When I was growing up in New York City, many institutional buildings, including my school, had radiation trefoils affixed to the doorframe, indicating that they were fallout shelters. While I was a little too young for the "duck and cover" routine, I did learn that in the event of a nuclear attack, I should tuck my pant legs into my socks, and proceed to the basement at a smart clip. After the Berlin wall fell, even this advice was discontinued.
So was the advice crazy back then, and is it crazy now? The answers are "probably not," and "no." The snark, though understandable, is misplaced.
Even short-term sheltering (a day or two) before attempting to evacuate the area will dramatically increase the number of survivors. The difficulty, as the planning document puts it, will be overcoming people's "natural instincts to run from danger and reunify with family members." Overcoming those instincts will require preparation and education on the part of public health and school authorities.
When Americans think about nuclear war, we tend to think about the apocalyptic scene at the end of Dr. Strangelove, a war involving thousands of megaton-yield hydrogen bombs. (A megaton is the equivalent of a million tons of TNT, or about 60-70 times the power of the Hiroshima atomic bomb, which had an explosive power of around 15 kilotons, the equivalent of 15,000 tons of TNT). But in 1951, neither the United States nor the Soviet Union had yet tested a hydrogen bomb, and the duck-and-cover era authorities were basically preparing people for a rerun of Hiroshima and Nagasaki with us on the receiving end of relatively small numbers of (relatively) small nuclear weapons. "Duck and cover" advice is particularly effective there.
An atomic explosion can blind you, burn you, crush you with explosive power, or poison you with radiation. The "duck and cover" advice, based in no small part on the experience of Hiroshima and Nagasaki survivors, was designed to do what could be done to minimize that.
When an atomic bomb explodes, several things happen in short order. First is a flood of "prompt" radiation created by the nuclear fission that produces the explosion. The good news -- if you can call it that -- is that if you are close enough to get a lethal dose of prompt radiation, you're close enough that you're likely to be killed by other bomb effects before it becomes an issue. Next comes the "flash," a brilliant pulse of light created as the air around the bomb is heated to millions of degrees; this starts out as ultraviolet, falls quickly into the visible light range, and then into the heat-ray infrared range within a few seconds. The flash can blind, or burn exposed skin, and start fires. Next comes the blast, as the superheated air expands outward, initially at supersonic speeds. The blast is dangerous on its own, and also because it crushes buildings and creates clouds of flying glass and debris.
Given that light travels almost instantaneously, for everyone outside the immediate vicinity of the bomb the flash will arrive before the blast. Furthermore, the fire-setting infrared part of the flash peaks a few seconds later than the initial burst of light. So those who see a brilliant flash of light -- and know what it means -- have a few seconds to get under some sort of cover to protect themselves from what comes next.
After these "prompt effects" of initial radiation, flash, and blast have passed, there is an additional hazard. A nuclear explosion sucks air, dust -- and, if it's close to the ground, vaporized soil, buildings, etc. -- up into the fireball, where some components are transformed into radioactive isotopes that then fall out of the cloud and back to earth over the next few hours, hence the term "fallout." The radiation from fallout can be severe -- the bigger the bomb, and the closer it is the the ground, the worse the fallout, generally -- but it decays according to a straightforward rule, called the 7/10 rule: Seven hours after the explosion, the radiation is 1/10 the original level; seven times that interval (49 hours, or two days) it is 1/10 of that, or 1/100 the original, and seven times that interval (roughly two weeks) it is 1/1000 the original intensity. Because it is dust, fallout travels with the wind.
A terrorist bomb is likely to be relatively small -- possibly only a fraction of the Hiroshima bomb's explosive power -- and likely exploded at ground level. This means that the area totally destroyed by the explosion is likely to be much smaller than the area exposed to lesser damage or to fallout radiation (this nuclear weapons effects calculator from the Federation of Atomic Scientists will let you see the effect of different sized bombs burst at different heights). Because of this, Homeland Security people in the Obama Administration have been encouraging a duck-and-cover approach, followed by advice to "shelter in place" against fallout rather than trying to evacuate the area.
As the professor would say, read the whole thing.