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Case Study:
Montgomery County, Maryland
Conclusions
Limitations
Accessibility measures such as those used by Montgomery County do require some general cautions. These include:
- Summing the destinations accessible within an X-minute travel time (in this case, 45 minutes) is one of the easiest accessibility measures to understand intuitively, but also oversimplifies the concept. For example, the measure does not distinguish the benefits of proximity to jobs that are 20 minutes from home vs. those that are 40 minutes from home. The Tren Urbano
and San Francisco Bay Area
case studies illustrate the use of a gravity-based measure, which is less intuitive but eliminates the problem of choosing an appropriate time threshold. (In the gravity-based measure, jobs are weighted by travel time.)
- Employment is taken to represent activity destinations (origins in Montgomery County's case, due to the nature of their travel demand model) in a generic sense, and accessibility for specific destinations such as retail or community facilities is not identified.
- The measure does not identify any "spatial mismatches" that are based on employment or household characteristics. For example, if low-cost housing is not available near low-wage jobs, people in low-income categories may have fewer employment opportunities or may need to travel long distances. The Appendix to the San Francisco Bay Area case study shows how socioeconomic data can be matched with employment types to measure differences in accessibility across socioeconomic groups.
One specific caution from this case study is that Montgomery County did not include land use feedback in their 2020 analysis, although different land use scenarios were tested in the 2050 scenarios. The county's assumption was that development will generally be fixed in accordance with their comprehensive plan. The county intends to account for feedback between transportation supply and land development in the future, recognizing that developers still have some flexibility within the master plan framework.
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