Charley, Jarrett Walker says that that’s exactly what they do in Paris, and even Los Angeles. More discussion at Streetsblog LA.

“Why built dependency on transit, when that only creates exploitable single-points-of-failure? If western society can’t work without the commuter trains, that’s the easy and obvious target.”

What about auto-bridges and tunnels? These are points of failure. Countless stretches of elevated roadway are vulnerable to attack. To the extent that automobiles can route around destroyed passages, trains can do the same if parts of their infrastructure are taken out. (Trains were used through the last century’s world wars, enduring attacks to which modern terrorism pales in comparison.) But this ‘failure’ framing of terrorism is fundamentally wrong; terrorism doesn’t work by physically disabling a system. It works by attacking some part of it, killing some number of civilians, and scaring the masses away entirely. If even a non-catastrophic explosion were achieved on a bridge into Manhattan, the city would shut down all bridges and tunnels for a time and most people would be too terrified to use them for much longer than that. Americans would be afraid to drive across bridges all over the country. In fact, 9/11 gave the perfect example of how terrorism cripples a decentralized system when it first disabled, and later severely impaired airline transportation (which we then got to bail out, hooray).

But leaving the false premise aside, the answer the series of rhetorical Why?s is the following: Because spreading out and depending primarily or highway transportation kills a lot more people in crashes every year than terrorism has in total. So, trying to accommodate terrorism by avoiding density would be a dumb move even if it did stop all attacks—which it would not do in the least. It would still be possible to terrorize the suburbs, which have shown themselves in the past decade to be particularly easy to terrify. To reframe your scary question, just because there hasn’t ever been a coordinated, large scale attack on the malls of America doesn’t mean that won’t be the next plot. If anything, global terrorists have shown a bias towards the unprecedented.

I don’t have anything against smaller urbanizations like suburban mixed-use developments and any surviving, functional American towns; to the contrary I think they’re great! But to say that terrorism is a good reason to avoid arbitrary levels of density and arbitrary kinds of transit is severely myopic. Apply that strategy of target reduction inductively, and you have everyone moving back into caves.