That was a hell of an effort to achieve a result that a) could easily have been exceeded by a small, simple low-explosive IED; b) Ended up having the opposite effect of the goal of terroism - it highlighted how difficult and relatively ineffective WMDs are.
http://en.wikipedia.org/wiki/Tsar_Bomba
"Thus certain bombs were designed to destroy an entire large city even if dropped five to ten kilometers from its center. This objective meant that yield and effectiveness were positively correlated, at least up to a point. However, the advent of ICBMs accurate to 500 meters or better made such yields unnecessary. Subsequent nuclear weapon design in the 1960s and 1970s focused primarily on increased accuracy, miniaturization, and safety. The standard practice for many years has been to employ multiple smaller warheads (MIRVs) to «carpet» an area, resulting in greater ground damage."
So sprawling won't save you neither from the initial "nuclear carpet" (that can be quite wide), nor from the radioactive fallout that have always spread over a much larger area than the radius of initial explosion(s).
But in these novels, the Earth was living in huge underground cities; big, noisy and somewhat dirty. The funny thing is that in Asimov's future Earth, population density was very extreme, people did have very little privacy (their apartments didn't even have own bathrooms, regular citizens used only public bathrooms and toilets). This was for 8 billion people, which in early 1950's looked like a huge population. This was supposedly achieved by megacities covering entire continents.
But today, the world has 7 billion people, and will have 8 billion people in a couple of decades - and life will not be that much different from what it is right now.
Urban designers for densely-populated countries should plan instead for people and industry to spread out as much as possible to eliminate the effective damage of WMDs. I think there was a sci-fi novel (by Asimov?) where people no longer lived or even met with each other, fearing death through either disease or malfeasance.
And the way drones are progressing, we'll soon have some like those in Dune - "bugs" that fly to their target, identify him/her and kill only that individual. I haven't heard of anyone working on a bug spray yet.
One solace: all the above was brought to us, one way or another, by democratically-elected large powerful national governments. Guess we wanted these things, eh?