Case Study:

San Francisco Bay Area, California

Conclusions

General Limitations

The MTC analysis represents a state-of-the-practice comparison of the distributional implications of alternative regional transportation plans. While the results provide valuable insights, there are nonetheless certain limitations that need to be recognized. These limitations are common to other accessibility analyses, such as those documented in the Montgomery County and Tren Urbano case studies. Specifically:

• The analysis by necessity assumes that the distribution of population by income, race, mobility limitations, and other measures used to define "disadvantaged populations" will remain constant in the future (in this analysis, in 2020 as compared to 1990, the most recent census year). As neighborhoods evolve over time, this is not likely to be the case. On the other hand, methods have not been developed to forecast demographic and socioeconomic population shifts on a small-area basis, and doing so accurately will be difficult. Household income and auto ownership are variables that could potentially be forecast to assess future-year equity.

• The accessibility measures do not account for potential "spatial mismatches" in housing and employment, for example, a lack of affordable housing near lower-paying jobs. A recent study by researchers in the Bay Area (see Appendix) illustrates how the accessibility index can be refined to include matching of residents to jobs by occupational class. This technique, known as "occupational matching," can easily be applied using census data for a base-year analysis. Conducting a similar analysis for a forecast year would be difficult, however, given that methods do not currently exist for forecasting the distribution of population or employment by occupation type.

• The accessibility measures do not provide an explicit way of valuing transit vs. automobile accessibility benefits, or of matching benefits by mode to mobility needs. For example, the definition used to identify "disadvantaged" vs. "not disadvantaged" neighborhoods does not directly correspond with the ability (or willingness) to own a car, and thus the importance placed on transit vs. automobile benefits.

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