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

Transportation’s Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
An alternative “bottom-up” approach to estimating GHG benefits from signal
optimization can be developed based on a signal timing evaluation conducted in Portland,
Oregon. This study estimated that approximately 50 metric tons of CO2 were saved each
year per traffic signal retimed in the city, although the study did not account for induced
demand effects. With approximately 3,300 traffic signals in the State, of which 70 percent
could benefit from retiming, the potential Statewide CO2 savings would be 115,000 metric
tons in one year (Peters, McCourt, and Hurtado, 2009). Further extrapolating these results
to the entire United States, based on ITE’s estimate of 300,000 traffic signals, provides an
estimated benefit of 11 mmt CO2e annually. This is somewhat higher than the top-down
nationwide estimate for developed for Moving Cooler, which assumed phased
implementation over time and a lower benefit per signal and accounted for induced
demand. 12
Another important consideration is that the future effectiveness of traffic management
strategies will depend upon vehicle technology. With hybrid and electric-drive vehicles,
driving cycles have a much lesser impact on fuel consumption and emissions. For
example, today’s hybrid-electric vehicles have urban fuel economy ratings that typically
exceed highway fuel economy ratings. All of the benefits estimates cited above assume
current vehicle technology, albeit with improved fuel economy under the baseline
assumptions described in Appendix A of this report.
As previously noted, State and local agencies are at varying stages of deploying traffic
management technologies. Some technologies (such as signal coordination and incident
management) are well-developed, and further widespread implementation could occur
within the next 5 to 10 years if financial, institutional, and political barriers are overcome
thereafter, for a total long-run elasticity of -0.8 (see Appendix A). An elasticity of -0.8 means that
a 1 percent decrease in total travel costs will result in a 0.8 percent increase in VMT.
11 The 10 mmt figure reflects an adjustment to the Moving Cooler study results to account for the fact
that the induced demand effect may have been overstated compared to the estimates for capacity
expansion and bottleneck relief strategies, which were based on different parameters. The Moving
Cooler study estimated that 93 to 95 percent of the 2030 benefits of traffic management strategies
would be offset by induced demand, but only 70 to 80 percent of capacity expansion and
bottleneck relief strategies. The difference appears to be at least in part due to the fact that
induced demand parameters for traffic operations strategies were not updated to be consistent
with the final parameters used for capacity expansion and bottleneck relief strategies. Therefore,
a range of induced demand effects is shown here, and the range of GHG benefits for the “with
induced demand” case is adjusted to show a maximum benefit that reflects only a 70 percent
reduction compared to the “without induced demand” case.
12 The Moving Cooler benefits for signal control management are based strictly on type of signal
control, in increasing order of sophistication: pre-timed/actuated, central control, and real time
traffic adaptive. Signals are assumed to be upgraded one level, with a coverage rate of up to 2,000
miles of roadway per year. Delay savings per signal without induced demand are estimated to be
in the range of 5 to 12 percent, much smaller than the 15 to 40 percent reported in the Portland
study. These smaller delay savings account for the fact that at high levels of delay (as the entire
intersection approaches exceeds capacity), improved signal timing will not result in GHG benefits
because delay is controlled by capacity, rather than signal timing.
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