Guide for Pavement Friction (2009)

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123 CHAPTER 7. CONCLUSIONS AND RECOMMENDATIONS While the main focus of highway agencies more than a generation ago was building an expanded highway system, the priorities have changed such that revamping existing roads and making them safer and less congested, now tops the priority list. With its strong link to safety, pavement friction is a tremendously important facet of highway transportation. However, achieving and maintaining adequate pavement friction can be very difficult to accomplish by agencies responsible for making roads safer. Though there are many reasons for this difficulty, three of the more apparent ones are: (a) the complexity of the pavement–tire friction interface, (b) controversy concerning the agency’s level of responsibility for ensuring user safety, and (c) uncertainty regarding the costs and benefits of a proactive and effective pavement friction program. This trend has been reversing gradually, motivated by the staggering levels of fatalities, injuries, and damage due to crashes. This has led to the development of improved pavement friction management, design, and maintenance methods, all of which are important for enhancing highway safety. Although it is impossible for an agency to correct all pavement deficiencies immediately, it is critical for agencies to implement a program that identifies deficiencies, warns the public about potential hazards, and uses reasonable care to correct hazards. The concepts and mechanisms behind pavement friction are quite involved and not easily understood. Moreover, because there are many factors that affect friction, it is more of a process than an inherent characteristic of the pavement. Thus, while highway engineers can control some factors (e.g., surface texture, speed), conditions and circumstances will arise that may put adequate friction beyond reach. CONCLUSIONS This study examined both past and on-going research and the current state-of-the-practice regarding pavement friction. The principles and methodologies of every aspect of friction were investigated in order to develop practical policy and how-to guidance for SHAs. The guidance, which is provided in a stand-alone Guide for Pavement Friction, covers the importance of pavement friction in highway safety; the fundamental concepts of friction; how friction is measured, reported, and managed in the field; and how friction is incorporated in design via the selection of aggregates and surface textures. This report provided most of the background and supporting information used in developing the new Guide. It discussed the efforts to gather and review important information on friction and related matters, and presented the governing ideas and methods for ensuring adequate friction.

124 Major conclusions of the study are summarized as follows: • Laboratory Testing—Several tests are available for assessing aggregates’ potential to provide adequate initial and long-term micro-texture. The tests can be categorized according to the characterization of mineralogical/ petrographical properties, physical and geometric properties, mechanical properties, and durability properties. Testing protocol and associated performance criteria vary from agency to agency, primarily according to the types of aggregates available and the conditions (traffic, climate, etc.) in which they’re used. • Field Testing—Several tests are available for measuring pavement friction and texture. For friction, there are over a dozen commercially produced devices that can operate at fixed or variable slip, at speeds up to 100 mi/hr (161 km/hr), and under variable test tire conditions, such as load, size, tread design, inflation pressure, and construction. Micro-, macro-, or mega-texture can be measured using a variety of laser devices (including CTM), volumetric techniques (SPM), water drainage rates (OFM), and sliding rubber pad apparatus (BPT, DFT). The literature review and interviews conducted as part of this study show a majority of agencies test their highway networks frequently for friction. The locked-wheel skid trailer test method (ASTM E 274) has been and continues to be the most common method for testing friction. The ribbed tire (ASTM E 501) is the main tire used with the locked-wheel testers, however, there is an increasing use of the smooth tire (ASTM E 524), as it has been shown that the ribbed tire does not see macro-texture and, thus, can miss very slippery conditions. While most agencies do not measure pavement surface texture on a routine basis for friction management or design purposes, they generally recognize the benefits of texture testing and several agencies are investigating the applications. • Surface Mixes and Texturing Techniques—Pavement macro-texture is primarily determined by the size and gradation of the aggregate in asphalt mixes (and exposed concrete mixes) and by the type of texturing applied to the surface of concrete mixes. Large-sized open-, gap-, or uniformly graded mixes generally provide the highest levels of texture depth, whereas small-sized dense-graded mixes provide the lowest levels. Although greater texture depth improves friction at higher speeds and reduces splash/spray, it can produce increased noise—particularly if the texture is positive—and greater tire wear. For concrete surfaces, burlap, broom, and most turf drag finishes provide the lowest levels of macro-texture, whereas exposed aggregate and porous concrete surfaces (which are rarely used) provide the highest levels. While tining, grinding, and grooving all yield significant levels of macro-texture, the orientation, spacing, and width of the grooves can impact friction and other pavement–tire interactions. For instance, transverse grooves provide drainage paths for water, thereby reducing hydroplaning potential. In addition, random, skewed grooves result in less noise than uniform, perpendicular grooves. Although longitudinal grooves provide improved resistance to lateral skidding and reduced noise, they yield less stopping friction and greater splash/spray than transverse grooves.

125 • Friction Management—While most SHAs have some components of a friction management program in-place, only a few (e.g., Texas, Maryland, Virginia) have a comprehensive program that utilizes all of the basic components. Such agencies have invested significant resources into developing and improving their friction management policies to tackle highway safety issues. • Friction Design Policy—Current friction design policies vary widely, with some states having only basic control measures and others having full-scale testing programs and performance databases. The scope and level of detail of friction design policies are largely determined by the availability of good quality aggregates and the general perception of whether wet-weather safety is an issue within the state. • Friction Investigatory and Intervention Levels—Most agencies do not clearly define minimum friction demand requirements for various site categories. RECOMMENDATIONS Although an enormous amount of information on pavement friction, texture, and related topics was gathered and analyzed in this study, there still remain many issues to be resolved concerning how to design, test, and manage pavements for friction. Provided below is a list of the most pressing issues and recommendations for addressing them in the future. • Laboratory Testing—While several tests have been identified as being good indicators of friction performance, there is room for improvement in the predictive capabilities of the tests. Such improvements could focus on the reduction of variability in test results, better simulation of in-service conditions, and closer controls in trying to relate aggregate type/source to field performance, via test properties. Additionally, research into new test methods, for both aggregates and mixtures, should continue, along with the development of databases to better link both design micro-texture and macro-texture with friction (and other characteristics, such as noise and splash/spray) in the field. • Field Testing—The locked-wheel friction test method (ASTM E 274) has been tried and tested over the years, and has proven to be reliable and accurate. It is strongly recommended that agencies consider use of the smooth tire (ASTM E 524) with locked-wheel testers. Pavement surface texture, on the other hand, is measured using various test equipment and associated protocols. The standard indices are MTD for the SPM volumetric method and MPD for laser-based measurements. The MTD and MPD are well correlated. The use of a universal friction/texture measuring index, such as the IFI, is one method of standardizing friction/texture test results.

126 • Surface Mixes and Texturings—Several on-going research studies and programs (NCHRP Project 10-67, NCAT, FHWA Concrete Pavement Surface Characteristics, and the Institute for Safe Quiet and Durable Highways [ISQDH]) are examining the issue of macro-texture, as provided by surface mixtures and texturing applications. Models developed to estimate texture depth based on mixture gradations and volumetrics should be evaluated and, as appropriate, incorporated into future Guide documents. Similarly, efforts to model the texture depth of different types of texturing on concrete pavement should be monitored. The effects of macro-texture on noise and other pavement–tire interaction issues should be a key part of the monitoring process. • Friction Design Policy—Although considerable guidance on friction design was developed and presented in the Guide, pavement designers could benefit from a comprehensive, systematic procedure for screening aggregates for use. • Friction Management Policy—Although very few agencies have a comprehensive friction management program in-place, this situation can be reversed by adopting, as needed, the guidance on friction management presented in the Guide. Also, there is a vast amount of knowledge available in published literature worldwide that can be used along with local experience to improve agency friction management policies. The development of such policies will help agencies reduce risk of tort litigation.