US 15 continues north through Haymarket, where the highway expands to a four-lane divided highway immediately south of its intersection with SR 55 (John Marshall Highway). The U.S. Highway passes through a diamond interchange with I-66 and runs through a suburban area between Catharpin to the east and the Bull Run Mountains to the west. At Woolsey, US 15 meets the northern end of SR 234 (Sudley Road) and reduces to two lanes. North of Bull Run, where the road enters Loudoun County, the highway passes through a pair of roundabouts at Gilberts Corner. US 15 meets US 50 (John Mosby Highway) at the second roundabout. The U.S. Highway runs along the east flank of the Bull Run Mountains as it passes James Monroe's estate of Oak Hill just south of the Little River and Oatlands Plantation in the community of Oatlands at its crossing of Goose Creek.