Guide for Pavement Friction (2009)

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ACKNOWLEDGMENT This work was sponsored by the American Association of State Highway and Transportation Officials (AASHTO), in cooperation with the Federal Highway Administration, and was conducted in the National Cooperative Highway Research Program (NCHRP), which is administered by the Transportation Research Board (TRB) of the National Academies. COPYRIGHT PERMISSION Authors herein are responsible for the authenticity of their materials and for obtaining written permissions from publishers or persons who own the copyright to any previously published or copyrighted material used herein. Cooperative Research Programs (CRP) grants permission to reproduce material in this publication for classroom and not-for-profit purposes. Permission is given with the understanding that none of the material will be used to imply TRB, AASHTO, FAA, FHWA, FMCSA, FTA, Transit Development Corporation, or AOC endorsement of a particular product, method, or practice. It is expected that those reproducing the material in this document for educational and not-for-profit uses will give appropriate acknowledgment of the source of any reprinted or reproduced material. For other uses of the material, request permission from CRP. DISCLAIMER The opinion and conclusions expressed or implied in the report are those of the research agency. They are not necessarily those of the TRB, the National Research Council, AASHTO, or the U.S. Government. This report has not been edited by TRB.

i TABLE OF CONTENTS LIST OF FIGURES...........................................................................v LIST OF TABLES .......................................................................... vii ACKNOWLEDGMENTS ..............................................................viii ABSTRACT ...................................................................................... ix SUMMARY .........................................................................................x 1. INTRODUCTION.........................................................................1 BACKGROUND..................................................................................................................1 DESCRIPTION OF THE PROBLEM .............................................................................1 PROJECT OBJECTIVES AND SCOPE.........................................................................2 OVERVIEW OF REPORT ................................................................................................3 2. INFORMATION GATHERING .................................................5 INTRODUCTION...............................................................................................................5 LITERATURE SEARCH...................................................................................................5 STATE FRICTION SURVEY ...........................................................................................6 STATE AND INDUSTRY INTERVIEWS .......................................................................7 3. PAVEMENT FRICTION AND HIGHWAY SAFETY .............9 HIGHWAY SAFETY...........................................................................................................9 RELATIONSHIP BETWEEN WET-WEATHER CRASHES AND HIGHWAY PAVEMENT SURFACE CONDITION .........................................................................11 Wet-Weather Crashes and Pavement Friction..................................................11 Wet-Weather Crashes and Splash/Spray ............................................................16 4. PAVEMENT FRICTION AND SURFACE TEXTURE ........19 PAVEMENT FRICTION .................................................................................................19 Definition...................................................................................................................19 Friction Mechanisms ..............................................................................................24 Factors Affecting Available Pavement Friction...............................................26 PAVEMENT SURFACE TEXTURE..............................................................................30 Definition...................................................................................................................30 Factors Affecting Texture......................................................................................32 FRICTION AND TEXTURE MEASUREMENT METHODS.....................................33 Overview....................................................................................................................33 Friction.......................................................................................................................34 Texture .......................................................................................................................36 Summary of Test Methods and Equipment .......................................................37

ii TABLE OF CONTENTS (CONTINUED) FRICTION INDICES.......................................................................................................45 Friction Number ......................................................................................................45 International Friction Index.................................................................................45 Rado IFI Model.........................................................................................................47 Index Relationships ................................................................................................49 5. PAVEMENT FRICTION MANAGEMENT............................53 INTRODUCTION.............................................................................................................53 FEDERAL MANDATES..................................................................................................53 Highway Safety Act of 1966 (23 USC Chapter 4)...............................................53 Highway Safety Program Standard 12 (HSPS No. 12) of 1967 ......................54 FHWA Instructional Memorandum 21-2-73 .......................................................54 1975 Federal-Aid Highway Program Manual ....................................................54 1980 FHWA Technical Advisory T 5040.17 ..........................................................54 Intermodal Surface Transportation Efficiency Act (ISTEA) of 1991...........55 National Highway System (NHS) Designation Act of 1995 ............................55 Transportation Equity Act for the 21st Century of 1998 (TEA-21)...............55 2005 FHWA Technical Advisory T 5040.36 ..........................................................57 PAVEMENT FRICTION MANAGEMENT FRAMEWORK.......................................57 ESTABLISHING THE PAVEMENT FRICTION MANAGEMENT PROGRAM....60 Defining Network Pavement Sections................................................................60 Establishing Friction Demand Categories ........................................................66 Data Collection.........................................................................................................69 Data Analysis ............................................................................................................73 Detailed Site Investigation....................................................................................80 Selection and Prioritization of Restoration Treatments ...............................84 Pavement Friction Management Approach and Framework .......................84 6. PAVEMENT FRICTION DESIGN..........................................87 INTRODUCTION.............................................................................................................87 FRICTION DESIGN POLICIES....................................................................................87 Consideration of Aggregate in Friction Design................................................87 Surface Mix Types and Texturing Techniques.................................................99 Design Policy for Friction and Texture..............................................................99 PROJECT-LEVEL FRICTION DESIGN ...................................................................108 Step 1—Determining Design Friction Level....................................................108 Step 2—Selecting Aggregates..............................................................................109 Step 3—Establishing Surface Mix Types and/or Texturing Techniques ..110 Step 4—Development of Construction Specifications ..................................119 Step 5—Formulation of Design Strategies.......................................................120 7. CONCLUSIONS AND RECOMMENDATIONS .................123 CONCLUSIONS .............................................................................................................123 RECOMMENDATIONS ................................................................................................125

iii TABLE OF CONTENTS (CONTINUED) REFERENCES ..............................................................................127 APPENDIX A. BIBLIOGRAPHY…………………………………A-1 APPENDIX B. STATE FRICTION SURVEY FORM.............B-1 APPENDIX C. SUMMARY OF STATE FRICTION SURVEY RESPONSES..................................................................................C-1 APPENDIX D. SUMMARY OF STATE AND INDUSTRY INTERVIEWS ................................................................................D-1 APPENDIX E. PRIMER ON FRICTION .................................E-1

iv LIST OF FIGURES Page Figure 1. Agencies participating in the project survey........................................................... 7 Figure 2. Total crashes (from all vehicle types) on U.S. highways from 1990 to 2003 (NHTSA, 2004) ................................................................................. 10 Figure 3. Total fatalities (from all vehicles types) on U.S. highways from 1990 to 2003 (NHTSA, 2004) ................................................................................. 10 Figure 4. Relationship between wet-weather crash rates and pavement friction for Kentucky highways (Rizenbergs et al., 1973) ....................................................... 12 Figure 5. Ratio of wet-to-dry pavement crashes versus pavement friction for Kentucky highways (Rizenbergs et al., 1973) ....................................................... 12 Figure 6. Relationship between wet crashes as compared to pavement surface friction (Schulze et al., 1976)............................................................................................... 15 Figure 7. Relationship between pavement friction and crash risk (Viner et al., 2004) ...... 16 Figure 8. Simplified diagram of forces acting on a rotating wheel ...................................... 19 Figure 9. Rolling resistance force with a free-rolling tire at a constant speed on a bare, dry paved surface (Andresen and Wambold, 1999) .............................................. 21 Figure 10. Forces and moments of a constant-braked wheel on a bare, dry paved surface (Andresen and Wambold, 1999)............................................................................. 21 Figure 11. Pavement friction versus tire slip.......................................................................... 22 Figure 12. Dynamics of a vehicle traveling around a constant radius curve at a constant speed, and the forces acting on the rotating wheel............................................... 23 Figure 13. Brake (Fx) and lateral (Fy) forces as a function of longitudinal slip (Gillespie, 1992) ...................................................................................................... 24 Figure 14. Lateral forces versus longitudinal force at constant slip angles (Gillespie, 1992) ...................................................................................................... 24 Figure 15. Key mechanisms of pavement–tire friction........................................................... 25 Figure 16. Micro-texture versus macro-texture (Flintsch et al., 2003).................................. 27 Figure 17. Effect of water film thickness on pavement friction (Henry, 2000) ..................... 28 Figure 18. Simplified illustration of the various texture ranges that exist for a given pavement surface (Sandburg, 1998) ...................................................................... 31 Figure 19. Texture wavelength influence on pavement–tire interactions (adapted from Henry, 2000 and Sandburg and Ejsmont, 2002)................................................... 31 Figure 20. Effect of micro-texture and macro-texture on pavement–tire wet friction at different sliding speeds (Flintsch et al., 2002) ...................................................... 32 Figure 21. The IFI friction model ............................................................................................ 47 Figure 22. The IFI and Rado IFI friction models (Rado, 1994).............................................. 48 Figure 23. Model skid crash reduction program (FHWA, 1980) ............................................ 56 Figure 24. Procedure for identification and prioritization of sites (Highways Agency, 2004) ....................................................................................... 58 Figure 25. Overview of a proactive strategy to manage friction on road networks (Austroads, 2005).................................................................................................... 59 Figure 26. Simplified example of a friction strategy currently operated by a state road authority in Australia utilizing SCRIM (Austroads, 2005) ......................... 59

v LIST OF FIGURES (CONTINUED) Page Figure 27. Change in highway cross-section as the horizontal alignment transitions from a tangent to a curve ....................................................................................... 62 Figure 28. Lateral forces that act on a vehicle as it travels along a curve............................ 62 Figure 29. Lateral sliding......................................................................................................... 62 Figure 30. Relationship between friction demand, speed, and friction availability ............. 66 Figure 31. Illustration of Iowa’s highway safety data integration and analysis system (Iowa DOT, 2005).................................................................................................... 72 Figure 32. Setting of investigatory and intervention levels for a specific friction demand category using time history of pavement friction ................................................. 74 Figure 33. Setting of investigatory and intervention levels for a specific friction demand category using time history of friction and crash rate history............................. 75 Figure 34. Setting of investigatory and intervention levels for a specific friction demand category using pavement friction distribution and crash rate–friction trend..... 76 Figure 35. Determination of friction and/or texture deficiencies using the IFI .................... 80 Figure 36. Example PFM program .......................................................................................... 85 Figure 37. Example illustration of matching aggregate sources and mix types/ texturing techniques to meet friction demand.................................................... 107 Figure 38. Example of determining DFT(20) and MPD needed to achieve design friction level ....................................................................................................................... 111 Figure 39. Asphalt pavement friction design methodology (Sullivan, 2005)....................... 112 Figure 40. Vehicle response as function of PSV and MTD (Sullivan, 2005) ....................... 112 Figure 41. Typical asphalt mix aggregate gradations (WAPA, 2004).................................. 114

vi LIST OF TABLES Page Table 1. List of hazards that can potentially be caused by splash and spray (NHTSA, 1998)........................................................................................................ 17 Table 2. Factors affecting available pavement friction (modified from Wallman and Astrom, 2001) .................................................................................. 26 Table 3. Factors affecting pavement micro-texture and macro-texture (Sandburg, 2002; Henry, 2000; Rado, 1994; PIARC, 1995, AASHTO, 1976) ...... 33 Table 4. Overview of highway-speed pavement friction test methods............................... 38 Table 5. Additional information on highway-speed pavement friction test methods ....... 39 Table 6. Overview of pavement friction test methods requiring traffic control ................ 40 Table 7. Additional information on pavement friction test methods requiring traffic control........................................................................................................... 41 Table 8. Overview of highway-speed pavement surface texture test methods.................. 42 Table 9. Additional information on highway-speed pavement surface texture test methods............................................................................................................ 42 Table 10. Overview of pavement surface texture test methods requiring traffic control........................................................................................................... 43 Table 11. Additional information on pavement surface texture test methods requiring traffic control........................................................................................................... 44 Table 12. Example criteria for highway features/environment factors corresponding to different friction demand levels (modified from TXDOT, 2004) ...................... 64 Table 13. Typical friction demand categories........................................................................ 67 Table 14. Summary of key issues to be considered in standardizing test conditions ......... 70 Table 15. Examples of useable crash statistics ..................................................................... 71 Table 16. Levels of pavement friction required for various friction demand categories (VicRoads/RTA, 1996)............................................................................................. 77 Table 17. Levels of pavement friction required for various friction demand categories (Transit New Zealand, 2002) ................................................................................. 77 Table 18. Friction demand categories used by Maryland SHA (Chelliah et al., 2003) ....... 78 Table 19. U.K. site categories and investigatory levels (Viner et al., 2004) ........................ 79 Table 20. Assessment of hydroplaning potential based on vehicle speed and water Film thickness......................................................................................................... 82 Table 21. Recommended actions for detailed site investigations (Chelliah et al., 2003; TXDOT, 2004; Austroads, 2005; Viner et al., 2004) ............................................. 83 Table 22. Effect of aggregate angularity, shape, and texture properties on pavement friction ..................................................................................................................... 89 Table 23. Test methods for characterizing aggregate frictional properties......................... 91 Table 24. Typical range of test values for aggregate properties........................................... 96 Table 25. Asphalt pavement surface mix types and texturing techniques........................ 100 Table 26. Concrete pavement surface mix types and texturing techniques ...................... 102 Table 27. Recommended levels of aggregate PSV for various site and friction requirement categories (Flintsch et al., 2002) .................................................... 105 Table 28. Pairs of MPD and DFT(20) needed to achieve design friction level of 40.......... 111

vii ACKNOWLEDGMENTS The research described herein was performed under NCHRP Project 1-43 by the Transportation Sector of Applied Research Associates (ARA), Inc. Dr. Jim W. Hall, Jr., was the Principal Investigator for the study. Dr. Hall was supported in the research by ARA Research Engineers Mr. Kelly Smith, Mr. Leslie Titus-Glover, and Mr. Lynn Evans, and by three project consultants—Dr. James Wambold (President of CDRM, Inc. and Professor Emeritus of Mechanical Engineering at Penn State University), Mr. Thomas Yager (Senior Research Engineer at the NASA Langley Research Center), and Mr. Zoltan Rado (Senior Research Associate at the Pennsylvania Transportation Institute). The authors gratefully acknowledge all of the individuals with state departments of transportation (DOTs) who responded to the pavement friction survey conducted for this project. The authors also express their gratitude for the valuable input provided by knowledgeable representatives of DOTs, paving associations, academia, and manufacturers of friction measuring equipment, vehicle tires, and trucks.

viii ABSTRACT This report documents the research performed under NCHRP Project 1-43. It describes the work activities undertaken in the study and presents the results of those activities toward the development of the Guide for Pavement Friction. The information provided in this report serves as the basis for many of the guidelines and recommendations contained in the Guide. The information will be of interest to highway materials, construction, pavement management, safety, design, and research engineers, as well as others concerned with the friction and related surface characteristics of highway pavements. Using information collected through detailed literature reviews and surveys/interviews with state highway agencies, this report discusses a variety of aspects regarding pavement friction. It describes and illustrates the importance of friction in highway safety, as well as the principles of friction, as defined by micro-texture and macro-texture. It identifies the factors affecting friction and examines the ways that friction can be measured (equipment and procedures) and expressed (reporting indices). Most importantly, it presents valuable information on (a) the management of friction on existing highway pavements and (b) the design of new highway surfaces with adequate friction. This information focuses on techniques for monitoring friction and crashes and determining the need for remedial action, as well as identifying combinations of aggregate (micro-texture) and mix types/surface texturing methods (macro-texture) that satisfy friction design requirements. The report includes various conclusions and recommendations based on the results of the study, and it features five appendixes containing supplemental information on friction.

ix SUMMARY This report describes and illustrates pavement friction (as defined by micro-texture and macro-texture) and discusses the importance of pavement friction in highway safety. It identifies the factors affecting pavement friction and examines the equipment, procedures, and indices used to quantify and report available friction. Most importantly, the report presents valuable information on (a) the management of pavement friction on existing highways and (b) the design of new highway surfaces with adequate pavement friction. The report is a useful resource for state personnel and others involved in managing, planning, and designing highway pavements. Pavement friction design is one of the key elements required for ensuring highway safety, as empirical evidence suggests that vehicle crashes are highly correlated to the amount of pavement friction available at the pavement–tire interface. Although comprehensive guidance covering both the policy and technical aspects of designing for and managing pavement friction was provided in Guidelines for Skid-Resistant Pavement Design, published by AASHTO in 1976, many significant improvements in design and material characterization have taken place since this time. Moreover, although more current information and guidance related to pavement friction is available, it is quite fragmented and has not been integrated into a comprehensive administrative policy and design tool for addressing friction issues. Thus, a new Guide for Pavement Friction is needed to assist highway engineers in (a) understanding the complex subject of pavement friction and its importance to highway safety and (b) instituting pavement management and design practices and processes that optimize friction safety, while recognizing and considering the effects on economics and other pavement–tire interaction issues (e.g., noise, splash/spray, visibility/glare). NCHRP Project 1-43, titled “Guide for Pavement Friction,” was conducted to address this need. Under this project, Applied Research Associates (ARA), Inc., was assigned the task of developing a new Guide—for consideration and adoption by AASHTO—that addresses the frictional characteristics and performance of pavement surfaces constructed with asphalt and concrete and considers pavement–tire noise and other relevant issues. This report describes and documents the work done to achieve the Project 1-43 objectives. Such work consisted of (a) information gathering through an immense literature search, state surveys, and state and industry interviews, (b) development of pavement friction management principles, and (c) development of pavement friction design procedures. Information gathering focused primarily on collecting national and international literature pertaining to pavement friction, texture, and other related surface characteristics. Also targeted in the search was information on user safety, as impacted by deficiencies in pavement friction, and economic considerations in the design of pavements with adequate friction. To determine the state-of-the-practice regarding the evaluation and design of pavement friction, texture, and noise, a six-page questionnaire survey was developed and distributed to all State Highway Agencies (SHAs) in the U.S., as well as Puerto Rico and Washington D.C. A substantial amount of information was obtained from these questionnaires and follow-up interviews, and the details of each discussion were fully

x documented in interview memoranda. The information gathered was used to develop pavement friction management and design procedures. In establishing a pavement friction management (PFM) program, a full understanding of federal and state legislative mandates regarding highway safety is needed, along with an understanding of the agency’s management/operational practices and resources (people, equipment, materials) available. Also of importance is an understanding of the following: 1. Factors that affect friction demand, categorized by highway alignment, highway features/environment, highway traffic characteristics, and driver/vehicle characteristics. 2. Strategies for establishing pavement friction demand categories. 3. Friction testing protocols (including equipment). 4. Crash data collection methodology and analysis. A practical approach to friction management and design developed and presented in this report is based on the principle that an appropriate level of pavement friction must be maintained across all pavement sections within a given highway network. The level of friction considered appropriate must be determined based on each section’s friction demand and it is imperative that friction supply meet or exceed friction demand at all times. This design approach ensures the provision of adequate friction levels economically for a variety of roadway (intersections, approaches to traffic signals, tight curves) and traffic conditions across a given network. The adequacy of friction (for both management and design) is assessed using two distinct threshold levels defined by the agency—investigatory and intervention. The establishment of investigatory and intervention friction levels requires detailed analyses of pavement surface micro-texture and macro-texture data, and crash data, if available. Presented in this report are three feasible methods for setting investigatory and intervention friction levels. It is recommended that one of the three methods be used. Pavement sections with measured friction values at or below an assigned investigatory level are subject to a detailed site investigation to determine the need for remedial action, such as erecting warning signs, performing more frequent testing and analysis of friction data and crash data, or applying a short-term restoration treatment. For pavement sections with friction values at or below the intervention level, remedial action may consist of immediately applying a restoration treatment or programming a treatment into the maintenance or construction work plan and/or erecting temporary warning signs at the site of interest. Pavement friction design is basically a process of selecting the right combination of pavement surface micro-texture and macro-texture to optimize available pavement friction for a given design situation. For both asphalt and concrete surfaces, micro-texture is defined by the surface aggregate material properties. The important aggregate properties that influence short- and long-term micro-texture are: