Or those "u-store-it" complexes. A lot of those are located near highways and commercial areas. It would be interesting to see if there are u-store-it complexes close to where the attacks took place. It would be simple to have a front person go around and rent a bay in one of several of the complexes, then drive into the closest one in a manner of minutes as soon as the shooting is done.
How else to explain that it seems that no one (neighbors, co-workers, family, etc.) has reported these vehicles? The vehicles are going underground and staying there until shortly before and after the shootings. They're being hidden.
They could well be driving right into a semi. Someone on another thread thought of that, and it makes the most sense to me. The semi is probably parked just out of sight, and they are probably inside with the door closed before the first 911 call is completed.