Transportation's Role in Reducing U.S. Greenhouse Gas Emissions ...

Transportation’s Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
A different study for the U.S. Public Interest Research Group (USPIRG) concluded that
transit reduced GHG emissions by nearly 26 mmt CO2e in 2006. This study used slightly
different assumptions than the APTA study, including accounting for lower fuel economy
of automobile trips removed during congested periods, and additional fuel savings from
reduced highway congestion. The study also accounted for “leveraged” benefits from
more compact land use patterns (Baxandall, Dutzkik, and Hoen, 2008).
For any data using national average GHG emissions by mode or total GHG reductions, the
caveat must be included that the results are heavily influenced by the most heavily used
and productive services in a few major cities, such as Boston, Chicago, New York,
Philadelphia, San Francisco, and Washington, D.C. For example, the USPIRG study found
that nearly half of the GHG reductions from transit—11.8 mmt CO2e—were from New
York State alone, with another 10.4 mmt from six other States; and that 26 States saw
reductions of less than 0.01 tonnes or even slight increases in GHG from their transit
services. It is not clear whether future transit expansion will result in the same level of
GHG reduction productivity.
The net GHG effect of future improvements to transit will depend on factors including the
amount of new ridership attracted from automobiles, number of passengers per transit
vehicle, relative efficiency of transit vehicles and automobiles, and carbon content of fuels.
Transit ridership growth will also depend on a variety of factors that are difficult to
predict, such as fuel prices, economic growth, socioeconomic and demographic trends, and
land use patterns. One recent study developed three possible scenarios for future
ridership growth: a continued 2.4 percent increase; a 3.52 percent increase, which
represents a doubling of transit ridership in 20 years, and would require aggressive
strategies to grow ridership; and a target growth rate of 4.63 percent (TCRP, 2008). 17
Based on these three scenarios, the Moving Cooler study estimated future GHG reductions
from additional transit investment. The study assumed that load factors would increase
from 10.5 passengers per bus in 2006 to 12 passengers per bus in 2030. Savings were
estimated separately for three different strategies: reduced fares; improvements to
18
headways and level of service; and expanded fixed-guideway services. The study also
17 The target growth rate assumes a variety of potential factors that could cause public
transportation ridership to grow more rapidly, including higher energy prices, implementation of
policies to promote development around public transportation services, increased concern for the
impacts of climate change, and stronger emphasis on the relationships between land use and
transportation.
18 Fare reductions range from 25 to 50 percent. For LOS improvements, signal prioritization,
limited-stop service, and other enhancements are implemented to improve travel speeds by 10 to
30 percent. Increased service levels and fixed guideway expansion occur at a rate of 2.4 to
4.67 percent annually, consistent with ridership forecast scenarios; investments are assumed to be
targeted in areas with at least 4,000 persons per square mile or that otherwise facilitate high
passenger loads per vehicle revenue-mile.
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