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    Measuring Road Surface Slipperiness

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    Twenty-one agencies cooperated in comparative tests with trailers, stopping-distance vehicles, and portable testers on five specially prepared surfaces ranging in coefficient of friction (C/F) from extremely low to relatively high. Several secondary experiments were conducted to determine the effects of such variables as water film thickness, changes in load of vehicles, operator experience, etc. The test data were statistically analyzed to determine significant differences between and within groupe of machines. The analysis revealed the following: 1. Many types of trailers are capable of determining accurately the C/F resulting from a steady-state skid. There was good agreement among trailers that included three different types of force measuring systems. 2. The refinement of certain techniques or equipment or both could bring the results of the Tappahannock study even closer. Two of these factors are trace interpretation and attainment of desired speed during testing. 3. Size of passenger vehicle, from compact car to bus, does not widely influence the ability of the vehicle to stop during an emergency skid if all vehicles utilize the same general type of tire tread and a similar rubber composition. The ability of various size vehicles to stop when utilizing their normal type of tire was not established in the Tappahannock study and is a question needing further research. 4. The technique and equipment used in making stopping distance tests should be standardized because of the wide divergence of results obtainable. 5. The slope of the regression line which compares the British portable tester and the stopping distance method of measuring the C/F is steeper for this study than was the regression line for a similar study held in Virginia in 1960. (Possible explanations for this are included in the text.) 6. The British portable testers do not differ from each other in the values which they give to any practical significant degree. This is best shown in the variance components analysis of the secondary experiment, Machine-Operator-Site Variables (Latin Square). 7. In the same analysis it was also found that the operator differences and, generally, the experimental error, were negligible. 8. In the secondary experiment, Experienced Versus Nonexperienced Operators, it was found that with little instruction a new operator can obtain valid data with the British portable testers. 9. When the British (natural) rubber sliders are used on the British portable testers lower readings are recorded than when American E-17 X276O (synthetic) rubber is used. 10. The newly developed PSU-PDH dragtester paralleled the values of the vehicular machines across all sites better than did the other portable testers. 11. Both the NCSA bicycle wheel and the dragtester give what is believed to be a more realistic value on Site I than do the British portable testers.

    Author Information:

    Dillard, Jack H.
    Highway research engineers, Commonwealth of Virginia,

    Mahone, David C.
    Highway research engineers, Commonwealth of Virginia,

    Committee/Subcommittee: E17.31

    DOI: 10.1520/STP48230S