This plan was laid out by Inspire, an English-language magazine by Al Qaeda in the Arabian Peninsula. The magazine stated that drivers of the customized trucks could look for crowded areas, then motor onto the sidewalk and cause major carnage. It's a sick plan, and apparently bin Laden felt the same way. He called it "indiscriminate slaughter," and felt it wasn't in line with what Al Qaeda does.
Yes, seriously. And no, we're not making this up. Read more about this aborted plan and the way bin Laden managed the terror organization from his secluded home at this link.