United States Department of Transportation
i
Accelerating Roundabout Implementation in the United States - Volume III of VII: Assessment of the Environmental Characteristics of Roundabouts
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2015-09-01
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[PDF-2.13 MB]
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Abstract:This volume is third in a series of seven. The other volumes in the series are: Volume I – Evaluation of Rectangular Rapid‐ Flashing Beacons at Multilane Roundabouts, Volume II – Assessment of Roundabout Capacity Models for the Highway Capacity Manual Final Report, Volume IV – A Review of Fatal and Severe Injury Crashes at Roundabouts, Volume V – Evaluation of Geometric Parameters that Affect Truck Maneuvering and Stability, Volume VI – Investigation of Crosswalk Design and Driver Behaviors, and Volume VII – Human Factor Assessment of Traffic Control Device Effectiveness. These reports document a Federal Highway Administration (FHWA) project to investigate and evaluate several important aspects of roundabout design and operation for the purpose of providing practitioners with better information, leading to more widespread and routine implementation of higher quality roundabouts. This research sought to develop a simple methodology for estimating pollutant emissions generated at roundabouts and comparing them to those generated at signalized intersections. The premise for this research is that the environmental performance is tied to the operational performance of the roundabout, with emission levels being sensitive to levels of traffic volume and congestion. The research resulted in empirically-based vehicle activity models and emissions models for roundabouts and signalized intersections. The activity models take into account driver behavior, traffic conditions and infrastructure design, and were produced under consideration of three trajectory types though the intersections as a function of the demand volume and, when appropriate, signal timing. The emissions models were based on Portable Emissions Measurement System (PEMS) data, and consider vehicle specific power (VSP) distributions for vehicles measured at roundabouts and signalized intersections. Combining the two models via the VSP approach resulted in emission estimates near the intersections at various temporal and spatial scales. All models are macroscopic in nature and were implemented in a spreadsheet-based emissions computational engine. The analysis approach is most suitable in a planning-level assessment of roundabouts in relation to traffic signals. Results indicate that emissions rates at roundabouts tended to be lower than those at intersections (1) for demand-to-capacity (d/c) ratios less than 0.7, (2) for higher d/c ratios if signal progression at the intersections was poor, and (3) in general for oversaturated periods.
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