops that are also within a one-quarter-mile walking distance (a measure of how well the street layout is designed for transit access);

• Average population or employment densities in transit stop areas;

• Population or employment accessible to a given level of transit service, e.g., buses running on 15-minute or shorter headways in the peak hour;

• Changes in any of the above measures as a result of changes in transit service or pedestrian access improvements.

In developing route-level or system-level accessibility measures, care needs to be taken so that populations in adjacent, overlapping catchment areas are not counted twice. Two overlapping areas may be combined into a single area, or may be split into two separate areas halfway between the stops.

The following example illustrates how regional transit accessibility can be evaluated by plotting the population accessible to OCTA's fixed-route service within a one-quarter-mile walk. This analysis relies on the disaggregation of socioeconomic data as well as the walk network analysis described above. Figure 9 illustrates the population accessible to existing transit service throughout the county. Figure 10 demonstrates the change in accessibility as a result of a proposed change in bus service. The changes as identified from this analysis are shown in Table 2.

Figure 9. Population Accessible to Existing OCTA Bus System

Figure 10. Change in Population Accessible to OCTA Bus System

Table 2. Pedestrian Access for Existing and Proposed Systems

Using other methods, relationships between residential and employment areas can be evaluated to determine different measures of accessibility or mobility. For example, the number of jobs within a 60-minute bus ride of a particular census tract or bus stop can be estimated. This would require estimates of employment accessible from each bus stop, as well as GIS representations of the transit network (bus routes), transfer locations, headways, and travel speeds. Some of these data may be obtained from travel demand models with a transit network. Travel demand models are used to calculate similar measures, but being more generalized, they may not provide results as detailed as GIS-based methods.

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