Case Study:

San Francisco Bay Area, California

Methodology

Calculation of Gravity-Based Accessibility

The gravity-based accessibility measure is computed as follows:

Where A_ik represents the employment accessibility for zone of residence i and mode k. Total regional accessibility by mode k is then computed as for the threshold-based measure.

In addition to the data required for the threshold measure, a "distance decay" parameter or exponent for the travel time function (b) is required. MTC selected a value for b of 2.0 as typical for home-based work travel (the methodology has since been updated using the gravity model coefficient from MTC's trip distribution model). A different value could also be used that is consistent with an area's local travel demand model.

A technical complication in computing the gravity-based measure is that some zones will not have transit service, and thus travel time to these zones is not defined. This problem can be solved in two ways:

Select an arbitrarily large number for travel time that will produce a very small (but non-zero) number when raised to the given power; or
Remove these zone pairs from the computation of A_ik for each zone.

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