Accelerating Roundabout Implementation in the United States - Volume III of VII: Assessment of the Environmental Characteristics of Roundabouts

Salamati, Katy; Rouphail, Nagui M.; Frey, C.; Schroeder, Bastian J.; Rodegerdts, Lee A. · 2015 · ROSA P / United States. Department of Transportation. Federal Highway Administration. Office of Safety

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Summary

This report, Volume III of a seven-part series sponsored by the Federal Highway Administration (FHWA), addresses the environmental performance of roundabouts compared to signalized intersections. While roundabouts are recognized for safety benefits, their environmental impact has been debated, with prior studies yielding mixed results due to a lack of empirical field data. The research aimed to develop a simplified, macroscopic methodology for estimating and comparing pollutant emissions at both intersection types, grounded in actual vehicle trajectories rather than simulation assumptions. The study posits that emission levels are directly tied to operational performance, specifically traffic volume and congestion-induced stop-and-go patterns. The methodology combined empirical data collection with modeling. Researchers utilized Portable Emissions Measurement Systems (PEMS) and GPS to record second-by-second vehicle trajectories and tailpipe emissions for over 1,980 vehicles. They categorized intersections into high-speed (≥35 mph) and low-speed (<35 mph) bins. The approach involved two primary components: activity models predicting the frequency of three trajectory types (no-stop, one-stop, and multiple-stops) based on demand-to-capacity ratios and signal timing; and emissions models using Vehicle Specific Power (VSP) distributions derived from the PEMS data. These models were integrated into a spreadsheet-based computational engine to estimate hourly emissions for carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx), and carbon dioxide (CO2) for passenger cars and trucks. The results indicate that roundabouts generally produce lower emissions than signalized intersections under specific conditions. Emissions at roundabouts were lower when the demand-to-capacity (d/c) ratio was less than 0.7. For higher d/c ratios, roundabouts still outperformed signalized intersections if the signals had poor progression. Additionally, roundabouts demonstrated lower emissions during oversaturated periods. The study found that the environmental advantage of roundabouts is contingent on traffic volume and signal efficiency, with the VSP-based approach providing a robust link between vehicle dynamics and emission rates. The significance of this work lies in providing practitioners with a validated, planning-level tool to assess the environmental trade-offs of intersection design. By establishing empirically based models, the report resolves inconsistencies in previous literature and offers a deterministic framework for comparing roundabouts and signals. This supports the FHWA’s goal of accelerating roundabout implementation by quantifying their environmental benefits alongside their known safety advantages, particularly in scenarios where traffic congestion or poor signal coordination would otherwise increase emissions.

Key finding

Emissions rates at roundabouts tended to be lower than those at signalized intersections for demand-to-capacity ratios less than 0.7, for higher demand-to-capacity ratios if signal progression at the intersections was poor, and in general for oversaturated periods.

Methodology

field_study

Sample size: 1980

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

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• intersection design