New tool estimates transit’s effect on VMT and emissions

By Eric Sundquist
Transit often fails to get the credit it deserves for reducing traffic and emissions. In most U.S. cities, transit’s mode share is in the single digits, so the direct effect of ridership seems small. And while it’s clear that even in places with low mode share transit plays a role in raising densities—and thereby reducing travel distances—this relationship has been hard to quantify; conventional demand models simply take land use as an input.
Filling this gap is a report and tool from TRB’s Transit Cooperative Research Program, “Quantifying Transit’s Impact on GHG Emissions and Energy Use—The Land Use Component.”
The authors examine VMT, fuel consumption, and greenhouse gas emissions from transportation across the nation and in metro areas. The big takeaway: While both transit-related densification and direct replacement of car trips tend to lower VMT, energy use, and emissions, the latter effect dominates. “The average ratio of land use benefits to ridership benefits across all U.S. cities is 4:1, but the ratio varies substantially across different urban areas,” the report finds.
“Taking the entire U.S. urban population in aggregate, gross population densities would be lower by 27 percent without transit systems to support compact development. In other words, U.S. cities would consume 37 percent more land area in order to house their current populations,” the report states. “The land use effect of existing transit makes U.S. cities more compact.”
The report provides findings for many areas, but it also comes with a calculator based on data for those areas. It is the opposite of demand models that force the user to input land use; the tool calculates transit’s densifying effect. For example, at SSTI Central, a.k.a. metro Madison, WI, it finds that the existing transit system (Madison Metro, with an annual ridership of 15 million) has the effects shown in Figure 1.

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Figure 1. Click for larger image.

The tool also estimates the effects of increased transit service, by region or corridor, and station area improvements. It also calculates the effects of additional freeway and surface roadway lane-miles. Based on the research documented in the report, these all add VMT, energy consumption, and GHG emissions. Running a few examples, again in Madison, a 10 percent increase in transit service reduces annual GHG by 13 million pounds, while a 10 percent increase in highway lane-miles increases GHG by 69 million pounds.
Eric Sundquist is Managing Director of SSTI.