It was really the female passenger. I was cruising down I-40 in my dairy truck when another big truck moved to the left lane to pass. We were starting up a slight grade so when it didn't pass right away I just figured the truck was loaded and was having trouble coming around.
When it moved slowly beside me I looked over and saw her. She had her top pulled down and had her boobs shoved against the door window. She was smiling and waving. When I smiled and gave her a "thumbs up" the driver gassed on it and they were gone.
Later when I was walking around my trailer I laughed out loud when I noticed the sign on the back. Maybe she wasn't flashing me but just answering the question: