“Spillover” effects reinforce VMT effects from transit and parking subsidies

By Eric Sundquist

We know that employer-provided free parking tends to increase auto-commute trips and that employer-provided transit passes tend to reduce auto-commute trips.

Research is less clear about the effect on vehicle-miles traveled (VMT), however, in part because we don’t know whether or how such employment practices might affect non-work travel. For example, auto commuters might have more opportunity to drive to non-work destinations because they have cars available at work. And transit commuters might choose to live in higher-accessibility neighborhoods.

A new paper by Eun Jin Shin of Sungkyunkwan University, using travel survey data from the Seattle area, sheds new light on the VMT question.

Compared to the median VMT of 19.2 miles per day, employees who were offered free or subsidized parking drove an extra 3.1 miles to commute and 1.0 miles for non-work trips. Employees who were offered free or subsidized transit drove 3.2 miles less for commuting and 1.2 miles less for non-work trips. A complementary model controlling for built-environment variables reduced the magnitudes of the effects but showed the same direction for each.

The study was motivated in part by research on similar “spillover” effects of telecommuting, which tend to undercut the benefits in reduced auto travel for commuting. In the case of transit and parking subsidies, however, it appears that the spillover effects tend to reinforce the commuting VMT effects.

Eric Sundquist is Director of SSTI.