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Impact Methodologies - Accessibility

Overview

Accessibility can be defined for personal travel as the ability to reach desired destinations such as jobs, shopping, or recreational opportunities. For goods movement it can be defined as the ability to reach suppliers or buyers of products. Key determinants of accessibility include:

The resulting benefits and impacts of accessibility changes include:

Some travel models use accessibility as an input variable. Auto ownership models developed in Philadelphia, Portland, and other places include accessibility as a variable. Portland's model has included transit accessibility for many years, while a model recently developed for Philadelphia includes both highway and transit accessibility. Trip generation models have also been developed using accessibility variables, including models in San Francisco and New Orleans.

While accessibility measures are a basic element in the development of both transportation models and land use models, they are rarely used directly in transportation decision-making. The significance of accessibility improvements may not be as easy to interpret as (for example) travel time savings. Given the growing recognition of its importance, however, the use of accessibility as a performance measure in regional planning is increasing. One area in which it has a particularly promising role is in the measurement of differences in benefits among population groups. For example, accessibility changes can be compared among income groups or for transit versus auto users, to compare the effectiveness of alternative transportation investments.

The included case studies illustrate how accessibility measures are currently being used in the evaluation of transportation and land use alternatives.

Case Studies

Montgomery County, Maryland

During 1998 and 1999, Montgomery County analyzed five alternative year 2020 transportation networks. Planning staff chose to use accessibility as one of a few key indicators of the performance of each alternative transportation scenario.

Orange County, California

The Orange County Transportation Authority in California applied GIS to analyze transit accessibility and ridership based on street patterns and land use data.

San Francisco Bay Area, California

The Metropolitan Transportation Commission used measures of job accessibility to compare the impacts of the Regional Transportation Plan among different population groups.

Tren Urbano - San Juan, Puerto Rico

Regional accessibility measures were applied to compare the performance of rail and bus transit investment alternatives and transit-oriented development policies.

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.

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.

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.

General References

General discussions of the measurement of accessibility can be found in Vickerman (1974), Weibull (1980), and Handy and Niemeyer (1997).

Updated: 04/24/2012
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