US 15 is the descendant of a pair of turnpikes that connected Frederick with Emmitsburg to the north and Buckeystown to the south. These turnpikes were reconstructed as state roads in the 1910s north of Frederick and in the early 1920s from Frederick south to Tuscarora. When US 15 was assigned in 1927, the Tuscarora – Point of Rocks highway had yet to be improved; this section was paved in the early 1930s. The modern Point of Rocks Bridge was built in the late 1930s after its predecessor was destroyed in a flood. The Frederick Freeway was constructed in the 1950s. US 15 was relocated to part of the freeway; the old route of the U.S. Highway through downtown Frederick became part of Maryland Route 355 (MD 355). US 15's present highway between Point of Rocks and Jefferson was constructed in the late 1960s; the old road south of Frederick was replaced with MD 28 and MD 85. North of Frederick, the U.S. Highway bypassed Thurmont and Emmitsburg in the late 1950s and mid-1960s, respectively. US 15 was upgraded to a divided highway north of Frederick in the early 1980s.