At various points in time, every segment of Route 1A except the one in Westerly served as part of U.S. 1, which historically was the lower routing of the Boston Post Road mail route. The Route 1A designation came after the U.S. 1 designation was moved to a different alignment. This led to most segments of Route 1A being named Old Post Road or Post Road, while much of US 1 is now named the Commodore Perry Highway.
