Toward greener transit: Carbon-efficient density thresholds for public transit vs. private vehicles

Document Type

Journal Article

Publication Date

2025

Subject Area

place - north america, place - low density, place - urban, technology - emissions, land use - impacts, land use - urban density, planning - environmental impact

Keywords

Public transit, CO2 emissions, population density

Abstract

Despite its potential as a sustainable transportation mode, public transit in many low-density urban and suburban areas in the U.S often generates more CO2 emissions per passenger-mile than privately operated vehicles (POVs), primarily due to low ridership and passenger loads. Using Gradient Boosting Decision Trees (GBDT) and spline regression models, this study investigates the non-linear relationship between population density and the relative carbon efficiency of transit compared to POVs across the 136 largest U.S. urban areas. This study found that the minimum density required for public transit to be more carbon-efficient than driving, while controlling for other factors, is around the lowest 10th percentile of population-weighted density (PWD)—approximately 3.4 persons per acre. Further, a critical density threshold was identified at around the 80th percentile of PWD, about 8.6 persons per acre, beyond which the positive impact of population density on transit’s carbon efficiency significantly shifts up.

Rights

Permission to publish the abstract has been given by Elsevier, copyright remains with them.

Comments

Transportation Research Part D Home Page:

http://www.sciencedirect.com/science/journal/13619209

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