Impact Methodologies

Accessibility

Forecasting Methods

Method 1. Regional Accessibility

At the regional level, accessibility can most easily be measured using data from a regional travel demand model. Various measures of accessibility can be derived based on the number of jobs (or other opportunities) accessible within X minutes of the average person living in the region, or the number of residents accessible within X minutes of a typical employment site. The data required to compute this measure include population and employment by TAZ and zone-to-zone travel times. Accessibility can also be distinguished by mode of travel, by the median income of the TAZ of residence, or along other dimensions. Differences in accessibility by population subgroup can be compared among transportation alternatives to measure relative benefits.

• The use of regional accessibility measures is demonstrated in the Montgomery County, San Francisco Bay Area, and Tren Urbano case studies. The San Francisco case study also includes an appendix that illustrates the use of occupational matching. Occupational matching is a refinement of the population-to-employment accessibility measure to include consideration of compatibility between the occupations of residents and the types of nearby employment.

• Rood (1997) describes a Local Index of Transit Accessibility (LITA), which compares zone-to-zone transit versus automobile travel times, weighted by overall trip volumes between zones. LITA utilizes data from an MPO's regional transportation model. If these data are not available, an alternative index is proposed which reflects qualities of the transit service being provided to a geographic area, based on transit operator data.

Method 2. Access to Transit

Access to transit is a mode-specific component of regional accessibility. It can serve as an indicator of the availability of modal alternatives, particularly to transportation-disadvantaged populations. Measures of transit accessibility can also indicate the extent to which land use-transportation patterns make alternatives to automobile travel feasible.

• Measurements of access to transit generally rely on GIS to analyze the spatial relationship between transit stops or routes and population and employment. A basic measure of transit access is the number or percentage of people or employment sites within a reasonable walking distance (one-quarter to one-half mile) of a transit stop. This measure can be calculated in approximate terms by drawing a buffer around transit lines or stations and overlaying this buffer with TAZ-level population and employment data.

• More precise methods of transit station area analysis have recently been developed that utilize street network data and/or parcel-level land use data. Network data can be used to identify the actual street network within one-half mile walking distance of transit stations, while parcel data can be used to identify the number of dwelling units or square footage of development that can be reached within this distance. Examples are provided in the Orange County and Tren Urbano case studies. Jasciewicz and Russ (1998) provide an additional example of this type of analysis for light rail transit planning.

Transit access is most meaningful when combined with a level of service and/or accessibility measure. Transit level of service depends on a variety of factors including peak and off-peak headways, hours of service, and the coverage of the system. A detailed transit network in a regional travel model can provide a starting point for estimating transit levels of service between zones, as well as accessibility (i.e., to jobs) by transit. The addition of micro-level analyses such as those detailed above can help refine the analysis by providing a more precise estimate of population or employment convenient to transit.

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