Like traffic lights, manhole covers (including this one in Chapel Hill, N.C.) have their own secrets. To learn what the arrows have to tell us, explore the photo gallery.
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For the past 15 years, writer Brian Hayes has made a hobby out of studying -- and photographing -- the manmade. He is the author of Infrastructure: A Field Guide to the Industrial Landscape.
Inspired by his daughter's questions, he set out to learn more about the workings of power lines, fire hydrants and other mysteries of the industrial age.
His subject on a recent field trip to the traffic command center in the nation's capital: How do stoplights work?