West Boulevard and East Boulevard mark the west and east borders of downtown, and numbered streets from First to Ninth run north and south between them. Mt. Rushmore Road is sometimes called Eighth Street by longtime residents. Since it is between Seventh and Ninth, that's quite accurate. Follow it south of Rapid City and it will indeed lead you to Mount Rushmore. With a touch to the screen you can find information on attractions, accommodations, and routes, as well as helpful facts on specific cities and regions.
Some centers also stock travel tapes.