Friday, January 24, 2014

Bay Bridge, San Francisco–Oakland:

Bay Bridge

The San Francisco–Oakland Bay Bridge is a complex of bridges spanning San Francisco Bay of the U.S. state of California. As part of Interstate 80 and the direct road route between San Francisco and Oakland, it carries approximately 240,000 vehicles per day on its two decks.It has one of the longest spans in the world.The toll bridge was conceived as early as the gold rush days, but construction did not begin until 1933. Designed by Charles H. Purcell, and built by American Bridge Company, it opened for traffic on November 12, 1936, six months before the Golden Gate Bridge. It originally carried automobile traffic on its upper deck, and trucks and trains on the lower, but after the closure of the Key System transit lines, the lower deck was converted to road traffic as well. In 1986, the bridge was unofficially dedicated to James B. Rolph.The bridge consists of two sections of roughly equal length; the older western section connects downtown San Francisco to Yerba Buena Island and the newer eastern section connects the island to Oakland. The western section is a double suspension bridge. Originally, the largest span of the original eastern half was a cantilever bridge. During the 1989 Loma Prieta earthquake, a section of the eastern section's upper deck collapsed onto the lower deck and the bridge was closed for a month. Reconstruction of the eastern section of the bridge as a causeway connected to a self-anchored suspension bridge began in 2002; the new bridge opened September 2, 2013 at a reported cost of over $6.5 billion and is currently the world's widest bridge, according to Guinness World Records.The bridge consists of two major crossings connecting each shore with Yerba Buena Island, a natural outcropping located mid-bay that is part of the city of San Francisco. The Western crossing lies between Yerba Buena and downtown San Francisco. It is composed of two complete suspension spans connected at a center anchorage.The top of the Rincon Hill neighborhood serves as the western anchorage and touch-down for the San Francisco landing of the bridge connected by three shorter truss spans. The eastern crossing, between Yerba Buena Island and Oakland, was a cantilever bridge that consisted of a double-tower span, five medium truss spans, and a 14-section truss causeway. Due to earthquake concerns, the eastern crossing was replaced by an entirely new crossing that opened on Labor Day 2013.On Yerba Buena Island, the double-decked crossing consists of a 321-foot concrete viaduct east of the west span's cable anchorage, a 540-foot tunnel through the island's rocky central hill, another 790.8-foot concrete viaduct, and a longer curved high-level steel truss viaduct that spans the final 1,169.7 feet to the cantilever bridge.The viaduct sections east of the tunnel are at present being modified, bypassed and replaced as part of the seismic safety work that will eventually transition traffic onto and off of the self-anchored suspension bridge of the new eastern bay crossing.The toll plaza on the Oakland side has eighteen toll lanes, of which six are dedicated FasTrak lanes. Mainline metering signals are located approximately 1,000 feet west of the toll plaza. Two full-time bus-only lanes bypass the toll booths and metering lights around the right side of the toll plaza; other high occupancy vehicles are permitted to use these lanes during weekday morning and afternoon commute periods. The two far-left toll lanes are operated as high-occupancy vehicle lanes during weekday morning and afternoon commute periods. During the morning commute hours, traffic congestion on the Oakland approach stretches back onto the three feeder highways, Interstate 580, Interstate 880, and Interstate 80 toward Richmond, California.Since the number of lanes on the San Francisco approach is structurally restricted, backups are frequent in the eastbound direction during evening commute hours. The western half is currently restricted to motorized freeway traffic. Pedestrians, bicycles, and other non-freeway vehicles and devices are not allowed to cross this section. A California Department of Transportation bicycle shuttle operates during peak commute hours for $1.00 each way.Freeway ramps next to the tunnel provide access to Yerba Buena Island and Treasure Island. Because the toll plaza is on the Oakland side, traffic between the island and the main part of San Francisco can freely cross back and forth without ever paying a toll. Those who only travel from Oakland to Yerba Buena Island, and not the entire length to the main part of San Francisco, must still pay the full toll.

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