Existing Roundabout Assessment and Recommendations for Improvements (Billie Road and Frank Shore Road) Kimley-Horn June 30, 2023 Existing Roundabout Characteristics Existing Inscribed Circle Diameter= 90 ft. A 90 ft Inscribed Circle is consistent with Mini-roundabouts according to FHA NCHRP Report 672, Roundabouts: An Informational Guide. Mini-Roundabout Standard Characteristics: o Speed limits on approaching roads should not exceed 30 mph. o Recommended roundabout entry speed is 20 mph. o Circulatory Roadway Speed for this facility should be 15 mph. o Designed to accommodate passenger cars and maximum design vehicle SU-30. o Mini-Fully traversable central island with mountable curb for any vehicle equal to or larger than a passenger bus (B-40). o Minimum area of splitter islands is 50 sf. If the existing roundabout island location and the existing asphalt footprint (approaches and total inscribed circle diameter of 90 ft) are to remain unchanged, a series of improvements need to be made to improve roundabout efficiency. Below is a list of deficiencies and recommendations to prevent vehicles driving at a fast speed through roundabout and/or driving over the concrete apron while maintaining the current asphalt footprint. 1. Circulatory Roadway Width - Widen Circulatory Roadway Width from 13.5 ft to 17 ft. The Standard width ranges from 16 ft to 20 ft. 2. Concrete Apron Width - Reduce Concrete Apron Width from 15 ft to 12 ft (including 6” back curb). The allowable range is 3 ft to 15 ft. 3. Approach Deflection - Introduce 2-degree deflections on east and south leg approaches (prior to roundabout entry). This will allow the roadway alignment to align left of the roundabout center instead of the right side (existing condition). This change will result on restriping of the approaches (double yellow line). 4. Entry Radius - Increase entry radii to 120 ft. The restriping of these transitions will help maintain a minimum size for the splitter islands and extend the length of the roundabout entry. transitions. The standard entry radius varies from 60 ft to 120 ft. The resulting entry widths range from 15’ to 16’ and are within the standard range of 12 ft to 17 ft. 5. Splitter Island – While the current splitter islands meet standard criteria, at least two of them (east and west legs) will need to be reconstructed to accommodate the new entry radius that is directly related to the new circulatory roadway width. 6. Signage – The exiting roundabout is missing roundabout signs and keep right signs. The following are recommended: a. Roundabout approach signs (W2-6 and W13-1P) with corresponding roundabout speed on approaches to roundabout. A second assembly is recommended on east leg for the limited sight distance approach from Reservation Road. b. Keep Right Signs (4-7) on splitter Islands 7. Pavement Markings – the current pavement markings in entry approaches to the roundabout provide little to no deflection for vehicles to slow down and access roundabout. Recommendation Items 3, 4, & 5 will result in restriping portions of the roadway at multiple locations.
