What's with truckers wanting to be flashed?

Truckers often motion for me to flash them when I'm out driving...well actually when I'm out and I'm a penger, but you know what I mean (I know that sounds wierd but I'm not the only one who this happens to apparently, other friends have commented on the same) and I was just wondering what the deal is-like why truckers are so big on seeing the of people in cars? I know s are great and all..but conductors in trains don't seem to ask to be flashed, or the guys at toll boths, and I was just wondering what the "trucker/" thing was all about