A Longitudinal Analysis on the Environmental and Travel Behavior Effects in Suburban and Urban Areas from Transit-Oriented Development in the Denver Region

Abstract

Transit-oriented development (TOD) is an established method for reducing greenhouse gas (GHG) emissions and vehicle miles traveled (VMT). TOD strategies include higher density residential housing with retail and commercial mixed-use, and walkable neighborhoods coupled with high-capacity transit services such as commuter or light rail service. The existing literature on TOD and environmental benefits has mostly centered on large, urban core areas, with limited studies evaluating the impacts in suburban communities which this research addresses. The objectives of my research were threefold: 1) to evaluate how TOD affects the environment, specifically carbon dioxide (CO2) and VMT, differently in suburban and urban TOD zones; 2) to evaluate how commuter travel modes in TOD zones change by age and density level; and 3) to understand how changes in VMT correlate with ridership, density, and commuter travel modes. My research plan included a longitudinal analysis of two rail lines (the A and H Lines) in the Denver Regional Transit District jurisdiction over an approximate 10- and 20-year period between 2009–2022 and 2000–2022 using U.S. Census data. A half-mile buffer around each rail station was used as the TOD zone and was classified by density level and age. I calculated CO2 and VMT based on the average kilograms (kg) of CO2 emission factors per passenger mile and commute distances by travel mode (automobile, bus, rail, bike, and walk) and period. I hypothesized that urban TOD zones had substantially higher reduced VMT than suburban TOD zones; non-motorized travel alternatives (biking and walking) changed at a higher rate than public transit; and that substantial changes in VMT and traveler behavior required at least 10 years of rail service and maturity. The results revealed that the urban TOD zones saw a reduction in VMT per capita that was two to three times higher than their suburban equivalents. The TOD zones with the oldest stations (built in 1994) saw declines in VMT per capita that were more than double those built in 2006 and 2016. I also determined that it took at least 10 years to reduce the automobile mode share by 10 percentage points. Overall, between 2009–2022, both lines combined saw an increase in CO2 emissions and total VMT of 25% and 35%, respectively, but also saw a decline in VMT per capita of 9%. The significance of these findings stems from the strong correlation between the change in VMT and TOD variables. The policy implications stress the value of cohesive transportation and land use plans at regional and local levels. Moreover, a longitudinal analysis, or before-and-after study, such as this underscores the importance for TOD programs and projects to be monitored and evaluated as they can yield vastly different results by location and typology, and when leveraged with census data can deepen the understanding of commuter travel behaviors by demographic groups.