0-6414-P1 TEXAS ROUNDABOUT GUIDELINES Authors: Alison Mills Jen Duthie Randy Machemehl Travis Waller TxDOT Project 0-6414: Development of Guidelines for Implementation of Roundabouts in Texas AUGUST 2011 Performing Organization: Sponsoring Organization: Center for Transportation Research Texas Department of Transportation The University of Texas at Austin Research and Technology Implementation Office 1616 Guadalupe, Suite 4.202 P.O. Box 5080 Austin, Texas 78701 Austin, Texas 78763-5080 Performed in cooperation with the Texas Department of Transportation and the Federal Highway Administration. Table of Contents 1. INTRODUCTION ............................................................................................................................... 1 1.1. WHAT IS A ROUNDABOUT ............................................................................................................... 1 1.1.1. Roundabout Elements ............................................................................................................ 1 1.1.2. Categories of Roundabouts .................................................................................................... 3 1.1.3. How Roundabouts Differ from Other Types of Circular Intersections .................................... 8 1.2. RESOURCES ................................................................................................................................ 11 1.2.1. NCHRP 672 ........................................................................................................................... 11 1.2.2. Supplemental State Roundabout Design and Implementation Guides ................................ 13 1.3. ROUNDABOUT RESEARCH FINDINGS ................................................................................................ 14 1.3.1. Safety.................................................................................................................................... 14 1.3.2. Operations ............................................................................................................................ 16 1.3.3. Geometry .............................................................................................................................. 18 1.4. EVALUATION TOOLS ..................................................................................................................... 19 1.4.1. Analytical Models ................................................................................................................. 19 1.4.2. Microsimulation ................................................................................................................... 19 1.5. EXPERIENCE FROM OTHER AGENCIES ............................................................................................... 20 1.5.1. Planning Process, Review Process, Work Allocation ............................................................ 21 1.5.2. Preferred Guidance Documents and Software ..................................................................... 21 1.5.3. Public Education Efforts ....................................................................................................... 22 1.5.4. Words of Wisdom ................................................................................................................. 22 2. SAFETY ASSESSMENT TECHNIQUES ................................................................................................ 23 2.1. SAFETY AND PREDICTION EVALUATION ............................................................................................. 23 2.1.1. Safety Prediction Models ...................................................................................................... 23 2.1.2. Empirical Bayes (EB) Method ............................................................................................... 23 2.2. INTERSECTION‐LEVEL SAFETY PREDICTION MODEL .............................................................................. 25 2.2.1. Recommended Intersection‐Level Safety Prediction Models for U.S. Roundabouts ............ 26 2.2.2. Applying Intersection‐Level Safety Prediction Models ......................................................... 27 2.3. APPROACH‐LEVEL SAFETY PREDICTION MODEL .................................................................................. 28 2.3.1. Applying Approach‐Level Safety Prediction Models ............................................................. 30 3. TRAFFIC OPERATIONS ASSESSMENT TECHNIQUE ........................................................................... 31 3.1. INTRODUCTION ........................................................................................................................... 31 3.2. ANALYSIS TECHNIQUES ................................................................................................................. 31 3.3. PLANNING METHOD .................................................................................................................... 32 iii 3.3.1. Lane Requirement Estimates at a Planning Level ................................................................ 32 3.3.2. Space Requirements ............................................................................................................. 34 3.4. HIGHWAY CAPACITY MANUAL METHODS ......................................................................................... 35 3.4.1. Capacity ................................................................................................................................ 35 3.4.2. Effect of Pedestrians on Vehicular Operations at the Entry ................................................. 38 3.4.3. Delay Model and Queue Length Model ................................................................................ 38 3.5. TRAFFIC OPERATIONS SOFTWARE TOOLS .......................................................................................... 39 3.5.1. Deterministic Method .......................................................................................................... 40 3.5.2. Simulation Method ............................................................................................................... 40 4. GEOMETRIC DESIGN ...................................................................................................................... 41 4.1. INTRODUCTION ........................................................................................................................... 41 4.2. ROUNDABOUT SIZE ...................................................................................................................... 43 4.2.1. Inscribed Circle Diameter ..................................................................................................... 43 4.2.2. Central Island ....................................................................................................................... 45 4.3. HORIZONTAL ALIGNMENT CONSIDERATIONS ...................................................................................... 45 4.3.1. Angle between Approach Legs ............................................................................................. 46 4.3.2. Path Alignment ..................................................................................................................... 47 4.4. DESIGN VEHICLE .......................................................................................................................... 48 4.5. SPEED MANAGEMENT .................................................................................................................. 49 4.5.1. Design Speed ........................................................................................................................ 49 4.6. SIGHT DISTANCE .......................................................................................................................... 52 4.6.1. Stopping Sight Distance ....................................................................................................... 52 4.6.2. Intersection Sight Distance ................................................................................................... 54 4.7. ENTRY AND EXIT DESIGN ............................................................................................................... 55 4.7.1. Entry Curve ........................................................................................................................... 55 4.7.2. Exit Curve .............................................................................................................................. 56 4.8. VERTICAL ALIGNMENT CONSIDERATIONS .......................................................................................... 57 4.9. SPLITTER ISLAND DESIGN ............................................................................................................... 58 4.10. PEDESTRIAN AND BICYCLIST CONSIDERATIONS ................................................................................. 60 4.10.1. Pedestrians Consideration .................................................................................................... 60 4.10.2. Bicyclist Considerations ........................................................................................................ 62 5. TRAFFIC DESIGN ............................................................................................................................ 63 5.1. PAVEMENT MARKINGS ................................................................................................................