Traffic Signal Design and Plan Review
Reading a plan set like a reviewer
Design and plan review questions test whether you can follow a signal from justification to the last electrical note on the sheet.
The sequence is: warrant justification, then geometrics, then phasing and timing, then the electrical package. Know the MUTCD signal warrants and that satisfaction of a warrant does not automatically mandate a signal - engineering judgment and an alternatives analysis still apply. Be comfortable with corner radii and curb returns for design vehicles, stopping sight distance to the stop line, left-turn bay storage, and pedestrian refuge widths. Signal head placement: at least one primary head and one secondary head per approach, mounted and aimed so the lens is visible from the required distances, with backplates where specified. Phasing questions hinge on ring-barrier diagrams, protected versus permissive left turns, and pedestrian intervals (walk plus flashing DON'T WALK sized at the crossing distance divided by walking speed).
The electrical sheet is where candidates lose points: service and panel sizing, cabinet location out of the clear zone, conduit fill, voltage drop limits, grounding electrode and bonding of all metallic parts, and conflict-monitor wiring.
Traps: confusing shall and should provisions; omitting the all-red clearance interval; using the wrong walking speed; forgetting that a warrant study needs current (not stale) traffic data.
Worked example - yellow change interval: Y = t + 1.47V / (2a). At 45 mph on level grade with t = 1.0 s and a = 10 ft/s²: Y = 1.0 + (1.47 × 45)/20 = 1.0 + 3.3 = 4.3 s. Round up to 4.5 s, then add an all-red sized to clear the intersection.