Modeling, Simulations Can Help a City Offer More Efficient Exodus looks at traffic in an emergency:
Under realistic conditions, a freeway carries about 2,000 vehicles per lane each hour past any given point. Doubling the number of lanes by making southbound ones northbound, as on I-45 from Galveston to Dallas, is therefore a crucial first step. Houston officials ordered the reversal only at midday Thursday, after vacillating for more than a day.
That ‘contra-flow’ will increase a road’s capacity 60% to 70%, calculates traffic engineer Avi Polus of Technion-Israel Institute of Technology, Haifa. It doesn’t double capacity because left-hand exits, drivers’ confusion over going the ‘wrong’ way and signs turned backward gum up the works.
Houston officials admit that contra-flow was not even part of their emergency planning. If it had been, says Prof. Polus, “it shouldn’t take more than two or three hours to convert freeway lanes to a contra-flow” and change the traffic signals, exit ramps and feeder roads.
Bottom line: 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.
One final word of advice: motorcycle.