I once drove a rig.
It was back in college, when I worked at the Baskin-Robbins Ice Cream factory (sometime when I'm not hijacking someone else's thread, I'll tell the story of the time I was buried knee-deep in pistachio ice cream). One of the jobs I held there was loading the outgoing trucks. We needed to get a now-full trailer out of the way to make room for the next one to fill. Which gets me to the time I drove an 18-wheeler.
The boss asked another worker to move the truck. I asked if I can come along. When we got to the truck, I asked the other guy if I could try moving it, and he said OK. He showed me how the shifter works, and off we went, clear over to the other side of the parking lot. I drove that truck a good hundred yards, but I don't think I ever got out of 1st gear, or turned the steering wheel at all.
BTW, what's the right terminology: rig, truck, 18-wheeler, etc.?