Bigger nukes increase the area of total destruction but they also increase the area of partial destruction. Furthermore, the area of total destruction is still not going to be big enough to cover an entire metropolitan area unless you're talking about something along the lines of Tsar Bomba. The point remains that civil defense procedures like "duck and cover" would save many lives.
As would "run for underground shelters", admittedly.
But then you're assuming reasonable and conscious plebes. Personally, I'd readily picture rednecks going "Meh, they won't finance these shelters on my dime lest it turn into yet another .gov fiasco, and I wouldn't budge anyway because .gov is just scaremongering us into living in rabit holes! I'm ME and I'm invincible. [Roar!]"
Plus, your story doesn't say if whoever goes for a subterranean shelter or ducks eventually digs her way back out. (I'd hope we never know if it does.)
I'm not sure I understand your point here. It doesn't take a whole lot of reasonableness to take cover when you know you're about to be hit by a massive blast wave. As for digging out, many of the survivors will just be able to climb out from the rubble. Escaping death by fallout then becomes interesting for many, but that won't affect many others.
