Appendixes to NCHRP Report 572: Roundabouts in the United States (2007)

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APPENDIX G DEFINITIONS FOR ESTIMATING FASTEST VEHICLE PATHS The following outlines the general approach used in this study for sketching the fastest paths and predicting the speeds through a roundabout. The method is generally consistent with that described in the FHWA Roundabout Guide (G1). 1. The vehicle was assumed to be 1.8 m (6 ft) wide. 2. The path was drawn with the following offsets to the particular geometric elements (except where a larger offset will create a faster path) acting as outer limits of the path: a. 1.5 m (5 ft) from concrete curbing and splitter islands, b. 1.5 m (5 ft) from a roadway centerline (marking the boundary with traffic traveling in the opposite direction), and c. 1.0 m (3 ft) from a painted edge line (marking the boundary with either a painted median or traffic in the same direction). 3. The vehicle path was constructed beginning at a point not less than 50 m (164 ft) upstream of the yield line, per the British method. 4. The vehicle path was drawn by hand to allow natural transitions between tangents and curves. Several radii at various points along the vehicle paths are of interest. These are based in principle on the five measurements as shown in the FHWA guide (G1). For analysis purposes, the entry path radii and exit path radii have been differentiated between left-turn and through movements, and right-turn path radii have been separated into entry and exit path radii. The complete list is as follows and is diagrammed in Figure G-1. • R0: Approach path radius – measured along the path at a point upstream of the entry yield line for the through movement. • R1: Through movement entry path radius – measured along the path prior to the entry yield line for the through movement. • R2: Through movement circulating path radius – measured around the central island in the circulatory roadway. • R3: Through movement exit path radius – measured on the exit for the through movement. • R1L: Left-turn movement entry path radius – measured along the path prior to the entry yield line for the left turn movement. In practice, this is often very similar to R1. • R4: Left-turn movement circulating path radius – measured on the circulatory roadway upstream of the conflict point with the opposing through movement. • R6: Left-turn movement exit path radius – measured on the exit for the left turn movement. In practice, this is often very similar to R3. • R5: Right-turning entry path radius – measured along the path prior to the entry yield line for the right-turn movement. • R5x: Right-turning exit path radius – measured on the exit for the right-turn movement. In practice, this is often identical to R5. NCHRP Web-Only Document 94: Appendixes to NCHRP Report 572: Roundabouts in the United States G-1

Figure G-1. Definition of Vehicle Path Radii. The research team estimated predicted speeds for each roundabout using the FHWA methodology as described previously. For each site selected for the analysis, the team plotted plans to a scale of 1 inch = 50 ft and sketched fastest paths for each movement through the roundabout. For each path radius, the team estimated a speed by using the basic speed-curve relationships as defined in the AASHTO Policy on Geometric Design of Streets and Roadways (G2). When predicting the speeds at roundabouts, unless otherwise known, the assumed superelevation was –2 percent for the circulatory roadway in a roundabout and +2 percent for the entry and exit legs, consistent with the methodology used in the FHWA Roundabout Guide. The side friction factor was based on the curve radii as defined by the AASHTO Policy and plotted in the FHWA Roundabout Guide. For multilane roundabouts, the fastest paths were assumed to occur irrespective of the presence of any striping on the approach, circulatory roadway, or departure. This deviates from the practice suggested in the FHWA Roundabout Guide but is consistent with the UK practice for estimating entry path curvature. This distinction was made to enable data extraction with the parameters used in the UK safety equations. In practice, the estimated difference in speeds between the two methods for estimating multilane paths is usually quite minor (1 to 3 mph). NCHRP Web-Only Document 94: Appendixes to NCHRP Report 572: Roundabouts in the United States G-2

References G1. Robinson, B. W., L. Rodegerdts, W. Scarbrough, W. Kittelson, R. Troutbeck, W. Brilon, L. Bondzio, K. Courage, M. Kyte, J. Mason, A. Flannery, E. Myers, J. Bunker, and G. Jacquemart. Roundabouts: An Informational Guide. Report FHWA-RD-00-067. FHWA, U. S. Department of Transportation, June 2000. G2. American Association of State Highway and Transportation Officials. A Policy on Geometric Design of Highways and Streets. AASHTO, Washington, DC, 2005. NCHRP Web-Only Document 94: Appendixes to NCHRP Report 572: Roundabouts in the United States G-3