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Maximizing Surface Friction for Road Safety Improvements Project 04 Incorporating Road Safety into Pavement Management: Maximizing Surface Friction for Road Safety Improvements Project 04 - 04 June 2007 Midwest Regional University Transportation Center College of Engineering Department of Civil and Environmental Engineering University of Wisconsin, Madison Authors: David A. Noyce, Ph.D., P.E., Hussain U. Bahia, Ph.D., Josue Yambo, Jeremy Chapman, and Andrea Bill University of Wisconsin-Madison Traffic Operations and Safety Laboratory Principal Investigator: David A. Noyce, Ph.D., P.E. and Hussain U. Bahia, Ph.D. Department of Civil and Environmental Engineering Technical Report Documentation Page 1. Report No. 2. Government Accession No. 3. Recipient’s Catalog No. MRUTC 04-04 4. Title and Subtitle 5. Report Date INCORPORATING ROAD SAFETY INTO PAVEMENT MANAGEMENT: June 1, 2007 MAXIMIZING SURFACE FRICTION FOR ROAD SAFETY IMPROVEMENTS 6. Performing Organization Code 7. Author/s 8. Performing Organization Report No. David A. Noyce, Hussain U. Bahia, Josue Yambo, Jeremy Chapman, and Andrea Bill 2007-005 9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) TRAFFIC OPERATIONS AND SAFETY LABORATORY University of Wisconsin-Madison 11. Contract or Grant No. Department of Civil and Environmental Engineering 0092-03-20 1415 Engineering Drive, Madison, WI 53706 12. Sponsoring Organization Name and Address 13. Type of Report and Period Covered Wisconsin Department of Transportation Final Report, 8/2003 – 6/2007 Hill Farms State Transportation Building 4802 Sheboygan Ave 14. Sponsoring Agency Code Madison, WI 53707 15. Supplementary Notes Project performed in cooperation with the Midwest University Transportation Center and the Wisconsin Department of Transportation. Report url: http://www.mrutc.org/research/0404/ 16. Abstract This research explored the relationship between asphalt mix design, skid friction, and roadway safety. Initial tasks attempted to find a relationship between pavement skid resistance (friction) and crash frequency, particularly wet weather crashes. Friction and crash data collected over 10 years at six study sites in Wisconsin were analyzed. The results of the analysis did not indicate a relationship between crash frequency and pavement skid friction. Although some evidence suggests that the number of wet pavement crashes increased as the pavement life increased (and skid friction values decreased), the frequency of crashes was not sufficient to statistically support. Nevertheless, the fact that the relationship seems to behave inversely proportional, that is to say more crashes occurred at low friction numbers (FNs), is an important indication that skid resistance may indeed be a factor affecting wet weather crashes. It was not possible to determine a skid friction threshold value that indicates the critical point where pavement maintenance would be needed. Although the data obtained in the research could not support a specific value, it is clear that friction values less than 35 are problematic from a safety standpoint. A possible indicator of friction on high-speed roadways is macrotexture. Therefore, macrotexture (measured as MTD) combined with friction data was of great interest in this research. Plots of MTD and FN values did not show a clear relationship between the two values, although it was evident that the larger FNs were concentrated in low MTD values. Skid resistance is an important feature which should be considered while evaluating roadway safety. An effective asphalt pavement asset management approach will include an annual testing program to monitor skid friction values. FN values less than 35 should trigger a safety monitoring program and those pavements scheduled for future rehabilitation or reconstruction. 17. Key Words 18. Distribution Statement No restrictions. Skid Friction, Road Safety, Pavement Management, Wisconsin, Road Surface 19. Security Classification (of this report) 20. Security Classification (of this page) 21. No. Of Pages 22. Price Unclassified Unclassified 240 Form DOT F 1700.7 (8-72) TOPS LAB REPORT 2007-005 Incorporating Road Safety into Pavement Management: Maximizing Surface Friction for Road Safety Improvements by David A. Noyce, Ph.D., P.E. Associate Professor and Director Hussain U. Bahia, Ph.D. Professor Josue Yambo Graduate Researcher Jeremy Chapman Graduate Researcher and Andrea Bill Graduate Researcher Research Project Number 144-ME92; 144-LZ06 Study Title: Incorporating Road Safety into Pavement Management Sponsored by the Wisconsin Department of Transportation In cooperation with U.S. Department of Transportation Federal Highway Administration June 2007 TRAFFIC OPERATIONS AND SAFETY LABORATORY University of Wisconsin-Madison Department of Civil and Environmental Engineering 1415 Engineering Drive Madison, WI 53706 www.topslab.wisc.edu iii iv DISCLAIMER This research was funded by the Midwest Regional University Transportation Center. The contents of this report reflect the views of the authors, who are responsible for the facts and the accuracy of the information presented herein. This document is disseminated under the sponsorship of the Department of Transportation, University Transportation Centers Program, in the interest of information exchange. The U.S. Government assumes no liability for the contents or use thereof. The contents do not necessarily reflect the official views of the Midwest Regional University Transportation Center, the University of Wisconsin, the Wisconsin Department of Transportation, or the USDOT’s RITA at the time of publication. The United States Government assumes no liability for its contents or use thereof. This report does not constitute a standard, specification, or regulation. The United States Government does not endorse products or manufacturers. Trade and manufacturers names appear in this report only because they are considered essential to the object of the document. v ACKNOWLEDGEMENTS This research project was conducted under a cooperative program between the Traffic Operations and Safety Laboratory and the Wisconsin Department of Transportation. The authors acknowledge and appreciate the guidance of the WisDOT in the skid friction data collection and use of the skid trailer. The contributions and sharing of information from Roger Larson of Applied Pavement Technologies is greatly appreciated. The authors would also like to thank the many graduate students and others associated with the Traffic Operations and Safety (TOPS) Laboratory who helped along the way. vi EXECUTIVE SUMMARY Traffic crashes and the associated injuries and fatalities pose a significant problem for transportation safety professionals. To overcome this problem numerous road crash investigations and statistical analyses have been done. These studies suggest that there is a distinct relationship between crash frequency and pavement surface conditions that should be taken into consideration to potentially improve safety on our highway network. This research study endeavors to find a relationship between pavement skid resistance (friction) on asphalt pavements and crash frequency, particularly wet weather skid crashes. The hypotheses developed were that: (1) crash frequency is inversely proportional to skid friction values; and (2) a skid friction threshold exists at which safety of the roadway system decreases with values at or below this threshold. Additionally, the research explored the relationship between pavement macrotexture (large scale texture) and friction for high speed roadways. To determine the correlation between pavement skid friction values and crash frequency, it was necessary to collect data pertaining to these two variables. A series of tasks were developed to acquire data and other information that provide guidance and support to achieve the objective of this research. Asphalt pavement skid resistance (measured as friction number at a given test speed and tire type, FN40R and FN50R) and crash data for study sites in Wisconsin from 1994 to 2004 were studied and analyzed. Results of the data analysis did not indicate a statistically significant relationship between crash frequency and asphalt pavement skid resistance. Hence, statistical support for either hypothesis was not found. However, there was some evidence which suggested that the number of wet pavement skid crashes increased as the pavement life increased (and consequently skid friction values reduced). The frequency of crashes was not sufficiently large to support this claim statistically. Nevertheless, the data shows an inversely proportional relationship between FN values and crash frequency indicating that skid resistance on asphalt pavements is a factor affecting wet weather crashes. Results of the study were hypothesized to show a directly proportionate relationship between macrotexture (measured as mean texture depth, MTD, use the Sand Patch Method) and skid resistance. In other words, the larger the MTD value, the larger the FN. However, the results of the data analysis for the study sections revealed the relationship was inversely proportional. There were no observable relationships shown on plots comparing MTD to FN data, although it was evident that the larger FNs were concentrated in low MTD values. If a statistically significant relationship existed between MTD and pavement FN, then skid resistance can be predicted from volumetric measurements. Volumetric measurements could then help in the design of pavement mixes that yield high FN, consequently increasing the safety of the road by maximizing grip in the tire-pavement
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