The Bay Bridge of today is not the same as the Bay Bridge of 1936. When the two-level bridge was originally built, it was designed to carry automobile traffic on the top level and trains and trucks on the bottom. Over the years, cars became increasingly popular and use of the railroad system waned. In 1958, the bridge was reconfigured to carry five lanes of westbound traffic on the top deck and five lanes of eastbound traffic on the lower deck [source:BayBridgeHistory]. An average of 270,000 cars cross the Bay Bridge each day [source: DOT.ca.gov].
One person died on the Bay Bridge in 1989 due to damage caused by the Loma Prieta earthquake [source: SFMuseum]. Since the earthquake, both east and west spans of the Bay Bridge have been refitted to protect against future damage from seismic activity. The eastern span is being replaced altogether with a new bridge, to be completed in 2013. Because engineers were concerned the current span would be susceptible to major damage from another earthquake, the new eastern stretch of the Bay Bridge will be able to sustain an 8.5 magnitude quake.