Marginal emission factors for public transit: Effects of urban scale and density.

The objective of this study is to determine the relationship between fundamental urban scale characteristics (population, area, density) and marginal emission factors (MEF) for public transit. Emissions intensity of travel is typically examined using average emission factors (AEF), but MEF (how emissions change with travel volume) are more important for understanding the effects of interventions. MEF and AEF are estimated and compared for transit systems across the U.S. using panel data from 376 urban areas over 27 years. Results show that both MEF and AEF vary substantially across cities and decrease with urban population, area, density, and transit system extent - but AEF are around 50% more sensitive to urban scale. The distinction between MEF and AEF is especially important for bus transit in smaller, less dense cities. Marginal analysis shows that mode shift from private vehicles to transit should be encouraged, even where average emissions from transit are higher.