Signal Controller Operation
Signal Controller Operation Most modern signal controllers are designed to operate in what is termed a dual- ring configuration. This configuration allows maximum flexibility for controlling phase duration and sequencing, which is necessary for operating in a fully actuated mode. The dual-ring concept can be best explained with a graphical illustration.
The three-letter notation in each numbered box refers to direction and movement type. For example, WBL means westbound left and NBT means northbound through. At an intersection with four approaches and separate/exclusive left-turn phases for each approach, a total of eight separate movements are possible. Any right-turn movements are implicitly considered to move with the corresponding through movement. The dual-ring terminology comes from four movements (1–4) being represented on the top ring, and four movements (5–8) being represented on the bottom ring. The movements in each of the numbered boxes must start and end at the same time; thus, the boxes represent phases. The logic behind this configuration is straightforward. Any phase in ring 1 can occur simultaneously with any phase in ring 2 as long as both are on the same side of the barrier (a term used figuratively to separate conflicting traffic movements). For example, the opposing WB and EB left-turn movements (phases 1 and 5) can move simultaneously. However, if no vehicles are detected in the EB left-turn lane, this movement can be skipped and the WB left-turn and WB through movements (phases 1 and 6) can move simultaneously. The same logic applies to movements on the right side of the barrier (phases 3, 4, 7, and 8).
