Booby trapped vehicles.......my nightmare is school buses. In my area shortly after 911, I noticed that bus drivers left their buses unattended in the far end of a Kroger parking lot, maybe upwards of two dozen. They parked them there after school drop off until afternoon pickup very close together, rows and rows, many with doors open. It would have been so easy to put bombs on or under these buses, all set to go off at the same time while kids were on them. I had to call the school district multiple times until they reluctantly listened. They stopped doing it for a while but right back to the same procedure. So to hell with it.
Why would any terrorists waste their time on airplanes when they are practically impenetrable and there are easy soft targets like the one you describe?
The DHS, CIA and FBI are incompetent if they can’t figure this out.