The Interstate 93 as the "Northern Expressway viaduct" travels roughly northwest–southeast and passes through the Sullivan Square area. The Interstate act as a boundary of Charlestown neighborhood with points heading west with only two roads heading westward: Cambridge Street in the north and Austin Street/Gilmore Bridge to the south. US 1 diverges with Interstate 93 at the Leonard P. Zakim Bunker Hill Memorial Bridge where US 1 becomes a toll road and passes below Paul Revere Park and City Square before becoming the Tobin Bridge to the City of Chelsea across the Mystic River. State routes passing through Charlestown include 38 to the City of Somerville and 99 to the City of Everrett. The sole city-owned road linking the neighborhood Charleston with Downtown Boston is North Washington Street to the Southwest.