When to walk

In The Upside of Down, Thomas Homer-Dixon writes that 11 September 2001 "won't be the last the last time we walk out of our cities".

But a hot tip from William J. Perry, Ashton B. Carter and Michael M. May (After the Bomb) is:

...for those downwind and more than a few miles from ground zero, the best move would be to shelter in a basement for three days or so and only then leave the area.

This is a hard truth to absorb, since we all would have a strong instinct to flee. But walking toward the suburbs or sitting in long traffic jams would directly expose people to radiation, which would be the most intense on the day after the bomb went off. After that, the amount would drop off day by day (one-third as strong after three days, one-fifth as strong after five days, and so on), because of the natural decay of the radioactive components of the fallout.