Terrorist: "Hi, Bob's business freight courier service? I've got a lead on some test petroleum product I need shipped from the seller's address in Indiana to my office here in Fort Worth, TX. It's all packaged up and ready to ship. Just arrive at the loading dock and use the dock phone to call my number and I'll be down to accept delivery."
Petroleum product meaning standard fertilizer/petroleum mixture so favored for explosives. Office being the target building. And the number he calls from the dock phone will be a call to the trigger.
The point of this is to say that there are so many ways that this can be accomplished today, that it is kind of silly to worry about AVs when the potential exists in a billion different ways today. The key is to catch them before they are ready to call Bob's Courier Service. Once it reaches that level of maturity, from a counterterror perspective, you've failed.