Walking is probably the worst thing they could do. If they survive the blast, then the main danger is exposure to fallout. A car will shield some of it directly, a building far more, and the biggest danger is ingestion(so you don't want to get it on your lips.) While length of exposure to certain types of radiation is also a factor, in most instances sheltering in place inside a home, showering off as much as possible, and eating only food from sealed containers after washing the containers is going to be far safer.
All of you are forgetting radiation hormesis: it has been repeatedly proven that 10,000 mrem a year would make us healthier and we'd live longer.
I would never evacuate. The risks are MUCH higher on the roads than closing windows, reducing air intake and waiting for the stupid little cloud of rad gas to dissipate. The fear-driven maniacs on the highways are guaranteed to kill more than if we all stayed put. Of course 30 years of mind-numbing propaganda by media and most textbooks has thoroughly mis-taught millions of Americans. As a result they will die needlessly in traffic jams.
Hormesis proof:
Is Chronic Radiation an Effective Prophylaxis Against Cancer?
at
http://www.jpands.org/jpands0901.htm
Afraid of Radiation? Low Doses are Good for You
http://www.lewrockwell.com/miller/miller12.html