National Academies Press: OpenBook

Roundabouts in the United States (2007)

Chapter: Summary

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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2007. Roundabouts in the United States. Washington, DC: The National Academies Press. doi: 10.17226/23216.
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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2007. Roundabouts in the United States. Washington, DC: The National Academies Press. doi: 10.17226/23216.
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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2007. Roundabouts in the United States. Washington, DC: The National Academies Press. doi: 10.17226/23216.
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S U M M A R Y Based on the findings of this study, roundabouts appear to be successful in a wide variety of environments in the United States. The following sections summarize the major conclusions from this study. Safety Performance In general, roundabouts have improved both overall crash rates and, particularly, injury crash rates in a wide range of settings (urban, suburban, and rural) for all previous forms of traffic control except for all-way stop control, for which no statistically significant difference could be found. In addition, single-lane roundabouts have better safety performance than multilane roundabouts. The safety performance of multilane roundabouts appears to be especially sensi- tive to design details. This study produced a number of major safety findings: • Intersection-level crash prediction models for the prediction of the overall safety performance of the intersection. These models relate the crash prediction to the number of lanes, number of legs, and the average annual daily traffic. • Approach-level crash prediction models that relate common types of crashes (e.g., exiting- circulating crashes) to average annual daily traffic volumes and key geometric parameters that were demonstrated to influence the prediction. • An updated comparison of the performance of roundabouts to other forms of traffic control, disaggregated to a greater extent than any previous study of U.S. roundabouts. Operational Performance Currently, drivers in the United States appear to use roundabouts less efficiently than models suggest is the case in other countries around the world. In addition, geometry in the aggregate sense—number of lanes—has a clear effect on the capacity of a roundabout entry; however, the fine details of geometric design—lane width, for example—appear to be secondary and less significant than variations in driver behavior at a given site and between sites. The following model is recommended for the entry capacity at single-lane roundabouts: where c = entry capacity (passenger car units [pcu]/h) vc = conflicting flow (pcu/h) c vc= ⋅ − ⋅1130 0 0010exp( ) (S-1). Roundabouts in the United States 1

2Because driver behavior appears to be the largest variable affecting roundabout performance, calibration of the models to account for local driver behavior and changes in driver experience over time is highly recommended to produce accurate capacity estimates. The exponential model parameters can be calibrated using locally measured parameters as follows: where c = entry capacity (pcu/h) A = 3600/tf B = (tc – tf /2)/3600 vc = conflicting flow (pcu/h) tf = follow-up headway (s) tc = critical headway (s) The recommended capacity model for the critical lane of a multilane entry into a two-lane circulatory roadway is as follows: where ccrit = entry capacity of critical lane (pcu/h) vc = conflicting flow (pcu/h) The recommended control delay model follows: where d = average control delay (s/veh) c = capacity of subject lane (veh/h) T = time period (h: T = 1 for 1-h analysis, T = 0.25 for 15-min analysis) v = flow in subject lane (veh/h) The recommended level of service (LOS) criteria are the same as those currently used for unsignalized intersections. The LOS for a roundabout is determined by the computed or mea- sured control delay for each lane. The LOS is not defined for the intersection as a whole. These models have been incorporated into an initial draft procedure for the Highway Capac- ity Manual, which the TRB Committee on Highway Capacity and Quality of Service will continue to revise until its eventual adoption. Geometric Design This study produced a number of major geometric design findings: • The application of acceleration and deceleration effects appears to significantly improve the ability to predict 85th-percentile speeds entering and exiting a roundabout. • The combination of the extensive field observations of critical gap and the revised speed predictions may be used to refine the current intersection sight distance procedure presented d c T v c v c c v c = + − + − ⎛⎝⎜ ⎞⎠⎟ + ⎛⎝⎜ ⎞⎠⎟3600 900 1 1 3600 4 2 50T ⎡ ⎣ ⎢⎢⎢⎢ ⎤ ⎦ ⎥⎥⎥⎥ ( )S-4 c vcrit c= ⋅ − ⋅1130 0 0007exp( . ) ( )S-3 c A B vc= ⋅ − ⋅exp( ) ( )S-2

in FHWA’s Roundabouts: An Informational Guide. These findings should be considered interim until a more comprehensive study of sight distance needs at roundabouts can be completed. • Anecdotal evidence suggests the importance of considering design details in multilane round- about design, including vehicle path alignment, lane widths, and positive guidance to drivers through the use of lane markings. Pedestrian and Bicyclist Observations This study produced a number of findings regarding pedestrian and bicyclist behavior at roundabouts: • This research did not find any substantial safety problems for non-motorists at roundabouts, as indicated by few crashes being reported in detailed crash reports. In addition, no crashes and a very small number of conflicts were observed from video recordings of interactions between non-motorists and motorists. Because exposure data were not available from before a roundabout was present, it is unknown whether pedestrians have altered their travel patterns because of the presence of a roundabout. • The ability of pedestrians and bicyclists to use the roundabout may be compromised if use of the roundabout by all modes and their subsequent interactions are greater than studied herein or if such interactions increase over time (i.e., as vehicle traffic and/or pedestrian traffic increases). • An emphasis needs to be placed on designing exit lanes to improve upon the behaviors of both motorists and pedestrians. • Multilane roundabouts may require additional measures to improve upon the behaviors of motorists, pedestrians, and bicyclists. 3

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 572: Roundabouts in the United States explores methods of estimating the safety and operational impacts of roundabouts and examines updated design criteria for roundabouts. Also available are appendixes to the report, published as NCHRP Web-Only Document 94, which includes detailed reviews of the literature on safety performance and operational models, the master inventory of U.S. roundabouts assembled for this project, and the results of the statistical testing of various models.

The materials linked to below may be adapted for use in presentations to the public on roundabouts.

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