SG30 system utilizes two units. Stop and go signaling systems alerts truck drivers and dock personnel to the safety status of docked or docking trucks. One outside (communicate with driver) and one inside (communicate with dock operator/attendant) SG10 system is available for applications where opposing signals are not required. Loading dock warning signs available: Increase dock safety. High contrast and visibility. Simple and direct instruction. Durable and light weight. Inside of building warning sign 12" x 12". Outside of building warning sign 12" x 24" (standard and mirrored text on same sign).