Inset map for St. Louis and St. Louis County from the 1937 official state highway map. This was still a couple of years before the opening of the Daniel Boone Bridge in west St. Louis County; thus, US 40 was still routed through the northwest part of the county. There were relatively few changes between 1937 and 1938, especially compared to Kansas City, but one notable change was along Olive Street Road. In 1937, it was designated as supplemental routes; the next year, it had become Missouri 340.