Virginia Department of Highways and Transportation Skid Testing Equipment Correlation Results, 1974, 1975, and 1978

About the report:

The purpose of this study was to modify, as required, the previously established relationships between the VHTRC skid trailer, the VHTRC stopping distance car, and the first VDHT skid trailer, and to develop appropriate relationships with the above three devices for the second, recently acquired VDHT trailer. Regression equations indicating the relationships between the trailers are given below. In general these equations indicate that 1. the relationships remain the same with changes in test speed, and from year to year; and 2. the relationships are essentially one to one, i.e., the slopes generally are not significantly different from 1.0 with some difference in average results as follows. VDHT Trailer #1 : VDHT Trailer #2 + 2.5 SN VDHT Trailer #1 VHTRC Trailer 3.0 SN VDHT Trailer #2 VHTRC Trailer- 7.0 SN As expected, the SN values obtained with the trailers were less than the SDN values obtained with the car at the same initial test speed and the relationships differ with test speed. However, it is important to note that for these relationships the slopes generally do differ significantly from 1.0; i.e., the difference in measured values between the car and trailers is not constant from low to high levels of skid resistance, with the difference being larger on low skid resistance sites. Since most of the survey skid data have been controlled with the VDHT Trailer #1, it seems reasonable to standardize results in terms of this test unit. Thus, survey data collected with the newer VDHT Trailer #2 should be corrected either in terms of adding 2.5 SN or by using equation 4 in Table 3 of the report. However, it should be pointed out that no correction of this type would result in a conservative (i.e. lower) interpretation of survey SN data. Predictions of SDN values, or stopping distances, should be made based on equations 17-21, 25, or 30-34 from Table 2 of the report, depending on the test trailer used.

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