Both the 2003^W1993 attack on the WTC and the Oklahoma City bombing attacks were delivered via truck. It's practically the only way to deliver a big explosive device.
The question was, "How often do we have vehicle bombs in the US?"
What do backscatter vans have to do with this? Want to know my position? Fine: I'm against the arbitrary deployment of rolling scanners.
Don't put words in my mouth. If you want to be snarky, you had better be willing to get it back.