Orange Discs in the Road

Q. We are noticing orange discs embedded in some Ann Arbor roads. Their location suggests traffic light management; however many are located after the intersection, leading me to think they are not changing the light for that intersection. What are they doing?

A. The discs are battery-powered sensors used for an adaptive traffic management system. “The sensors are placed on the departure side of intersections so they can gauge the amount of traffic flowing to the next intersection and therefore inform our traffic signal system,” reports city spokesperson Robert Kellar. “They are connected to our traffic control systems using wireless technology.”

According to a 2017 article in Wired magazine, Ann Arbor’s system uses data from the discs and from roadway camera systems to adjust the timing of multiple stoplights. A bulletin from manufacturer Siemens claims that installation of the system on the “Ellsworth Corridor” cut travel time there by 12 percent on weekdays and 21 percent on weekends.

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