had learned from doing so. From west (Point San Quentin) to east (Castro Point), the bridge consists of: A trestle structure supported by fifty-seven bents. The upper deck is , and the lower deck is slightly longer at . of girder spans, consisting of nineteen girder spans of truss spans, divided into fourteen trusses each long, on average. The western cantilever structure, with arms measuring each flanking a central span of . The western cantilever span is the main navigation channel and features a vertical clearance of . of truss spans, consisting of ten spans each long, on average. The eastern cantilever structure, with arms measuring each flanking a central span of . The eastern cantilever span is the secondary navigation channel and features a reduced vertical clearance of . of truss spans, consisting of twelve spans each long, on average. of girder spans Excluding approaches, the bridge structures comprise a total length of on the upper deck and on the lower deck. Despite the varying height of the bridge, roadway grades are limited to 3% or less. As completed, the bridge has two decks each capable of carrying three lanes of traffic. As of 2020, westbound traffic rides on the upper deck and is marked with two lanes of vehicle traffic, as well as a pedestrian/bicycle path separated from vehicles by a movable barrier. Eastbound traffic rides on the lower deck and features two lanes of vehicle traffic as well as a third lane that is activated during evening commute hours and serves as a shoulder when not in use. The extra lane features lights indicating that the lane is open or closed. The third lane has been used for various purposes other than traffic, such as carrying a water pipeline during a drought. The bridge stands on 79 reinforced concrete piers supported on steel H-piles. Nine piers stand on land, eight are in cofferdams near the Contra Costa terminus, and the remaining 62 are bell-type piers with a flared base. The original deck was a thick reinforced concrete slab, with a mortar wearing surface thick. To facilitate maintenance, the bridge was designed with two lines (carrying compressed air and potable water) extending from end to end. Each deck was also equipped with three overhead maintenance tracks. Public transit service Golden Gate Transit bus route 580 provides public transportation across the bridge. Formerly route 40, it runs between the