Traffic Signals

Timing, justifying and laying out a signal: change intervals, pedestrian timing, cycle length and splits, offsets, railroad preemption, the MUTCD warrants, and the hardware — signal faces, pedestrian heads and push buttons. It carries 5–8 of the 80 questions on the Transportation exam.

What the problems look like

Short timing calculations and MUTCD reading, often from a plan or a count table. Recurring shapes — yellow and red clearance intervals; pedestrian clearance and the minimum green it forces; Webster's cycle and green splits; an offset on a time-space diagram; detector minimum green; the dilemma zone; a warrant verdict from counts or crashes; preemption components; and face and push-button placement.

By exam day you should be able to —

• compute Y = t + v/(2a + 2Gg) and red clearance (w + L)/v with v in ft/s;
• time the walk and pedestrian clearance, and turn them into a minimum green;
• find each phase's critical flow ratio, the cycle length and the effective green;
• set an offset that clears the downstream queue before the platoon arrives;
• run a warrant check end to end — which conditions, which volume set, which curve.

Where people lose points

• Change intervals: only the approach grade enters the yellow, as a decimal, negative downhill. Red clearance has no reaction term. mph × 1.467 = ft/s.
• Critical flow ratios: each phase is governed by its largest v/s — not its largest volume, not the sum of its groups. Check whether a printed saturation flow is per lane or for the whole lane group before forming v/s; multiply by the number of lanes only in the per-lane case.
• Pedestrian distance runs from the curb to the far side of the traveled way: a near-side parking lane counts because timing starts at the curb, a far-side one does not. On a one-stage crossing it includes the median. The walk interval runs inside the green too.
• Warrants: alternative conditions each satisfy a warrant on their own. Check whether the site qualifies for the reduced-volume set, read the curve for the actual lane combination, and count only the crash types the warrant counts.
• Minimum vs maximum: mounting heights, symbol sizes and push-button offsets come as minimums, maximums or ranges — and differ for faces over the roadway. Read which one is asked.
• Offsets: use the progression speed, not the posted limit, and the right intersection spacing; subtract the standing queue's discharge time.

Have these at your fingertips

The ITE change-interval relations — and where they live in the Handbook — plus Webster's cycle length and flow-ratio arithmetic. Warrants, pedestrian intervals, signal faces and preemption come from the MUTCD; capacity concepts from the HCM. The edition depends on your appointment date: MUTCD 2009 (Revisions 1 and 2) and HCM 6th before April 2027, MUTCD 11th and HCM 7th from April 2027. Work from the edition you'll be given.

Going deeper

• Transportation design standards: the April 2027 HCM and MUTCD change
• Traffic Engineering — the capacity analysis behind the timing
• Intersection Geometry — the turn lanes a signal serves

Part of the Transportation exam. Practice this area with exam-real problems — the tutor walks any one you miss. → Start practicing free — free with an account.