the early 1940, the Muskegon Bypass was given the US 31 designation, and the route downtown was redesignated US 31A. During World War II, a bypass of downtown South Haven was built; the former route of US 31 through the heart of the city was designated Bus. US 31 at that time. After the war, the route of US 31 north of Charlevoix was realigned to follow the shoreline; this section opened by the middle of 1949. The route of the highway between Holland and West Olive was changed to run on a more angular course northwesterly in 1950. A few years later, a bypass to the south and east of Holland opened and the former route through down was redesignated as a business loop in 1954. Freeway era On November 1, 1957, the Mackinac Bridge opened to traffic. For the opening of the bridge, the highways coming into Mackinaw City from the south were realigned to connect to it; US 31 terminated at the southern approach to the Mackinac Bridge. In November 1960, sections of I-75's freeway opened from Indian River north to the southern Mackinac Bridge approaches, and US 31 was rerouted to follow segments of that freeway from the current northern terminus south of Mackinaw City northward. By the end of the decade, another freeway segment opened along the Muskegon Bypass as well. The next year, US 33 was extended northward along US 31 from St. Joseph for about . In 1962, a section of freeway along US 31 was opened between I-94 and the Berrien–Van Buren county line. This section was originally designated as part of I-96/US 31; the former route near the lakeshore became just US 33. The MSHD petitioned federal highway officials to switch the Interstate designations west of Grand Rapids, reversing the I-96 and I-196 numbers to their current configurations. After the designation switch was approved in 1963, an additional was opened from the northern end of the freeway near Benton Harbor to Holland as I-196/US 31. The freeway was also extended northward from Muskegon to the Muskegon–Oceana county line north of Montague in 1963. When I-196 was completed between Holland and Grandville in 1974, the BL I-196 designation was applied along US 31 and Bus. US 31. The next year, the US 31 freeway was extended northward into Oceana County to New Era. In 1976, this freeway was lengthened further to Hart.