US 15 was widened from downtown Thurmont north to Owens Creek starting in 1953. That same year, the highway was relocated as the first carriageway of an ultimate divided highway from Owens Creek to just south of the intersection of US 15 with St. Anthony and Orndorff roads south of Mount St. Mary's University. In 1956, construction began on the Thurmont Bypass, which was constructed as the first carriageway of an ultimate divided highway from south of Catoctin Furnace to just south of Owens Creek. The bypass, which included an interchange with MD 77, was completed in 1958. The MD 77 interchange originally consisted of one two-way ramp in the northeast quadrant of the junction. The old alignment of US 15 through Catoctin Furnace and Thurmont was designated MD 806 by 1960. US 15's interchange with MD 550 was built in 1972.