In the days before traffic signals and interstate highways, drivers traveling between New York and Florida passed through the intersection of Pratt and Light Streets in Baltimore, making it the busiest in the nation. It was up to the Baltimore Mounted Police and their faithful steeds to keep cars, trucks and people moving. No one did it better than Sergeant Bill McKeldin with his high-pitched whistle and the positioning of his horse Bob in the direction the traffic should flow. Called "The Whistler of Pratt and Light Street, " he became a legend. This is his story.