Car Bomb

Car Bomb in Times Square:

A fuel-air bomb of "an amateurish" sort is still a significant threat. America does not realize how lucky it has been to go this far without these things being common in our cities; if they are hard to stop in Baghdad, with divisions of the US and Iraqi armies controlling approaches and manning checkpoints, there's honestly nothing at all to stop them in New York except good luck.

One reason horrible crimes often set off copycats is just that there are always horrible people who hadn't thought of it yet. Seeing it done is enough to wake their minds to the possibility that it could be done.

A shooting rampage, or a stabbing rampage like the one cited above, can be stopped while in progress by armed citizens -- indeed, even just by brave citizens. Car bombs aren't like that. You can harden society against them -- look at Ireland or Israel, or Baghdad -- but they are a different order of threat.

UPDATE: Allah at Hot Air remarks:

Read this Time magazine piece from five years ago about Al Qaeda capo Dhiren Barot’s “Gas Limo Plot,” which involved packing limousines with tanks of compressed gas, driving them into underground garages, and detonating them to create a fuel-air concussion that would bring down the building. As I understand it, an enclosed place is ideal for maximum damage from a bomb like that, but obviously not essential.
Well, in fact that's true for any kind of explosive. The force of an explosion is the pressure wave, at the edge of which gas or shrapnel is being thrown away from the blast. If it hits a wall, that wave will reflect back upon you. Thus, if you get hit twice by the pressure wave, it roughly doubles the amount of pressure that you are subject to.

The force of the pressure of a bomb above regular atmospheric pressures is called "overpressure." Enclosure is one way to increase it, but not the only way. Fuel-air bombs have a longer pressure wave than many kinds of bombs, so the concept of generating overpressure by reflection is even more useful with them.

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