Slipperiness of Highway Pavements: Phase 1, Final Report

About the report:

To gain knowledge of the wet friction levels needed for various traffic conditions, this study evaluated the relationships between (1) accumulated traffic volumes and pavement friction, and (2) percent wet accidents and pavement friction. In addition, the study evaluated speed gradients and machine correlations, and selected thirty test sites for continued periodic testing under state funds. Tests were performed at two-tenth mile intervals on 11,650 lane miles of pavement. The test data will be passed on to the Highway Department to be used as the nucleus of its future road pavement friction inventory. In the study, sixty-two sites were tested at various speeds with the Council's skid trailer to provide data for developing friction speed gradient curves. Speed gradients were not found to be different for any of the paving mixes normally used in Virginia, and the average gradient was about 0.7. This conclusion is based on the conditions under which the data were obtained and would not necessarily be valid for varying water and tire tread depths. At sixty-one of the sites, stopping distance tests were also performed so that a correlation could be established between the values obtained with the trailer and those obtained with the stopping distance car. The regression analyses performed on these sets of data indicate that the best predicting equations for stopping distance numbers occur when the trailer tests at 30 mph. Also, it appears that the relationships did not change in any orderly fashion with time. Forty-six sites were tested to determine the relationship between accumulated traffic volumes and friction levels. It was found that non-polishing S-4 and S-5 mixes retain an average PSDN of 48 after 25-30 million vehicle passes. Non-polishing portland cement concrete mixes lost skid resistance more rapidly than do non-polishing S-4 and S-5 mixes, and on the average have a PSDN of about 44 after 20 to 25 million vehicle passes. I-2 and I-3 limestone mixes decrease in friction more rapidly than do the S-4, S-5, and portland cement concrete mixes, and reach an average PSDN of 42 after 16 million vehicle passes. Five hundred and twenty-one sections on Virginia's interstate system, totaling 312.8 miles, were studied to determine the relationship between the percent of wet accidents and predicted stopping distance skid numbers. The sites were separated into four categories open roadway, non-open roadway, open interchanges, and non-open interchanges. It was found that the minimum PSDN for the traffic lanes of interstate roadways with a mean traffic speed of about 65-70 mph should be 42. It can be demonstrated that this value probably is about the same as that recommended by Kummer and Meyers in NCHRP Report 37, assuming that a different relationship exists between skid trailers and the stopping distance method than the one they used.

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