U.S. Route 15 (US 15) is a part of the U.S. Highway System that runs from Walterboro, South Carolina to Painted Post, New York. In Maryland, the highway runs 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, Leesburg, Virginia, and Charles Town, West Virginia (via US 340), to the south and with Thurmont, Emmitsburg, and Gettysburg, Pennsylvania, to the north. US 15 is a four-lane divided highway throughout the state except for the portion between the Point of Rocks Bridge and the highway's junction with US 340 near Jefferson. 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 has an unsigned business route through Emmitsburg.