An excellent point. I was discussing this with some friends at work, and these former air force (US and Can) members were talking about the kind of destruction you get in a strategic yeild airbust.

OK, in fairness, a low airburst would be easy- put it in a light aircraft, take it up to 10K feet, and pull the pin. But we'd be most likely looking at a tactical device or a small strategic device, maybe 25kt. Yes, that is the class that ruined large parts of Hiroshima and Nagasaki, but in a ground burst a very significant portion of the blast and radiation effects are going to get soaked by the terrain. Sure, set off a FatMan in Central Park and you'll probably break windows in Brooklyn and maybe have a few small fires from the thermal pulse, but you are in a much greater danger from the fall out. (Brooklyn- spark plug sized fallout?) That's where the FEMA duct tape and drop cloths will come in handy. Hopefully.

If people shelter in place, (a) they aren't clogging the streets, (b) limiting exposure, and (c) probably staying put long enough for the radiation levels to drop to the point that it fairly survivable.

Of course, I expect them to panic.