What led to duck and cover drills and bomb shelters in the united states during the cold war?

Duck and cover is a method of personal protection against the near prompt effects of nuclear explosion. It is useful in conferring a degree of protection to practitioners outside the radius of the nuclear fireball but still within sufficient range that by standing upright death or serious injury is certain. As a countermeasure to the lethal effects of nuclear explosions, it is most effective in the event of both a surprise nuclear attack, which might come at any time without warning, and in the more likely event of sufficient warning of a few seconds to minutes given by a system such as the Pinetree Line, but not enough advance notice to permit an effective evacac. Bomb shelter protected there men from bombs.