Killed by a Traffic Engineer: Shattering the Delusion that Science Underlies Our Transportation System

by Wes Marshall

sciences. The original intent of design speed, for example, was to mathematically link design speed with curve radius, superelevation,h and the side friction factor. In fact, the title of Barnett’s 1936 paper in which design speed was first introduced is “Safe Side Friction Factors and Superelevation Design.”5 Design speed was not the main attraction. Instead, Barnett’s goal was a “balanced design” where all these factors would work in harmony to counteract centrifugal forces and allow drivers to take corners without slowing down. The same basic superelevation equations we use today date to 1920.6 At the time, “there was considerable public hostility to providing any superelevation due to the opinion that it encouraged high speeds and reckless driving on curves.”7 Despite the lack of consensus, AASHO made up its mind in 1930 that superelevation on curves was their official recommendation. The next step was to figure out the side friction factor, meant to represent the coefficient of friction between the vehicle’s tires and the road. In 1935, the Bureau of Public Works called on volunteers to participate in a study. “Several hundred drivers” were asked to take their own cars and ride along curves of known superelevation and radius.8 These drivers were then asked to report the speed at which they felt the car begin to “pitch outward.” As you might expect from such a study design, the results were all over the map. Nevertheless, the researchers averaged whatever drivers told ...