Superelevation Criteria for Sharp Horizontal Curves on Steep Grades (2014)

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N A T I O N A L C O O P E R A T I V E H I G H W A Y R E S E A R C H P R O G R A M NCHRP REPORT 774 Superelevation Criteria for Sharp Horizontal Curves on Steep Grades Darren J. Torbic Mitchell K. O’Laughlin Douglas W. Harwood Karin M. Bauer Courtney D. Bokenkroger Lindsay M. Lucas John R. Ronchetto MRIGlobal Kansas City, MO Sean Brennan Eric Donnell Alexander Brown Tejas Varunjikar ThoMas D. laRson PennsylvanIa TRansPoRTaTIon InsTITuTe aT The PennsylvanIa sTaTe unIveRsITy University Park, PA Subscriber Categories Design • Operations and Traffic Management • Safety and Human Factors TRANSPORTAT ION RESEARCH BOARD WASHINGTON, D.C. 2014 www.TRB.org Research sponsored by the American Association of State Highway and Transportation Officials in cooperation with the Federal Highway Administration

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C O O P E R A T I V E R E S E A R C H P R O G R A M S AUTHOR ACKNOWLEDGMENTS This report was prepared by Dr. Darren J. Torbic, Mr. Douglas W. Harwood, Ms. Karin M. Bauer, Ms. Lindsay M. Lucas, and Mr. John R. Ronchetto of MRIGlobal; Dr. Sean Brennan, Dr. Eric Donnell, and Mr. Alexander Brown of the Thomas D. Larson Pennsylvania Transportation Institute at the Pennsylvania State University; Mr. Mitchell K. O’Laughlin of Kiewit (formerly of MRIGlobal); Ms. Courtney D. Boken- kroger of Elanco (formerly of MRIGlobal); and Mr. Tejas Varunjikar of Nexteer Automotive (formerly of the Thomas D. Larson Pennsylvania Transportation Institute at the Pennsylvania State University). The authors wish to thank the state departments of transportation of California, Maryland, Pennsylvania, Washington, and West Virginia for their assistance in this research. CRP STAFF FOR NCHRP REPORT 774 Christopher W. Jenks, Director, Cooperative Research Programs Christopher Hedges, Manager, National Cooperative Highway Research Program David A. Reynaud, Senior Program Officer Megan A. Chamberlain, Senior Program Assistant Eileen P. Delaney, Director of Publications Natalie Barnes, Senior Editor NCHRP PROJECT 15-39 PANEL Field of Design—Area of General Design Norman H. Roush, West Virginia DOT (retired), Racine, OH (Chair) Kenneth T. Briggs, KCI Technologies Inc., Sparks, MD Antonette C. Clark, California DOT, Sacramento, CA James R. Kladianos, Wyoming DOT, Laramie, WY John Roccanova, California DOT, Stockton, CA Larry F. Sutherland, Parsons Brinckerhoff, Columbus, OH Jeffrey Shaw, FHWA Liaison Stephen F. Maher, TRB Liaison

F O R E W O R D By David A. Reynaud Staff Officer Transportation Research Board This report provides superelevation criteria for horizontal curves on steep grades. A series of field studies and vehicle dynamics simulations were undertaken to investigate combina- tions of horizontal curve and vertical grade design. The report should be of interest to state and local highway design practitioners. Sharp, horizontal curves on steep downgrades represent a potential safety concern for vehicles, especially heavy vehicles. Examples where this combination may occur are inter- change ramp movements, curves on mountainous roads, or high-speed downgrade curves on controlled-access roadways. At these locations, the complicating factors of grade, pave- ment cross slope, and pavement friction fully tax the driver’s ability to provide correct vehicle positioning without compromising control of the vehicle. Superelevation criteria, horizontal curvature, and other associated geometric criteria needed to be developed for situations where steep grades are located on sharp horizontal curves. The objective of NCHRP Project 15-39 was to develop superelevation criteria for hori- zontal curves on steep grades. Other criteria associated with design of horizontal curves (e.g., tangent-to-curve transitions, spiral transitions, lateral shift of vehicles traversing the curve, need for pavement widening, and determination of curve radii) were also considered. The research was performed by MRIGlobal and the Pennsylvania State University. Design criteria were developed based on a series of field studies and vehicle dynamic simulations. Field studies were conducted to collect vehicle speed and lane-changing maneuver data from locations across the United States, as well as representative samples of tire–pavement friction data for various pavement surface conditions. Vehicle dynamic simulations used AASHTO design criteria in combination with field-measured data. Three classes of pas- senger vehicles and three classes of trucks were considered for safety analysis. The report provides design guidance based on the analyses for sharp horizontal curves on steep grades.

C O N T E N T S 1 Summary 7 Section 1 Introduction 7 1.1 Background 8 1.2 Research Objective and Scope 8 1.3 Overview of Research Methodology 8 1.4 Key Terms 9 1.5 Outline of Report 10 Section 2 Literature Review 10 2.1 Horizontal Curve Design 14 2.2 Heavy Trucks 14 2.3 Driver Comfort 14 2.4 Friction Studies 16 2.5 Vehicle Dynamics Models 18 2.6 Current Practice 20 Section 3 Field Studies 20 3.1 Site Selection 21 3.2 Speed and Vehicle Maneuver Studies 31 3.3 Instrumented Vehicle Studies 36 3.4 Friction Testing 40 Section 4 Analytical and Simulation Modeling 40 4.1 Analysis Approach 43 4.2 Step 1: Define Basic Tire–Pavement Interaction Model(s) and Estimate Lateral Friction Margins against Skidding in AASHTO’s Current Horizontal Curve Policy 49 4.3 Step 2: Define Road Geometries and Variable Ranges for Use in Subsequent Steps 50 4.4 Step 3: Develop Side Friction Demand Curves and Calculate Lateral Friction Margins against Skidding Considering Grade Using the Modified Point-Mass Model 53 4.5 Step 4: Define Vehicles and Maneuvers to Use in Non-Point-Mass Models 55 4.6 Step 5: Predict Wheel Lift Using Quasi-static Models 58 4.7 Step 6: Predict Skidding of Individual Axles during Steady-State Behavior on a Curve 65 4.8 Step 7: Predict Skidding of Individual Axles during Braking and Lane-Change Maneuvers on a Curve 98 4.9 Step 8: Predict Skidding of Individual Axles during Transient Steering Maneuvers and Severe Braking 110 4.10 Step 9: Predict Skidding of Individual Wheels 139 4.11 Step 10: Predict Wheel Lift of Individual Wheels during Transient Maneuvers

143 4.12 Step 11: Analysis of Upgrades 155 4.13 Summary of Analytical and Simulation Modeling 157 Section 5 Crash Analysis 157 5.1 Data Description 159 5.2 Analysis Approach 159 5.3 Analysis Results 162 Section 6 Conclusions, Geometric Design Guidance, and Future Research 162 6.1 General Conclusions 164 6.2 Geometric Design Guidance 165 6.3 Future Research 167 References A-1 Appendix A Nomenclature B-1 Appendix B Vehicle Parameters Used in Simulation C-1 Appendix C Potential Changes Recommended for Consideration in the Next Editions of the Green Book and MUTCD Note: Many of the photographs, figures, and tables in this report have been converted from color to grayscale for printing. The electronic version of the report (posted on the Web at www.trb.org) retains the color versions.