US 15 continues north as a four-lane controlled-access expressway past the Monocacy Boulevard/Christophers Crossing interchange before leaving the city of Frederick by crossing Tuscarora Creek. The highway parallels the Monocacy River and has a junction with Biggs Ford Road on its way north to the hamlet of Hansonville, where the U.S. Highway intersects Old Frederick Road. US 15 crosses Fishing Creek to the west of Lewistown and is closely paralleled on the west by Auburn Road as the highway gradually approaches the eastern flank of Catoctin Mountain. At the northern end of Auburn Road, MD 806 (Catoctin Furnace Road) begins to parallel the northbound side of the U.S. Highway, which follows the eastern boundary of Cunningham Falls State Park just west of the village of Catoctin Furnace. Access to the main section of the state park is provided by Catoctin Hollow Road, which intersects the southbound lanes of US 15 and follows Little Hunting Creek up the mountain. The park's Manor Area is accessible from US 15 a short distance north of Catoctin Hollow Road.