U.S. Route 15 (US 15) is a part of the U.S. Highway System in Maryland that runs 37.85 miles (60.91 km) from the Virginia state line at the Potomac River in Point of Rocks north to the Pennsylvania state line near Emmitsburg. Known for most of its length as Catoctin Mountain Highway, US 15 is the primary north–south highway of Frederick County. The highway connects the county seat of Frederick with Point of Rocks to the south and with Thurmont and Emmitsburg to the north. US 15 is a four-lane divided highway throughout most of the state. The U.S. Highway is a freeway along its concurrency with US 340 and through Frederick, where the highway meets US 40 and Interstate 70 (I-70). US 15 is the descendent 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 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.
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