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

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.