Car Bombs & The Fourth Generation:
It's things like this that have spurred me to teach the military science classes online. It isn't enough to be horrified; you have to go the next step, and realize that these things are probably going to come to America sooner or later. Many military scientists think that in "Fourth Generation" warfare, the distinction between civilian and military will largely disappear: the enemy will attack civilian targets so often that it will no longer be possible to rely on professional forces -- military or police -- as the primary defense of targets.
In broad terms, fourth generation warfare seems likely to be widely dispersed and largely undefined; the distinction between war and peace will be blurred to the vanishing point. It will be nonlinear, possibly to the point of having no definable battlefields or fronts. The distinction between "civilian" and "military" may disappear.
The car bomb (or VBIED/SVBIED, in military-lingo) is by far the most deadly weapon in the hands of modern insurgents -- in Iraq or otherwise. Unless they get their hands on radiological / nuclear material, that will probably remain true; and even the radiological material is most effective when combined with a car bomb.
Car bombs can't be controlled by restricting access to explosives; the ones in Iraq often use tank or artillery shells just because there were so many weapons depots in Iraq, but it's just as easy to make them out of common household or farm chemicals. For the same reasons -- power and access to explosives -- they have been the most popular weapons for serious terrorists. The IRA's long-argued-over surrender of its guns is purely symbolic; the successes of the movement have been won with bombs, not guns.
There isn't a good way of countering these things. The standard way is to create checkpoints where you can stop and search every vehicle that passes, but these are so manpower intensive that you can't set up very many. As a consequence, the best you can usually do is to set them up randomly in the hope of catching the car bomber by surprise. Even then, a suicide car bomber will probably just detonate at the checkpoint, still managing to kill quite a few people. It is possible that in the future, technology solutions may arise to aid these problems: robots to man checkpoints, chemical sniffers to search cars (though again, the wide variety of potential explosives hampers the effort to develop such sniffers).
The other option is to erect barriers and create no-vehicle zones. There are some notable side benefits to doing so -- they create pleasant areas for families and children, a kind of "main street" feel for small-business friendly districts, etc.
However, their size is limited by pratical considerations (e.g., how do you get food to restaurants/grocery stores in these zones? Well, you have to carry it in, which you can only do over so much distance; therefore, the zone can only be so big). They end up being practical for small popular concentrations, such as small towns, but not for large cities (imagine trying to subdivide Manhattan into a series of no-vehicle zones). However, large cities are where they are needed most, b/c of the population concentration that makes it easiest to kill lots of people there.
The logical consequence is the finality of a trend that started fifty years ago: the death of big American cities. America is large enough, and rich, enough to redistribute its population into a series of no-vehicle exurb-style towns if the business becomes important. There is some indication that Americans would prefer that lifestyle anyway, and it's consistent with the service-based economy and increasing telecommunication. In fact, life in or near such a zone could be relatively more pleasant than in current, vehicle-based suburbs.
It won't work for nations whose economic base is manufacturing, however, where there has to be concentration of manpower in zones easy to reach by transportation. Technology solutions will have to serve there; but, of course, these relatively poorer nations are the least well-suited to developing those solutions.
