IMSA Traffic Signal I cheat sheet

A temporary traffic control zone has five parts: advance warning area, transition area, activity area (buffer space + work space + traffic space), and termination area.

Taper length: L = W × S for speeds of 40 mph or less; L = W × S² / 60 for speeds of 45 mph or more (W = width in feet, S = speed in mph).

Buffer space is the unoccupied longitudinal/lateral separation between traffic space and work space; it is sized to provide stopping sight distance for an errant vehicle.

STOP/SLOW paddle: minimum 18 in × 18 in, 6-in retroreflective letters, red octagon side reads STOP, orange side reads SLOW.

High-visibility apparel: ANSI/ISEA 107 Class 2 minimum for daytime work in the right-of-way; Class 3 for nighttime or high-speed roadway work.

OSHA 1926.651 requires a protective system (slope, shore, or trench box) for excavations 5 ft deep or deeper; spoil and equipment stay at least 2 ft back from the edge.

Fall protection is required at 6 ft in construction work; aerial bucket work requires a full-body harness with a lanyard attached per the manufacturer's instructions.

Treat all downed conductors as energized: maintain at least 10 ft approach distance for circuits up to 50 kV and notify the utility before approaching.

Apply lockout/tagout at the cabinet service disconnect before servicing controllers, load switches, or field wiring.

MUTCD Part 4 governs highway traffic signals; MUTCD Part 6 governs temporary traffic control; Part 4D covers signal face design and display requirements.

Nine signal warrants appear in MUTCD Chapter 4C, including eight-hour vehicular volume, four-hour volume, peak-hour volume, pedestrian volume, school crossing, coordinated signal system, crash experience, and intersection near a grade crossing.

Cycle length equals the sum of all green, yellow, and red-clearance intervals for every phase serviced in the sequence.

Yellow change interval: Y = t + V / (2a + 64.4g), with t ≈ 1.0 s reaction time, V in mph, a ≈ 10 ft/s², and g = grade as a decimal.

Red clearance is commonly 1–2 s and may be computed from intersection width plus vehicle length divided by approach speed.

Pedestrian change interval: flashing DON'T WALK = crossing distance ÷ walking speed, using 3.5 ft/s design speed (3.0 ft/s where elderly or children predominate).

Overlap: a signal display serving two or more phases simultaneously, such as a right-turn arrow running with a compatible through phase.

Standard NEMA ring-barrier structure is dual-ring, 8-phase; concurrency groups are 1-6, 2-5, 3-7, and 4-8.

Bottom of an overhead-mounted signal face must be at least 15 ft above the pavement; post-mounted or pedestal-mounted faces must be at least 7 ft above the pavement.

12-inch lenses are required for new installations where the 85th-percentile approach speed exceeds 40 mph; 8-inch lenses are permitted at lower speeds.

Backplates with retroreflective borders improve conspicuity of the indications against a complex or brightly lit background.

Signal cable: IMSA 19-1 is 14 AWG twisted pairs, IMSA 20-1 is 16 AWG, and loop lead-in cable is typically IMSA 51-5.

Conduit fill is limited to 40 percent of the conduit cross-sectional area for three or more conductors (NEC Chapter 9).

Ground resistance at the service or pole ground should be 25 ohms or less; if exceeded, add a second driven electrode (NEC 250.53).

Span-wire installations use messenger wire between strain poles with sag typically about 5 percent of the span; poles and wire are sized for wind and ice loading.

120/208 V conductor color code: black, red, and blue for phase conductors, white for neutral, green for equipment ground.

Each signal head needs a switched conductor for each color (R/Y/G) plus a neutral, terminated either at a pole terminal block or as a direct homerun.

NEMA TS 2 controllers use 24 VDC logic power with 120 VAC field wiring to signal heads and detectors.

The conflict monitor (MMU) places the intersection into flashing operation when it detects conflicting greens, a red failure, or other monitored faults.

Inductive loop detectors are typically 6 ft × 6 ft (or 6 ft × 50 ft) with inductance on the order of 100–300 μH; a vehicle reduces loop inductance and the amplifier reports a call.

Presence mode holds a continuous call while a vehicle occupies the detector; pulse mode produces only a momentary call.

Vehicle extension (passage time) is the added green time after a vehicle leaves the detector; a gap longer than this value permits the phase to gap out.

Minimum green should be long enough to clear vehicles queued between the detector and the stop bar, using about 2 s of start-up lost time per vehicle.

Coordination uses cycle length, offset, split, force-off, and permissive periods; offset is the time from the system reference point to local zero.

Control types include pretimed, fully actuated, and semi-actuated; volume-density control adds variable initial green and variable passage time.

Verify cabinet input voltage at 120 VAC nominal, confirm the neutral and equipment ground are bonded, and check for correct phasing before energizing.

Insulation resistance (megger) testing is performed with the circuit de-energized; megohm-range readings indicate sound insulation.

Loop troubleshooting: an open circuit reads infinite or off-scale inductance, a short reads abnormally low inductance; compare against as-built values.

An all-red or unexpected flash condition is investigated by reading the MMU fault log and checking field wiring for conflicting green outputs before resetting.

Test switches allow a technician to force an individual phase to green, yellow, or red for lamp-out, conflict, and wiring verification.

Preventive maintenance includes annual cabinet inspection, torqueing terminations, cleaning lenses and visors, checking surge arrestors and battery backup, and re-verifying ground resistance.

LED signal modules are replaced as complete modules; inspect for failed pixels and non-uniform color or brightness rather than replacing individual LEDs.

Confirm that each signal face is aimed and positioned to be visible from the required minimum sight distance on its approach.

Update as-built drawings and timing records after any field modification, then re-verify flash operation, detection, and coordination.