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Appendix D Page D-1 Roundabouts: An Informational Guide APPENDIX D DESIGN SUPPLEMENTAL MATERIALS CONTENTS D.1 DESIGN APPENDIX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D-3 D.2 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D-3
Roundabouts: An Informational Guide Page D-2 Appendix D LIST OF EXHIBITS Exhibit D-1 Side Friction Factors at Various Speeds . . . . . . . . . . . . . . . . . . . . . . . D-3
Appendix D Page D-3 Roundabouts: An Informational Guide D.1 DESIGN APPENDIX The relationship between travel speed and horizontal curvature is documented in the AASHTO Policy on Geometric Design of Highways and Streets (1). Equation D-1 can be used to calculate the design speed for a given travel path radius. where: V = design speed, mph; R = radius, ft; e = superelevation, ft/ft; and f = side friction factor. Superelevation values are usually assumed to be +0.02 for entry and exit curves and â0.02 for curves around the central island. Values for side friction factor can be determined in accordance with AASHTO standards for curves at intersec- tions [see AASHTO Exhibit 3-11 (1)]. The coefficient of friction between a vehicleâs tires and the pavement varies with speed, as shown in Exhibit D-1. V R e f= +( )15 Exhibit D-1 Side Friction Factors at Various Speeds 0.00 0.10 0.20 0.30 0.40 0.50 0.60 5 10 15 20 25 30 35 40 Speed (mph) Si de F ric tio n Fa ct or D.2 REFERENCE 1. A Policy on Geometric Design of Highways and Streets. AASHTO, Washington, D.C., 2004. Equation D-1
