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Suggested Citation:"Summary ." National Academies of Sciences, Engineering, and Medicine. 2014. Evaluating the Performance of Corridors with Roundabouts. Washington, DC: The National Academies Press. doi: 10.17226/22348.
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Suggested Citation:"Summary ." National Academies of Sciences, Engineering, and Medicine. 2014. Evaluating the Performance of Corridors with Roundabouts. Washington, DC: The National Academies Press. doi: 10.17226/22348.
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Suggested Citation:"Summary ." National Academies of Sciences, Engineering, and Medicine. 2014. Evaluating the Performance of Corridors with Roundabouts. Washington, DC: The National Academies Press. doi: 10.17226/22348.
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Page 4
Suggested Citation:"Summary ." National Academies of Sciences, Engineering, and Medicine. 2014. Evaluating the Performance of Corridors with Roundabouts. Washington, DC: The National Academies Press. doi: 10.17226/22348.
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Page 4
Page 5
Suggested Citation:"Summary ." National Academies of Sciences, Engineering, and Medicine. 2014. Evaluating the Performance of Corridors with Roundabouts. Washington, DC: The National Academies Press. doi: 10.17226/22348.
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Page 5

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Evaluating the Performance of Corridors with Roundabouts Page S-1 SUMMARY EVALUATING THE PERFORMANCE OF CORRIDORS WITH ROUNDABOUTS This study identified 58 roundabout corridors in the United States in 2011. There is diversity among these corridors in terms of length, roundabout spacing, number of lanes, surrounding land use, previous control (if not a new facility), and reasons for the selection of roundabouts. From this set of 58 corridors, nine were selected for detailed study: MD 216 in Scaggsville, Maryland La Jolla Boulevard in San Diego, California Old Meridian Street in Carmel, Indiana Spring Mill Road in Carmel, Indiana Borgen Boulevard in Gig Harbor, Washington SR 539 in Whatcom County, Washington Golden Road in Golden, Colorado Avon Road in Avon, Colorado SR 67 in Malta, New York Safety evaluations of roundabouts from other research projects indicate conversion of individual signalized and two way stop controlled intersections to roundabouts results in a reduction in the frequency and severity of crashes. Although safety data were not collected for this project, corridor interviews revealed that safety improvements with roundabout applications were generally consistent with documented research. As a result, the research team found no evidence suggesting the safety performance of a roundabout in series differs from the safety performance of an isolated roundabout. The following subsections highlight key findings for four topic areas: Corridor interviews; Travel time collection, analysis, and modeling; Comparison to equivalent signalized corridors; and Development of a Corridor Comparison Document. Corridor Interviews Interviews with the owners of each of the nine corridors provided an insight into the creation and history of these roundabout corridors, agency and community goals for the corridors, and their effectivness at meeting those goals. The interviews revealed a variety of contexts in which roundabout corridors have come into being. Some of the corridors were designed and constructed in their entirety at one time; others started with one or two roundabouts and more were Summary

Evaluating the Performance of Corridors with Roundabouts Page S-2 added over time. The variety of motivations for considering roundabouts, the variety of levels of interaction with the public, and the design treatments ultimately constructed reinforce the notion that each corridor is a unique installation. Specific themes and trends that emerged from the interviews include the following: Once several roundabouts are built on a corridor, new or upgraded intersections are more likely to be roundabouts than signalized intersections. Reasons for this include good performance of the roundabouts in place, increased public and agency awareness and acceptance of roundabouts, concerns about queue spillback from signals into roundabouts, access management, and consistency within the corridor. Traffic analysis of roundabout corridors prior to their construction was typically conducted by analyzing each roundabout in isolation. However, several corridors were analyzed with microsimulation. It is anticipated the predictive tools for operational performance developed in this project, combined with the new tools intended for the Highway Capacity Manual (HCM), will provide practitioners with a simpler alternative to microsimulation. The safety effect of each corridor was not studied in detail in this project. However, owners of two roundabout corridors constructed as retrofits stated crash frequency decreased on the corridor following the construction of roundabouts. The consistent safety findings reported elsewhere suggest this trend is likely to continue in a corridor context. An agency champion was often the key to a corridor being constructed with roundabouts. Travel-Time Collection, Analysis, and Modeling A data collection crew visited each of the nine corridors for two or three days. Floating car runs using probe vehicles equipped with GPS units recorded vehicle activity with one second resolution and produced speed and travel time trajectories. Field travel times were recorded during the a.m. peak, off peak, and p.m. peak periods. Aggregating data within these times periods together for each corridor indicates the following: Study corridors operated at level of service (LOS) A through C based on travel speed as a percent of free flow speed (the HCM 2010 performance measure for Urban Streets). Most routes had the same LOS for the three time periods. Some changed by one le‘er grade. Travel speed and LOS for through routes and left turn routes were generally similar, with no pa‘ern apparent of one performing be‘er than the other. Traffic operations models were developed from the field data for the purpose of enhancing the Urban Streets methodology in Chapter 17 of the HCM 2010. The methodology in the HCM 2010 was developed primarily for signalized corridors Summary

Evaluating the Performance of Corridors with Roundabouts Page S-3 and does not explicitly account for certain aributes of roundabout corridors such as geometric delay of a through movement at a roundabout. The four models specifically developed for roundabout corridors as part of this research are: A free flow speed model, which estimates the speed that most drivers would choose at mid segment locations beyond the extent of the roundabout influence areas when other vehicles are not present. A roundabout influence area model, which estimates the length of roadway upstream and downstream of a roundabout over which speeds are reduced due to the presence of the roundabout. In other words, the roundabout influence area includes the deceleration zone prior to the roundabout and the acceleration zone following the roundabout. A geometric delay model, which estimates delay incurred at a roundabout node due to speed limiting characteristics of the roundabout. The model was based on data collected by probe vehicles passing through roundabouts in the absence of other vehicles. An impeded delay model, which estimates delay incurred at a roundabout node due to the presence of other vehicles. Impeded delay includes control delay. These models were incorporated into a methodology for estimating travel speed and LOS of roundabout corridors. Field data from two of the nine corridors were reserved for validation, and one of these contains a mix of roundabouts and signals. The validation exercise showed the developed corridor methodology correctly predicted the LOS for all four analysis routes on the first validation corridor, and was within one leer grade for the second validation corridor. The TRB Commiee on Highway Capacity and Quality of Service was notified of this project, and a conceptual overview of the roundabout corridor methodology was presented to the relevant subcommiees. Comparison to Equivalent Signalized Corridors Comparisons of field measured vehicle travel times and simulated “equivalent” corridors with signal or two way stop control confirmed a need for case by case evaluations. Specific findings include the following: Neither roundabout nor signalized/stop controlled corridor configurations consistently result in reduced travel time or intersection delays for through routes. Approximately half of the through movement routes resulted in lower travel time under a roundabout configuration, and approximately half resulted in lower travel time under non roundabout configuration. Evidence suggests roundabout corridors have a good likelihood of improving travel time performance, but site specific operational conditions may favor signalization or stop control. This finding reinforces the need for a case by case evaluation. Corridors with irregular intersection spacing show a higher likelihood for having beer travel times under a roundabout configuration rather than a signalized configuration. Summary

Evaluating the Performance of Corridors with Roundabouts Page S-4 Corridors that can use two way stop controlled intersections (rather than signals) in the place of roundabouts generally produce beer end to end travel times, even if intersection delays are lower under a roundabout configuration. For corridors where turning movements entering or departing the corridor are of similar or greater importance than end to end travel times, roundabout corridors appear more likely to improve those travel times. This may be due to higher side street delays and the general practice of prioritizing signal timing for progression of through traffic over left turns and side street movements. Among left turn routes, the roundabout corridor usually had lower travel time than the non roundabout corridor. Some findings for specific corridors: o Approach delay was lower with roundabouts for all intersections in both major street directions except for SR 539. o Through route travel time (average of both directions) increased with roundabouts on La Jolla Boulevard, Old Meridian Street, and Golden Road; decreased with roundabouts on MD 216, Spring Mill Road, Avon Road, and SR 67; and remained virtually unchanged on SR 539. o Travel time for routes with a left turn off the major street (average of both directions) increased with roundabouts on La Jolla Boulevard; decreased on MD 216, Old Meridian Street, Spring Mill Road, SR 539, Avon Road, and SR 67; and remained virtually unchanged on Golden Road. o Travel time for routes with a left turn onto the major street (average of both directions) increased with roundabouts on La Jolla and decreased on the other corridors. o The La Jolla Boulevard corridor performs quite differently from the other corridors studied in this project. It is the most urban of the corridors studied, with considerable pedestrian, bicycle, and on street parking activity. As a result, through vehicular traffic experiences more friction than was observed for other corridors. As confirmed in the corridor interviews, this outcome is consistent with the multimodal focus desired for this particular corridor. In general, the findings of this project indicate a need for a corridor specific evaluation to determine which form of intersection control is preferred on a given corridor. Furthermore, there are many performance measures other than traffic operations that are used to choose intersection control on a corridor. Development of a Corridor Comparison Document Finally, a Corridor Comparison Document (CCD) was developed to provide an overall framework for users to compare alternative corridor configurations and objectively inform project decisions based on the unique context of each project. It has the following overall features: Summary

Evaluating the Performance of Corridors with Roundabouts Page S-5 The CCD provides a broad approach for helping to inform corridor solution concepts by enabling case specific comparisons and evaluations. The CCD is flexible to adapt to the broad range of potential catalysts that might be the impetus for a particular project. These are illustrated through a series of example applications. The CCD presents many performance measures for various project and corridor contexts and refers the reader to other documents (e.g., HCM and Highway Safety Manual [HSM]) for specific assessment techniques. Other performance measures are generally evaluated through a best practices approach or are more qualitative in nature. The CCD is intended to be an evaluation and decision making framework rather than a guideline or standard. The following specific elements are included in the CCD: Information on different users of arterials, including passenger cars, buses, pedestrians, bicycles, trucks, and emergency vehicles; An overview of the project planning process wrien from the perspective of a practitioner evaluating alternatives for reconstructing an existing corridor or constructing a new roadway where the alignment has already been determined; Typical performance measures, assessment techniques for performance measures, and methods for selecting and prioritizing performance measures, grouped into broad categories of quality of service, safety, environmental, costs, community values, and others; and Four example applications illustrating use of the CCD, three of which result in roundabouts being selected and one of which results in signals being selected. The example applications are as follows: o A new suburban arterial being built in a greenfield to create access to undeveloped land and to provide increased connectivity. o A community enhancement project on an existing urban arterial. o An existing two lane highway in a rural, context sensitive environment that is beginning to experience suburban style development as it transforms into a vacation and second home community. o An existing suburban corridor being evaluated for safety and operational improvements due to changing context and a need for pavement rehabilitation. Summary

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 772: Evaluating the Performance of Corridors with Roundabouts provides measurement and evaluation methods for comparing the performance of a corridor with a functionally interdependent series of roundabouts to a corridor with signalized intersections in order to arrive at a design solution.

For the purposes of this research, a “series of roundabouts” is defined as at least three roundabouts that function interdependently on an arterial.

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