A triangular arch is to be constructed over a busy street in order to place a sky bridge traversing one side of the triangle to the other. The base of the arch spans the width of the street, which is 20 feet. Each side of the triangular arch measures 30 feet in length. If the entrances of the sky bridge lie at the midpoints of each side of the triangular arch, and the bridge is to be parallel to the street below, how long will the sky bridge be?