On a cool December day in 1925, Charles Adler Jr. stood beside Falls Road, a state highway on Baltimore’s north side. He was there to test his latest invention: an electromagnetic apparatus that would automatically slow cars traveling at unsafe speeds. Adler had embedded magnetic plates in the road where it led into a precarious curve, and he was now waiting for a specially prepared car to drive over the magnets. The magnets would activate a speed governor connected to the vehicle’s engine, slowing it to 24 kilometers per hour.
Adler had developed this automatic speed-control system for railroad crossings, the scene of many deadly accidents at the time. But he soon came to imagine all sorts of applications for it: “Dangerous road intersections, streets on which schools are located, bad curves, and even steep down grades,” according to an article in the Baltimore News. (Source)