Southern Avenue is one of three boundary streets between Washington, D.C., and the state of Maryland. Following a southeast-to-northeast line, Southern Avenue begins at the intersection of South Capitol Street in Southeast, Washington, D.C., and Indian Head Highway on the Maryland side. It runs for approximately seven miles to its other end at Eastern Avenue in Northeast, Washington, D.C., with an uncompleted gap between Naylor Road SE and Branch Avenue SE. The Southern Avenue bridge over Suitland Parkway was the first orthotropic deck girder bridge built in Washington, D.C.