The alignment of US 13 in Wilmington has been shifted multiple times to improve traffic flow. By 1932, US 13 was split into the one-way pair of French Street northbound and Market Street southbound in the downtown area. Both directions of the route was shifted to use French Street by 1936. In 1952, a new bridge over the Christina River, the Walnut Street Bridge, was proposed to link Walnut Street in the downtown area with the Dupont Parkway section of US 13 south of the city. Under this plan, the Walnut Street Bridge would be used for northbound traffic while the South Market Street Bridge would be used for southbound traffic. By this time, US 13 was routed on a one-way pair in the downtown area, using Walnut Street northbound and French Street southbound. A year later, the Walnut Street Bridge project was under contract. Construction on the project began in 1955. Construction of the Walnut Street Extension, which included the drawbridge, approach roads, and a new bridge under the Pennsylvania Railroad (now Amtrak's Northeast Corridor), was completed in 1957. As a result, US 13 was split into the one-way pair of South Walnut Street northbound and South Market Street southbound between the south end of the city and downtown.