Abstract: A method, apparatus, and system for transmitting and receiving an electromagnetic data signal in a moving, linear chain. Each unit in a chain of units has a processing unit interconnecting a directional receiving element, a directional low power transmitting element, a deceleration detector for the unit, and a deceleration detector for preceding units in the chain. In a simple form, motor vehicles are the units. If the invention detects deceleration of a preceding vehicle, deceleration of itself, or an identifiable signal from a preceding vehicle such as a blue light, it causes a rearward facing blue light to illuminate. This blue light indicates to following motorists that the perceived vehicle is decelerating or that a vehicle somewhere in front from of it is decelerating. The blue light also may be received by another vehicle equipped with the invention and consequently be transmitted rearwardly. In a general form, a data encoded high frequency signal is transmitted rearwardly from a vehicle.