Once engineers design that fast road, they set a speed limit. This, Dumbaugh explains, is also based on those old rule books. The rules recommend setting a speed limit based not on safety or accident prevention but on a combination of factors that engineers should consider equally. As one engineering rule book puts it, “In selection of design speed, every effort should be made to attain a desired combination of safety, mobility, and efficiency within the constraints of environmental quality, economics, aesthetics, and social or political impacts.” Or, traffic engineers should consider whether people can get to Walmart without a traffic jam to be as crucial as whether people don’t die on the road.
As developments arrives on a new road, congestion follows, and the road slows down. Chief among the rules taught to traffic engineers is that congestion is a problem and slow is inefficient, so postdevelopment, traffic engineers will reevaluate the speed limit. They decide the new speed limits by conducting a study that looks at how fast everyone is driving on the street. Then, they chart those speeds by frequency. Almost always in these studies, the majority of people are found to be driving at a similar pace, but around 15 percent are found to be driving much faster than everyone else. Traffic engineers use this latter group as their limit—setting the speed limit at the low end of how fast the fastest 15 percent drive, which is the high end of how quickly the other 85 percent drive. They call this the 85th percentile speed. It is how engineers set speed limits on major roads nationwide.
“We look at how fast cars are going and we assume that is the safe speed of the roadway,” says Dumbaugh. “Note that this has no safety basis: it’s simply assured that most people don’t want to get into a crash and are thus doing what it is safe for them to do.”
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