On 9/11/2001, following the second plane crash into South Tower, the Port Authority of New York and New Jersey closed all of the bridges and tunnels under their jurisdiction. The MTA Bridges and Tunnels also closed their crossings into Manhattan for at least a day after the attacks.
With half a million people on Manhattan Island and the Twin Towers on fire and collapsing, why did New York/Port Authority shut down all evacuation routes except for the ferries and re-purposed water craft?
Apparently this decision necessitated the largest maritime evacuation in history, in a location that relies heavily on non-maritime travel (50% subway, 17% rail, 14% bus ).
Why did officials choose to do this? Was this standard procedure for a fire, or a special exception made on 9/11 and if so why?