What Contributes to Successful Commute Trip Reduction in the State of Washington? A Focus on Transit Accessibility

Abstract

Washington State passed the first version of the Commute Trip Reduction (CTR) Law in 1991 and the resulting CTR program has been run by the Washington State Department of Transportation (WSDOT). The primary goal of the program was to reduce congestion by promoting the use of alternative commuting modes to SOV. That intention has remained the same today, although much of the implementation has shifted from the state to city level. Rather than setting state-wide goals for all work sites, WSDOT now primarily serves as an advisor to local jurisdictions in charge of creating their own CTR plans. Existing data shows a concentration of sites in downtown Seattle meeting their state-wide CTR goals while work sites in suburban jurisdictions struggle to meet the same goal. The purpose of this research is to determine the primary contributors to a site’s ability to reduce SOV mode split and meet its CTR goal—including the effect of transit accessibility. Specifically, this research attempts to answer two questions: 1) Do the factors for which WSDOT currently collects data significantly contribute to a site’s success to reduce its SOV mode split and meet its CTR goal? and 2) How can transit accessibility be factored into these CTR goals? A transit accessibility indicator was developed by summing the number of transit stops within a quarter-mile radial buffer of a work site. WSDOT’s 2015/2016 employee CTR survey data as well as King County Metro transit data were used as the basis for this research. Each site’s transit accessibility score was compared to its SOV mode split, and an exponential regression resulted in an R-squared value of 0.71 for a sample of 294 CTR work sites. The transit accessibility indicator, along with other variables identified in WSDOT’s employee CTR survey, were then incorporated into logit and multivariable regression models that explain, respectively, whether a site has achieved an 18% reduction in calculated VMT set by WSDOT in 2007/2008, and its SOV mode share. The results of the analysis show the transit accessibility indicator having a significant association with both a work site’s ability to meet its VMT goal and its SOV mode split. Preliminary discussions with WSDOT indicate the analysis will inform future policymaking as wells as be useful for TDM plan evaluation and department resource allocation. The analysis may also alleviate some concerns from suburban jurisdictions and employers who feel that alternative modes to SOV are not as widely available in areas with lower population and employment densities compared to central business districts. This research demonstrates that an “expected” SOV percentage can be identified for each work site depending on the level of transit accessibility, which could then be used as a benchmark for evaluation within the CTR program.