U.S. Route 15 forges a 15-mile path through Frederick County, Maryland, from a bridge over the Potomac River out of Virginia to the Mason-Dixon Line into the Commonwealth of Pennsylvania. US 15 passes through the city of Frederick, has a concurrency with U.S. Route 340 and intersects Interstate 70, while also serving Thurmont, Emmitsburg and several Catoctin Mountain recreational areas. It can be regarded as a dividing line between the mountainous and piedmont regions of Maryland.