Accessibility vs Demand: Bringing a Map to a Modeling Fight

When planners quantitatively prioritize transit investments, they can use either accessibility measures or ridership measures, or some combination of the two. Accessibility calculators count how easily people are able to reach opportunities (e.g. jobs) by transit, and ridership models attempt to forecast likely passengers. Historically, agencies have used both methods for different tasks, though recent criticism of ridership models coupled with the availability of new web-based accessibility calculators has made a comparison of each method important for planners to understand.

As part of Transportation Camp DC 2018, we challenged Conveyal to debate the strengths of accessibility calculators, and Anson Stewart came forward. I defended ridership models. During the debate, we showed off Conveyal's Analysis software as well as gave the first demonstration of CityCast, our web-based travel demand model that estimates ridership.

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Introducing CityCast

To help cities figure out where they need roads, transit, buildings, sidewalks, and other infrastructure, transportation and urban planners must understand three components of mobility:

  1. households and the people in them,
  2. firms and their employees and customers, and
  3. the movements between them.

Traditionally, planners collect information about these using small-sample household surveys.

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