Complex sentence: Prior to the September 11th terrorist attacks, two of the bridge's four lanes were open to traffic during the festivals, but security concerns have now prompted the closing of the entire span to vehicles during these events.
Split into simpler sentences: Prior to the September 11th terrorist attacks, two of the bridge's four lanes were open to traffic during the festivals. Since 2001, security concerns have caused the entire span to be closed to vehicles during these events.