First Advisor

Miguel Figliozzi

Date of Publication


Document Type


Degree Name

Master of Science (M.S.) in Civil & Environmental Engineering


Civil and Environmental Engineering




Traffic congestion -- Environmental aspects, Air quality management, Transportation -- Management -- Environmental aspects



Physical Description

1 online resource (xix, 175 p.) : ill. (some col.)


Policy-makers, transportation researchers, and activists often assume that traffic congestion mitigation results in reduced vehicle emissions without proper justification or quantification of the benefits. If congestion mitigation is going to be tied to air quality goals, a better understanding of the impacts of traffic congestion on motor vehicle emissions is needed. This research addresses that need by investigating under which circumstances the commonly held assumption linking congestion mitigation to emissions reductions is valid. We develop and apply a mathematical framework to study the trade-offs between vehicle efficiency and travel demand that accompany travel speed changes. While the exact relationships among emissions, travel speed, and travel demand vary with location and pollutant, several consistent results arise. The potential for marginal emissions rate reductions through average travel speed adjustments is small for speeds between about 25 and 70 mph. Emissions rate sensitivity to speed increases with the fraction of heavy-duty vehicles and for certain pollutants (gaseous hydrocarbons and particulate matter), and decreases with the fraction of advanced-drivetrain vehicles, such as electric and gas-electric hybrid vehicles. But travel volume is also a key consideration for the total emissions impacts of congestion and congestion mitigation. While travel speed increases are generally expected to increase efficiency, they are also expected to increase vehicle travel volume as a result of induced demand. To explore efficiency and volume trade-offs we look at emissions break-even conditions for average speed and travel demand elasticity. Depending on the pollutant and the vehicle fleet, total emissions are only expected to decrease with increasing travel speed for initial conditions of both low demand elasticity and low average speed. Thus, higher levels of congestion do not necessarily increase emissions, nor will congestion mitigation inevitably reduce emissions. This result includes projects that seek to increase vehicle throughput from existing roadway supply through better traffic management and operations. Congestion mitigation through reduced vehicle volumes, on the other hand, presents the opportunity for additive emissions benefits through efficiency improvements and total Vehicle Miles Traveled (VMT) reductions. Comparing capacity-based congestion mitigation strategies with alternative emissions reduction strategies we show that where emissions reductions are possible through speed increases, the emissions benefits are likely to be more easily and cost-effectively attained by other strategies. A sketch analysis of vehicle-class segregated facilities shows that truck-only lane strategies consistently out-perform general-purpose/mixed-flow lane strategies in terms of emissions reductions. An analysis of several congestion-related performance measures shows that for reflecting emissions impacts, VMT is an essential component of performance. Thus, alternative congestion metrics such as total/excess travel distance and travel time are preferable emissions performance indicators to speed or distance-normalized delay. The Travel Time Index, in particular, poorly reflects emissions changes on congested roadways. This thesis offers several original contributions to the body of knowledge regarding congestion and emissions. First, it describes a parsimonious conceptual framework for assessing the effect of congestion on emissions. Then from that framework, several simple and original equations are presented which can be used for sketch-level planning to estimate emissions impacts from congestion mitigation. Finally, application of the framework provides quantitative support for the decoupling of congestion and emissions mitigations.


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Portland State University. Dept. of Civil & Environmental Engineering

Persistent Identifier