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As development 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 ineffcient, 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 assumed that most people don’t want to get into a crash and are thus doing what it is safe for them to do.”