Signal Operation, Timing, and Controllers
Operation questions are mostly vocabulary plus arithmetic. Learn the hierarchy: a controller runs a timing plan made of intervals; intervals group into phases; phases sit in rings and barriers; splits divide the cycle; offsets align that cycle with adjacent intersections. In coordinated operation the cycle length is common across the system, splits divide the cycle among phases, and offset is measured from a system reference point, often the start of coordinated green on the main street.
Know the difference between actuated and fixed-time operation, and between gap-out and max-out: a phase gaps out when detection stops calling, and maxes out when it reaches maximum green. Recall settings change how a phase behaves with no demand.
Clearance intervals are the highest-yield calculation. The ITE yellow change formula is y = t + v / (2a + 2gG), with reaction time t usually 1.0 s, v the approach speed in ft/s, a the deceleration rate, and G the grade. Red clearance is the clearing distance divided by speed.
Worked example: a 45 mph approach is 66 ft/s. With t = 1.0 s and a = 10 ft/s-sq, y = 1.0 + 66/20 = 4.3 s. If the intersection plus vehicle length is 100 ft, red clearance = 100/66 = 1.5 s.
Traps: treating MUTCD minimums (3 s yellow) as design targets, confusing cycle length with split, mixing phase numbering conventions between agencies, and treating preemption as identical to transit priority.