Originally Posted by Bart
I think the reason for the focus on aviation as opposed to any other means of transportation is because the idea of flying bombs has a much more devastating impact than a blown up bus, train or subway. Hate to reduce this down to a matter of acceptable and unacceptable casualties, but that's essentially what it all boils down to.
If air travel shuts down (whether officially, or just because peopel stop doing it because it is perceived as "too dangerous"), the country shuts down.
The same can't be said of bus or train travel. Train travel is probably moderately important in the Northeast, but it could be easily replaced by driving. Bus travel could end and the country (economically) wouldn't notice.