Right-size parking calculator showcased at TRB annual meeting

By Chris McCahill

How much parking is just the right amount? Developers and policymakers have mulled over this question for decades. Formulas are often passed along from town to town, or estimates are pulled from the Institute of Transportation Engineers’ Parking Generation report, now in its fourth edition. Despite the tremendous effort behind these estimates, however, critics have pointed out their failure to reflect the considerable variability in demand, particularly in compact areas with different travel options or when parking is priced. King County Metro in Washington state has taken a big step toward better understanding this variability through a study conducted with the Center for Neighborhood Technology, which they presented at a session focused on parking impacts at this year’s annual meeting of the Transportation Research Board in Washington, DC.

The study, funded through FHWA’s Value Pricing Pilot Program, began in 2012 when researchers observed overnight parking use at more than 200 multi-family housing locations throughout the Seattle area. Parking utilization ranged from roughly zero to two parking spaces per housing unit. After analyzing more than 100 independent factors, the researchers chose seven factors to include in a model explaining 81 percent of the variation among the sites. An earlier overview of the research showing key factors—including price, transit access, and land use density—was published in the ITE Journal and is available on the King County Metro website.

The parking demand model also forms the basis of the Right Size Parking Calculator, available online with an interactive map showing parking demand in and around Seattle. Users can estimate residential parking demand based on unit size, rent costs, parking price, population density, concentration of jobs, and access to transit. The calculator outputs the number of parking stalls needed, construction and maintenance costs, the influence of bundled parking costs, and the associated trip generation and emissions estimates. This work will be instrumental in revising zoning codes, involving more stakeholders in the development process, and getting parking supplies right. King County Metro is currently leading policy reform efforts in four cities based on this work and engaging several partners to develop parking pricing strategies for transportation demand management.

Chris McCahill is a Senior Associate at SSTI.