TRR No. 1260 Pavement Design, Management And Performance, pp. 135-146.
A fundamental scale is presented for evaluating appropriate levels of tangential and side friction factors with respect to design speed for new designs, redesigns, and rehabilitation strategies. The friction data used were obtained from the geometric highway design guidelines of the United States and several Western European countries, and from actual pavement friction inventories in New York State and in the Federal Republic of Germany (FRG). From the friction data of the countries in this study, relationships were developed between tangential or side friction factors and speed. The curves resulting from these relationships were then compared with percentile level distribution curves obtained from the actual pavement friction inventories. Analyses indicated that (a) the friction factors produced by the overall regression curves coincided with those obtained from the 90th-percentile level distribution curve of New York State, and with those derived from the 80th-percentile level distribution curve of the FRG; and (b) the friction factors derived from the 95th-percentile level distribution curve of New York State coincided with the friction factors derived from the 95th-percentile level distribution curve of the FRG. On the basis of these results, recommendations are provided for highway design for minimum stopping sight distances and minimum radii of curve. It is estimated that in applying the proposed friction factors for design, redesign, and rehabilitation strategis 95 percent of wet pavements will be covered in the United States and Europe. The recommendations provided should not be regarded as a final solution, but perhaps an international discussion of a larger dynamic safety supply for driving may be useful in reducing accidents on 2-lane rural highways. Because there are often inadequate safety factors in tire-road friction, friction demand often exceeds friction supply, causing more accidents than necessary.