Evaluating Parking Demand Management Interventions Using a Randomized Controlled Trial

This paper presents a trial aimed at reducing parking demand at a large urban employer through an informational campaign and monetary incentives. A six-week randomized controlled trial was conducted with (N=2000) employees at the Massachusetts Institute of Technology who fre- quently parked on campus. Split into four arms of five hundred each, one group received weekly informational ‘Commuter Digest’ emails highlighting MIT’s various new transportation bene- fits; a second group received monetary rewards proportional to any reduction in parking they ex- hibited during the prior week; a third group received both interventions, while a control group was monitored with no intervention. Success was measured as the extent to which drivers de- creased their frequency of parking and increased their use of alternative modes during and after the campaign. It was found that while the combined treatment group contained the highest num- ber of top-performing participants, no statistically significant differences-in-differences were ob- served amongst the treatment arms as a whole. A post-experiment survey indicated a widespread increase in awareness of employer transportation benefits, and a much larger stated shift from driving towards transit than was supported by revealed data. Survey results suggested that while intent to reduce car use existed, complaints of insufficient quality of transit service and relative convenience of driving suppressed long-term modal shifts.