What's that clicking noise?

This morning on the way to work, Debbie saw several 18-wheelers that had their left turn signal going at different spots along the interstate.  The trucks were just travelling along without turning off the signals.  Made me wonder if it was some sort of protest thing about high fuel prices?  Or, maybe all of the drivers were old guys who just forgot the signal was blinking.  Bet they kept asking themselves, "What IS that clicking noise?"