Accessibility

Aggregate Accessibility Measures

The topological or Shimbel measure is based on graph theory and the representation of a network as nodes (origins and destinations) and links (connections between nodes). This measure was introduced in geography in 1960 by William Garrison. Row sums in a filled impedance matrix represent the sum of shortest paths from one origin to all destinations and the accessibility of that origin.

The market or population potential measure of accessibility incorporates the attractiveness of destinations (such as the size or square footage of retail space), with higher accessibility resulting from being closer to more and larger destinations. As in the related gravity model of spatial interaction, distance is usually weighted to reflect the declining importance or utility of destinations farther away. There is no consensus about the specification of the weights attached to distance or the problem of self-potential, in which the distance of an origin to itself is zero and so does not allow the population or attractiveness of the origin to be included.

Simpler measures count the number of destinations within a certain radius of origin locations. These may use a single radius, such as a quarter mile, although more elaborate conceptualizations of distance are possible. In some cases, the counts of destinations may simply refer only to those within the origin zone.

Accessibility Studies

Accessibility studies became common in the 1960s during the quantitative revolution in geography. These studies provided a measure of the spatial structure of economies and cities, and they related socioeconomic indicators to accessibility to jobs or services at the intraurban level or economic variables at the interurban level. Before-and-after studies were (and remain) common. In this type of study, accessibility within a city or region is compared at multiple points in time before and after a major transport project, such as a freeway or railroad system, is completed. Accessibility improvements are then related to changes in economic development. Causality in these studies remains uncertain, and there is also the possibility that in developed countries the provision of transport infrastructure in the past half-century has lessened the importance of accessibility.

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