How to evacuate a doomed city

Hurricanes Katrina and Rita are forcing us to think carefully about evacuating cities.

Sharon Begley wrote a Wall Street Journal "Science Journal" column (September 30) on what we're learning from simulation modeling about the role of highways in evacuations:

• "Under realistic conditions, a freeway carries about 2,000 vehicles per lane each hour past any given point."
• Contra-flow (using in-coming lanes for outgoing traffic) doesn't double freeway capacity - it can increase it by about 60% to 70%.  "It doesn't double capacity because left-hand exits, drivers' confusion over going the "wrong" way and signs turned backward gum up the works."
• Secondary roads can be used to supplement and to "drain" traffic from highways.  This requires that traffic lights be adjusted "to give priority to evacuees."  However this can't be done on the fly.  To do it effectively you need a "central control station, which few cities have."  Jurisdictional conflicts complicate things, because "traffic signals at highway exits are typcially controlled by the state, but those on arterial roads are under municipal control."
• Design highways with turnouts on the median to let out-of-gas cars pull off.
• Staging departures by postal zip code may help even out flow and cut "the 'network clearance time' - how long it takes to get everyone to his destination - 47% to 57%."  But can you really see people cooperating with this?
• "If you have six lanes of freeway (of which three are contra-flow), then at 2,000 vehicles an hour per lane and 2.5 people per vehicle, you can get about 600,000 people out of a city every 24 hours.  You can load more people into each car or use buses and trains, but evacuating 1.5 million souls will take two to three days."
• "Getting people out of harm's way if there is no advance warning (after, say, a radiological bomb) is just not in the cards."

All of this assumes that a terrorist threat to the city itself isn't also accompanied by a supplementary or follow-up attack on the car-choked highways. 

Begley cites a simulation in which "an apparent terrorist bombing of a major bridge in Portland, Ore." led the player to order an evacuation in case of additional attacks.  "As evacuees sat in gridlock, a toxic cloud of chlorine gas - released by accident or design - killed hundreds at their wheels."