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67 Description A modern roundabout is a form of circular intersection in which traffic circulates counter- clockwise around a central island and entering traffic yields to circulating traffic (1). This technique is intended to reduce the delay and crashes associated with signalized intersections. Tables 47 through 50 follow. Quantitative Analysis Methods Motor Vehicle Operations Methods in Chapter 22 in the HCM6 can be used to estimate delay, capacity, queues, and other performance measures for motor vehicles at roundabouts (5). Those performance values can be compared with performance values for other intersection forms by using the appropriate HCM6 chapters for those intersections. Motor Vehicle Safety The table from the HSM, displayed on Page 68, provides crash modification factors related to converting a signalized intersection into a modern roundabout (2). The HSM also provides table data on crash modification factors related to converting a minor road, stop-controlled intersection into a modern roundabout (2). C H A P T E R 1 4 Roundabouts Source: Photograph provided by Washington State DOT.
68 Guide for the Analysis of Multimodal Corridor Access Management Access Management Technique Performance Trends and Documented Performance Relationships Operations Safety Install roundabout in lieu of traffic signal. â â â â â â â â Â Â Â Â Â Â Â Â Â Â Table 47. Multimodal operations and safety performance summary. Mode Operations Safety For identical traffic volumes, roundabouts operating within their capacity will generally produce lower delays (1). Facilitates U-turns (1). Random departure pattern at exits breaks up traffic progression and may reduce available gaps for midblock turns (1). The motorized vehicle crash rate is lower, and the injury crash rate is substantially lower (2). Depending on driver-yielding behavior, pedestrians may experience reduced delay relative to a traffic signal-controlled intersection (3). Longer travel distance through a roundabout than at a conventional signalized intersection. Fewer vehicleâpedestrian conflict points, shorter crossing distances per crossing, and lower vehicle speeds improve pedestrian safety (1). Visually impaired pedestrians may experience difficulty crossing roundabout approaches, as circulating traffic may mask the sound of exiting traffic (1). Longer travel distance through a roundabout than at a conventional signalized intersection, but potentially easier left-turning maneuvers. Fewer vehicleâbicycle conflict points and vehicle speeds similar to bicycle speeds improve safety, but separate bicycle facilities may be required at higher vehicular and bicycle volumes (1). Multi- lane roundabouts can be challenging for bicyclists (1). Precludes ability to provide a far-side stop where a single-lane exit is used unless a bus pullout is provided (1). Bus pullouts introduce delay for buses re-entering the roadway (4). No documented effect beyond that generally observed for motor vehicle and pedestrian traffic. Circulatory roadway and truck apron width should accommodate the design vehicle (1). No documented effect beyond that generally observed for motor vehicle traffic. Table 48. General trends associated with installing a roundabout in lieu of a traffic signal. Intersection Type Crash Severity Crash Modification Factor Urban, 1 or 2 lanes All 0.99 Injury 0.40 Suburban, 2 lanes All 0.33 All settings, 1 or 2 lanes All 0.52 Injury 0.22 Source: Highway Safety Manual, 1st ed., Table 14-3 (2). Table 49. Crash modification factors related to conversion of a signalized intersection into a roundabout.
Roundabouts 69 Pedestrian Operations The pedestrian method in Chapter 2 in the HCM6 can be used to evaluate pedestrian delay when crossing roundabout approaches. The use of a local value for driver-yielding behavior will produce the accurate results (5). Bicycle Operations Bicycles traveling through a roundabout as vehicles (i.e., using the circulatory roadway) will experience control delays similar to that of motorized vehicles. Bicycles traveling through the roundabout as pedestrians, or using a shared-use path around the exterior of the roundabout, will experience control delays similar to pedestrians (5). Additional Information ⢠Chapters 1 and 2 in this guide. ⢠Access Management Manual, Second ed.: Section 20.5.3. ⢠Access Management Application Guidelines: Chapter 14, Roundabout Access Spacing. References 1. Rodegerdts, L., J. Bansen, C. Tiesler, J. Knudsen, E. Myers, M. Johnson, M. Moule, B. Persaud, C. Lyon, S. Hallmark, H. Isebrands, R. B. Crown, B. Guichet, and A. OâBrien. NCHRP Report 672: Roundabouts: An Informational Guide, Second ed. Transportation Research Board of the National Academies, Washington, D.C., 2010. 2. Highway Safety Manual, 1st ed. American Association of State Highway and Transportation Officials, Washington, D.C., 2010. 3. Dixon, K. K., R. D. Layton, M. Butorac, P. Ryus, J. L. Gattis, L. Brown, and D. Huntington. Access Management Application Guidelines. Transportation Research Board, Washington, D.C., 2016. 4. Kittelson & Associates, Inc.; Parsons Brinckerhoff; KFH Group, Inc.; Texas A&M Transportation Institute; and Arup. TCRP Report 165: Transit Capacity and Quality of Service Manual, 3rd ed. Transportation Research Board of the National Academies, Washington, D.C., 2013. 5. Highway Capacity Manual: A Guide for Multimodal Mobility Analysis, 6th ed. Transportation Research Board, Washington, D.C., 2016. Intersection Type Crash Severity Crash Modification Factor All settings, 1 or 2 lanes All 0.56 Injury 0.18 Rural, 1 lane All 0.29 Injury 0.13 Urban, 1 or 2 lanes All 0.71 Injury 0.19 Urban, 1 lane All 0.61 Injury 0.22 Urban, 2 lanes All 0.88 Suburban, 1 or 2 lanes All 0.68 Injury 0.29 Suburban, 1 lane All 0.22 Injury 0.22 Suburban, 2 lanes All 0.81 Injury 0.32 Source: Highway Safety Manual, 1st ed., Table 14-4 (2). Table 50. Crash modification factors related to conversion of a minor road, stop-controlled intersection into a roundabout.
