Just once. It was three years ago, on the sixth day of a seven day tour that went from Philadelphia to Pittsburgh. We were on a rolling two lane highway with no shoulder. Apparently, we were told later, many 18-wheelers use this road instead of the PA turnpike. DH and I were going downhill fairly fast and I could hear a big truck gearing down behind me. DH was ahead of me by some distance. The truck driver couldn't pass because of oncoming traffic. I had nowhere to go to get out of his way. Finally when the coast was clear, he began to pass - so close I could have reached out and touched the truck. I could feel myself being pulled into his draft. But the worst thing was him blowing his air horn, loud and long while he passed, which scared the sh!t out of me and caused me to wobble. Fortunately I was able to stay in control, but as soon as I saw one of the SAG vehicles parked just ahead in a driveway, I pulled over, stopped, and had a complete meltdown. It was 12 miles into a 75 mile day and I was done. DH continued on after I had calmed down, and I spent the rest of the day in a very nice Lincoln Navigator.