Abstract: In a portable traffic control system in which receivers are controlled from a central transmitter the carrier signal employed is modulated by two different modulation signals in order to command a green light to be shown, reducing the possibility of this happening accidentally. The receipt of a carrier signal with only a single pilot modulation causes production of a red signal. An amber caution light if present is lit automatically upon turning off green, and flashing amber is produced by switching on green and then turning it off before the green can be lit. A drop in battery voltage below a predetermined level will cause a flashing red signal to warn and conserve the battery power. A non-resettable link in circuit with the green light is broken upon existence of a malfunction condition whereby another light is lit while green is alight, so that the green can no longer be lit until the unit is serviced to remove the malfunction.