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 performance of the transportation system. For a given land use pattern, quicker, more reliable, and/or lower-cost transportation alternatives provide greater accessibility.

• Land use patterns, including the density and mix of development. For a given level of transportation performance, a more dense arrangement of land uses means greater accessibility, since more activities can be reached within a given distance/time. The mix of land uses, for example jobs/housing balance or proximity to retail, also influences accessibility.

The resulting benefits and impacts of accessibility changes include:

• As an end goal in itself. Providing accessibility can be viewed as a fundamental objective for the transportation system, as it allows people to access desired activities.

• Economic activity. Businesses benefit from easier access to suppliers, a larger labor pool, and expanded consumer markets. These factors can reduce transportation costs both for business-related passenger travel and for the movement of goods. Access to larger numbers of workers, consumers, and suppliers also provides greater choice and allows greater specialization, thus increasing business efficiencies.

• Land use patterns. Feedback between accessibility and land use means that the relationship between transportation improvements and accessibility gains is complicated. An example is the construction of a new highway. The immediate accessibility improvements may lead to significant land development in the vicinity of the highway. Eventually, however, the traffic generated by new development can cause significant congestion, reducing some of the original accessibility benefits provided by the highway.

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.

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