Which number replaces the question mark?

A truck driver was driving along on the freeway. He passed a sign that said “low bridge ahead.”
Before he knew it, the bridge was right ahead of him and he got stuck under the bridge. You could say that he got a rock solid “Trucker's Wedgie.”
Cars were backed up for miles.
Finally, a police car pulled up. The cop got out of his car and walked around to the truck driver, put his hands on his hips and said, “Got stuck, huh?”
The truck driver said, “No officer,… I was delivering this bridge and ran out of gas!”