Guide for Pavement Friction (2009) / Chapter Skim
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53 CHAPTER 5. PAVEMENT FRICTION MANAGEMENT INTRODUCTION Highway safety management in the U.S.
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54 • Highway design and maintenance (including lighting, markings, and surface treatment)
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55 • Identifying and correcting sections of roadway with a high or potentially high incidence of skid-accidents. • Ensuring that the new surfaces have adequate and durable skid resistance properties.
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56 Select Sites Representing New and Typical Design Mixes Identify and List High Wet Weather Accident Sites Develop Representative Sampling Plan with Stratification by Highway Type, Area, and ADT Develop Wet and Dry Pavement Times for Highway Location Sample Analyze Wet Pavement Accident Rates Collect Skid Resistance Data List Selected Sites in Sample Calibrate Skid Tester at Test Center Collect Auxiliary Pavement Data as Needed Prepare Skid Number Distribution by Highway Type, Area, and ADT for Representative Sample Prepare Listing of Hazardous Sites by Priority Order Evaluate New and Typical Pavement Mixes Establish Performance of Mixes Conduct Cost-Effectiveness Analysis of Treatments for HighPriority Sites Schedule Highway Projects for Resurfacing and Other Remedial Treatments (within constraints of funds) Provide Feedback to Design, Operations, and Research Implement Projects in Coordination with Safety Improvements, 3R*
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57 2005 FHWA Technical Advisory T 5040.36 In 2005, the FHWA issued Technical Advisory T 5040.36, "Surface Texture for Asphalt and Concrete Pavements." This advisory issued (a) information on state-of-the-practice for providing friction and surface texture on pavements and (b)
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58 Figure 24. Procedure for identification and prioritization of sites (Highways Agency, 2004)
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59 Figure 25. Overview of a proactive strategy to manage friction on road networks (Austroads, 2005)
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60 ESTABLISHING THE PAVEMENT FRICTION MANAGEMENT PROGRAM Defining Network Pavement Sections In a traditional PMS, pavement sections within a network are defined for evaluation based on the consistency of structural capacity defined using characteristics such as structural composition (surface type, layer thicknesses, and so on) , construction history, and traffic.
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61 friction needed for a project, a rational estimate can be developed by evaluating the array of factors comprising three broad categories -- highway alignment, highway features/ environment, and highway traffic characteristics. A fourth category, driver/vehicle characteristics, which covers driver skills and age, vehicle tire characteristics, and vehicle steering capabilities, is difficult to assess in terms of friction demand.
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62 Figure 27. Change in highway cross-section as the horizontal alignment transitions from a tangent to a curve.
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63 The relationship between side-force friction for horizontal curves (the most critical horizontal alignment) , vehicle speed, radius of curvature, and highway cross-section (superelevation)
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64 While the first part of this equation is determined based on driver skill, experience, reaction time, and perception, the second part depends, to some extent, on the highway geometry (longitudinal grade) and available surface friction.
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65 Highway Traffic Characteristics Traffic characteristics that influence friction demand include traffic volume, composition, and speed. Discussions of each are provided below.
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66 Figure 30. Conceptual relationship between friction demand, speed, and friction availability.
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67 Table 13. Typical friction demand categories.
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68 Table 13. Typical friction demand categories (continued)
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69 Data Collection Three key data inputs are required for an effective PFM program: pavement friction, pavement texture, and crash rates. Procedures for collecting these data are presented in this section.
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70 Table 14. Summary of key issues to be considered in standardizing test conditions.
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71 Equipment Calibration and Maintenance Proper calibration and maintenance of the friction testing equipment is essential to the collection of reliable friction data. To this end, agencies should follow the manufacturerspecified regime or guidance for calibration and maintenance.
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72 Crash data must be stored in a structured databank, so that each individual crash location can be related to a unique PFM pavement section. The databank must be compatible with an agency's PFM program and must contain sufficient amounts of data for meaningful analysis (i.e., contains crash data for a minimum of 10 years)
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73 Data Analysis Establishing Investigatory and Intervention Friction Threshold Levels Because conditions and circumstances along a highway change, there is no one friction level that defines the threshold between "safe" and "potentially unsafe." Although the ideal situation is to have friction supply meet or exceed friction demand over the entire system, such a practice would be prohibitively expensive (as well as largely unnecessary) and would not generate the cost-benefits associated with a better-targeted strategy.
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74 • Step 1 -- Plot pavement friction versus age/time for a given friction demand category (figure 32)
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75 Figure 33. Setting of investigatory and intervention levels for a specific friction demand category using time history of friction and crash rate history.
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76 Figure 34. Setting of investigatory and intervention levels for a specific friction demand category using pavement friction distribution and crash rate–friction trend.
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77 Table 16. Levels of pavement friction required for various friction demand categories (VicRoads/RTA, 1996)
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78 Table 18. Friction demand categories used by Maryland SHA (Chelliah et al., 2003)
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79 Table 19.
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80 Figure 35. Determination of friction and/or texture deficiencies using the IFI.
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81 Splash and Spray While there is currently very little data/information on the relationship between highway crashes and splash and spray, it is obvious that these occurrences can reduce a driver's vision and increase the risk of crashes. Splash and spray from passing and/or leading vehicles make seeing ahead, behind, and to the sides more difficult, particularly at night.
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82 • Step 2 -- Compute WFT using agency-established models or procedures or the WFT prediction models (and accompanying software) developed in NCHRP Project 1-29 (Anderson et al., 1998)
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83 Using the micro-texture and macro-texture results and the data listed above, the exact cause of friction loss can be determined. Common causes of friction loss include polishing of coarse aggregates and excessive wearing of the pavement surface resulting in a loss of macro-texture.
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84 Selection and Prioritization of Restoration Treatments The final step in a PFM program is to analyze the collected data to identify sites requiring more frequent monitoring or forensic investigation, and sites requiring friction restoration. Highway agencies normally use pavement friction and other condition data to identify and prioritize sites to be included in a program for: • Short-term remedial (maintenance)
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85 Figure 36. Example PFM program.
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86 • Network-Level Data Analysis -- Analyze friction and/or crash data to assess overall network condition and identify friction deficiencies. ¾ Establish investigatory and intervention levels for friction.
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