Philadelphia will be painting optical illusion speed bumps at 100 dangerous intersections. These are designed to look like raised triangles, making drivers stop and think and hopefully slow down.

Despite the opinions of ignorant tech blog readers who fall into the same assumptions as classic traffic engineers, visual cues are very effective at slowing traffic. Shorter sight lines and a narrower feel of a road make drivers proceed more carefully, as do painted murals in intersections.

In Chicago, officials grappled for years with how to slow traffic at a dangerous S-curve on Lake Shore Drive. Lower speed limits didn’t help, and straightening the curve didn’t cut accidents. They finally hit on another optical illusion: painting a series of horizontal strips, which become closer together as you reach the curve. This produces a visual effect that looks like speeding up; drivers instinctively slow down to keep their perceived speed the same, and voil&agrave: slower traffic and a safer curve.

Image from Nudge, page 37

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Tagged: chicago, philadelphia, roads, traffic

David Alpert created Greater Greater Washington in 2008 and was its executive director until 2020. He formerly worked in tech and has lived in the Boston, San Francisco Bay, and New York metro areas in addition to Washington, DC. He lives with his wife and two children in Dupont Circle.