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

by Wes Marshall

5, but for now, I want to introduce one of the most insidious offenders of this mentality: design speed. What is design speed? It represents the theoretical safe limit that we are designing the street to, akin to a design load for a bridge or building. For most of the years since the design speed concept originated in the mid-1930s, we defined it as the “maximum safe speed” for a given section of roadway. The idea behind design speed is simple enough. Let’s say I’m trying to design a major street where I’m expecting the speed limit to be 30 or 35 mph. As a designer, I’m aware that some people will drive faster than that. My guidelines3 tell me that I need to make sure those people are safe too: “The selected design speed should fit the travel desires and habits of nearly all drivers expected to use a particular facility.”e That is good to know, but what does it mean for design speed? What the guidelines say is that other than on local streets, “every effort should be made to use as high a design speed as practical.”4 So after setting the design speed as high as possible, I then design everything to make sure people can safely drive at that speed.