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Regulation in the U.S. requires it: “After several additional delays, the National Highway Traffic Safety Administration issued its final ruling in February 2018. It requires hybrids and electric vehicles travelling at less than 18.6 mph (30 km/h) to emit warning sounds that pedestrians must be able to hear over background noises. The regulation requires full compliance in September 2020, but 50% of "quiet" vehicles must have the warning sounds by September 2019.” https://en.m.wikipedia.org/wiki/Electric_vehicle_warning_sou...