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

Summary of Impacts
Transportation’s Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
Highway operations and management strategies include traffic management (signal
coordination, ramp metering, faster clearance of incidents), real-time traveler information,
and highway bottleneck relief. These strategies have an emission reduction effect by
reducing congestion and smoothing traffic flow, thus reducing inefficient vehicle operation.
However, the improved conditions they create also result in some amount of additional travel
and thus additional emissions through a phenomenon known as induced demand (see section
4.1.4 and Appendix A). Whether the emission-increasing effect of induced demand
outweighs the emission reducing effect of congestion reduction and traffic smoothing
depends on the amount of induced demand and on the particulars of the strategy.
Quantifying the magnitude of induced travel demand and the impact of this extra traffic on
travel speeds and traffic flow is particularly challenging, and further research is needed. 1 An
additional source of uncertainty relates to the vehicle technology or fuels used by future
vehicles, which could further reduce any benefits from congestion reductions strategies. 2
Because of these uncertainties, numerical estimates are not included for highway operations
and management strategies in Volume 1 of this report, though outside estimates are included
here in Volume 2 both in this summary section (directly below) and in the more detailed
Section 4.2. The estimates presented in this section are taken from outside studies and should
be considered illustrative. Despite these uncertainties, it is worth noting that congestion relief
strategies yield significant cobenefits by reducing the time travelers spend on the road due to
congestion and delay. Traffic management strategies are technologies and practices to
reduce congestion and smooth traffic flow through improved traffic operations and
management, such as signal coordination, faster clearance of incidents, and freeway ramp
metering. Outside studies have incorporated simplified assumptions regarding induced
demand to estimate the GHG impacts of highway operations and management strategies.
Analysis for the Moving Cooler study (Cambridge Systematics, 2009) suggests that widespread
deployment of traffic management strategies would reduce transportation GHG emissions by
a modest 3 amount, 0.5 percent or less in 2030. The study found that in 2050, there would still
be net emission reductions.
1 U.S. DOT is designing research to provide a better understanding of the role of induced demand
in offsetting GHG improvements from congestion reduction strategies.
2 If vehicle technology evolves to rely heavily on electric-drivetrain technology (such as hybrids, batteryelectrics,
or fuel-cell vehicles), the energy and GHG benefits of congestion reduction will be greatly
reduced or disappear altogether. Because the efficiency of these vehicles typically is not reduced at
lower speeds, their emission levels stay relatively constant even under congested conditions. In
addition, for any highway operations strategy, future improvements in fuel efficiency or reductions in
the carbon content of fuels beyond the baseline assumed in this report will lead to lower absolute
benefits from any of these strategies.
3 In this report, when referring to individual strategy effects, “modest” refers to reductions in CO2e
emissions of less than 0.5 percent of total transportation emissions, or 12 mmt in 2030; “moderate”
to reductions in the range of 0.5 to 2.5 percent of total transportation emissions, or 12 to 60 mmt in
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