Inset map for St. Louis and St. Louis County from the 1938 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.

From 1937 to 1938, there were not nearly as many changes in routes as there were in Kansas City. For example, city routes were still designated in St. Louis. But the addition of bypass routes was new. Bypass routes were designated for US 40 in the northern reaches of the metroplitan area; and for US 67 on the west and south sides along the route now known as Lindbergh Boulevard. Also making its first appearance in 1938 was Missouri 340, replacing supplemental routes E and K along Olive Street Road.