Correction:
"Vast majority" is probably inaccurate, depending on the attack scenario (military vs. civilian targets). "Majority" is probably still accurate. Urban/Non-urban split of human population is about 50/50 now.
Most fallout is generated by ground bursts or "burrowing" warheads. Military targets. A civilian target like a city would usually be an air burst (like Hiroshima) which generates much less fallout. I live near what used to be a major military target (it's not any more), so figured in an attack without warning I could kiss my ass goodbye. People who lived some distance outside the Bay Area, like the unsurpassed imbeciles in the movie "Testament," could survive quite well.