I don't know. If we're talking about trucks its a grey area. They have to inspected anyway. They have to go to weigh stations and can be pulled over for a weigh in and inspection of their log books. Its also likely that a smart terrorist would use a truck rather than an airplane. We're just lucky that we are fighting a bunch of Rube Goldberg wannabes who aim for the most complex solution to any problem. I mean simultaneous hijackings (okay that one worked), shoe bombs, ass bombs, underwear bombs, dirty bombs? Its like the CIA in the sixties trying to give Castro an exploding cigar.

On the other hand, two rednecks with a rental truck and some fertilizer got the job done. Its only a matter of time before Al Queda gets the message. Simpler is better. Add to this that the trucking world is a relatively small place. These guys know each other. They eat and get gas at the same truck stops, They go to the same distributors, they talk to each other on their CBs. Getting them to keep an eye out seems to make all sorts of sense to me. Plus, if they can put some sort of explosive sniffer at weigh stations it wouldn't be invasive.
Not sure I agree with everything I just wrote, but I'll throw it out for consideration.
FQ13