US 29 / 15 is joined by US 17 south of Warrenton in Fauquier County, and continues around the town, with US 17 splitting off. US 29/15 continues mostly eastward to Gainesville where US 15 splits and US 29 intersects Interstate 66 for the first time. US 29 continues into Fairfax County, where it passes along the boundary of the city of Falls Church, where the road has two different names. The portion of the street running northbound is located in the City of Falls Church is called Washington Boulevard, and has different street addresses than the other side running southbound in Fairfax County, where it is named Lee Highway. The road continues into Arlington, having intersected I-66 five more times before crossing into the District.