Signal

NCHRP Report 825, Section L, Signalized Intersections. That section is the planning application of HCM 6th Edition, Chapter 19. The procedure is a critical movement analysis in two parts: intersection sufficiency from the critical volume-to-capacity ratio, then delay and level of service.

The steps follow Section L:

1. Choose left-turn phasing from the volume checks (protected or permitted).
2. Convert each movement to through passenger-car equivalents. Adjustments cover heavy vehicles, the peak-hour factor, the permitted left-turn equivalent, a right-turn pedestrian equivalent, parking, and lane utilization.
3. Assign those flows to lane groups and find the critical lane group on each phase.
4. Set the cycle length and the through and left greens, or use the values supplied in the signal plan.
5. Compute lane-group volume-to-capacity, control delay, and level of service.

The right-turn pedestrian adjustment is the Section L through-car equivalent applied to the right-turn volume before lane grouping. None or low is 1.2, medium is 1.3, high is 1.5, and very high is 2.1. The same factor applies whether the right turn is shared or exclusive. Right-turn-on-red is not subtracted. This is not the HCM operational saturation-flow adjustment.

Lane groups follow the right-turn lane count:

• right: 0 is a shared right. The right-turn volume is added to the through lane group. The right-turn movement’s own flow and volume-to-capacity are 0.
• right greater than 0 is an exclusive right. That volume stays in its own lane group and can set the critical flow for the through phase.

Timing is permitted-right only. Greens are for through and left on each street. A right turn uses the through green. A signal plan cannot set a right-turn split. Protected and overlap right turns are not implemented.

The intersection volume-to-capacity reported by the API is the highest lane-group volume-to-capacity. Cycle length and lost time are on Signal timing.