Page:America's Highways 1776–1976.djvu/353



Skid accidents, which result from slippery pavements, especially when wet, are of such severity that they have received considerable attention in research and safety programs for more than four decades. Major skid factors include: water-on-pavement conditions, tires and their treads, pavement surface characteristics, aggregates and mixes used, pavement age and surface wear, speeds and vehicle operations, and highway geometries. There have been many contributing research studies on individual factors and their various combinations, but the complex interactions involved have inhibited easy and low-cost solutions.

In the late 1920’s Iowa State College began measuring road skid resistance with a towed trailer. Extension of these studies were reported in 1934; this proved to be a milestone report on the theory of skidding and means for measuring. Within the next few years many States, universities, industries and other groups were engaged in related research. Both field measurements and laboratory studies on aggregates, mixes, their polishing features, and means for measurement were under study. The Hybla Valley and other test roads included these features. In the 1950’s hydroplaning was identified as one of the skid conditions of much concern on airports as well as roads. Also the first steps toward grooving of concrete pavements were being made to reduce slipperiness.

Skid prevention attained nationwide attention and emphasis at an important international conference in Charlottesville, Virginia, in 1958. This conference dealt with many new research and development studies to attain better skid resistance and served to redirect the research that has continued since then. Because of the magnitude and severity of the problems in recent years, FHWA has instituted a large scale research project under its federally coordinated program. It deals with friction requirements, pavement materials and surfaces, accident and cost effectiveness studies, and a national program to standardize the skid measurement process.

Early results from the research have provided: (1) Three regional test centers and the calibration of dozens of State skid trailers; (2) a standard two-wheel test trailer and testing procedure; (3) guidelines to design appropriate variable message signs under adverse weather conditions; and (4) improved laboratory methods to select skid-resistant materials.

Accident research studies have often been deficient because of limited data and the difficulties in obtaining the data. Despite these limitations, key accident studies have provided much useful information that has dispelled certain myths and provided support for research and practical day-to-day programs.

One such myth prevalent during the 1940’s and 1950’s that has been exploded is that of the so-called “accident prone driver.” A study in 1962-63, showed that “. . . in any one year 0.5 percent of the drivers have two or more reported accidents and that they account for 13.9 percent of all accidents.” The accident prone concept suggested that these drivers be removed from the road, thereby reducing accidents by 13.9 percent. Such a conclusion, however, was erroneous. Nearly all of those who were “accident repeaters” in any one year became so by chance alone; the research revealed that 87 percent of the so-called accident repeaters would not have even one reportable accident the second year.

This trailer measures pavement skid resistance. A prescribed film of water is spread on the pavement junk prior to brake application. Instruments on the trailer measure wheel motion and traction during the entire braking cycle. 347