What signaling system must buses, trucks, and tractor-trailers be equipped with?

• Only turn signals for left and right turns
• A system that flashes both front and rear turn signals simultaneously
  Explanation:

  "Buses, trucks, and tractor-trailers must be equipped with a signaling system. In addition to signaling turning movements, these vehicles must have a switch or combination of switches that causes the two front turn signals and the two rear signals to flash simultaneously as a vehicular traffic signal warning. The signal must be capable of flashing simultaneously with the ignition on or off."
• A signal that only works when the ignition is on
• A system that flashes only the rear lights