Each day, thousands of cars—sometimes as many as 18,000—rolled along Elmwood Place’s streets, crossing the third-of-a-mile town to get to neighboring Cincinnati or major employers in bustling suburbs or heavily traveled Interstate 75. Many zipped by Elmwood Place’s modest homes and small businesses at speeds well above the 25 mph limit. Bedeviled by tight budgets, the police force was undermanned. The situation, villagers feared, was dangerous. Then the cameras were turned on, and all hell broke loose. Like hundreds of other U.S. communities big and small, Elmwood Place hired an outside company to install cameras to record traffic violations and...