On medium and heavy duty trucks, it's called a "Clearance Light Interrupter Switch". The switch on the dash is usually a spring loaded toggle switch that automatically returns to the On position when it's released by the driver. The switch controls a relay(s) under the dash that interupts the circuit(s) feeding the truck's clearance lights.
Bill
Not sure what it's called, I think I. C. C Flash. I would like to hook up a switch that flashes the clearance lamps to signal a passing truck or other pulling a trailer that it's ok to merge back over.
I just use my 4 way flashers to say "thanks". I use the flash to pass when the truck that is going around me is in the clear.
I hate it when people improperly use 4 way flashers for signaling, I never know if there stopping or just just new to driving.