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Access Type

Open Access Thesis

Document Type


Degree Program

Civil Engineering

Degree Type

Master of Science in Civil Engineering (M.S.C.E.)

Year Degree Awarded


Month Degree Awarded



Roundabouts have been gaining acceptance by city planners and traffic engineers alike. The number of roundabouts in the U.S. has risen to roughly 3,700 over the last 20 years. Due to this rise in use, there has been a need to better understand the characteristics roundabouts have and how they affect the performance of transportation networks. Sustainability-related advantages of roundabouts are of particular interest.

Field data were collected for the purpose of determining the critical gap at a double single-lane roundabout and whether that value is substantially different than other types of intersections like stop-controlled intersections. Critical gap values are used in micro-simulation software and the more accurate the input data are the more accurate the model behaves compared to reality.

VISSIM, a micro-simulation software was used to develop models for two roundabouts, a single–lane roundabout and a double single-lane roundabout. The single-lane roundabouts was previously a signalized intersection and the double single-lane roundabout used to be two stop-controlled intersections. Models of both the current conditions and the previous conditions of these two locations were developed for both the morning and evening peak periods of demand.


Due to lack of data for the previous condition entry volumes, the current volumes were used for the previous condition models. These eight models were used for a before and after comparison of emission levels to determine what types of intersections are most sustainable in terms of emissions. In addition, the after models were used for a safety evaluation of the two roundabouts.

The after condition models of the two locations were used to produce vehicle trajectory files that can be interpreted by the Surrogate Safety Assessment Model (SSAM), a conflict estimation software developed by the Federal Highway Administration. This was done to determine if SSAM is an adequate tool for estimating the total number, type, and location of conflicts that occur at both a single-lane roundabout and a double single-lane roundabout. To validate findings the SSAM data were compared to a safety analysis conducted on video data collected at each site.

Another field study was conducted that utilized the Intelligence to Drive (i2D) device that plugs into a vehicle’s on-board diagnostic port. This device can produce a vehicle’s speed profile and estimate the level of emissions produced while the car is running. The combination of this device with video data were used to attempt determining the relationship between pedestrian activity and the level of emissions at roundabouts.

This research showed that the critical gap at a roundabout is significantly different than the default values used in micro-simulation. The stop sign controlled intersections, which were the before condition of the double roundabout produced higher emissions than the double roundabout. The signalized intersection, which was the before condition for the single-lane roundabout, produced considerably fewer emissions than the single-lane roundabout. SSAM was shown to be a reliable tool for estimating the total number, type, and location of conflicts that occur at both locations. The relationship between pedestrians and high emission levels at roundabouts was difficult to determine due to the small number of data that were collected. However, based on the speed profiles it was shown that the presence of pedestrians at roundabouts could potentially lead to more aggressive deceleration rates causing them to produce higher levels of emissions.

Future research should include: 1) continuing all the studies presented above for a variety of roundabouts with various geometric and traffic conditions to determine if the observed patterns hold true, and 2) collecting more data with the i2D device to determine a clear relationship between the presence of pedestrians at roundabouts and the associated emission levels.


First Advisor

Eleni Christofa

Second Advisor

Micheal Knodler