Signalized Intersection with Pedestrian-Actuated Left Turns
A four-legged urban intersection operates under fully actuated control (8-phase, NEMA-style). The northbound left turn is protected-permissive with a 3-section head plus a protected left-turn arrow. Field observation over three weekday peaks shows the northbound left-turn queue frequently spills back 180 m into the through lanes, blocking the inside through lane. The pedestrian push button on the north leg is actuated and is pressed an average of 40 times per hour during the peak. The controller's pedestrian walk and clearance intervals for the north crosswalk total 22 s. The engineer observes that the protected left arrow is served only once per cycle, and that the permissive gap acceptance is failing because opposing through volume is 1,150 veh/h with a heavy platoon structure. The cycle length is 90 s.
1. Diagnose why the protected left arrow is being served only once per cycle and explain how the actuated pedestrian timing interacts with the left-turn service.
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The protected left arrow is gated by the pedestrian phase because the north crosswalk pedestrian movement must be served in a compatible phase. With 22 s of walk+clearance and a 90 s cycle, the pedestrian phase consumes a large fraction of the cycle. If the controller is set so the left-turn phase is only called when the pedestrian phase is not active, the left arrow gets locked out or deferred.
The core issue is phase sequencing and recall. Two mechanisms are plausible and must be checked in the controller's timing/sequence plan:
- The left-turn phase may be set to not recall (no memory), so it is only served if a detector call is present at the right point in the ring structure. If the detector is a presence loop that drops out, the call may be missed.
- The pedestrian phase for the north crosswalk may be assigned to a phase that conflicts with the left arrow, forcing the controller to serve them in different cycles.
Additionally, the permissive left is failing because the opposing through volume of 1,150 veh/h with platooning leaves few acceptable gaps; the left turn therefore relies on the protected arrow, and if that is served only once per cycle, the 180 m spillback results. The fix is to verify the ring/barrier structure, ensure the left-turn phase has recall to the main phase (or is set to recall), and confirm the pedestrian phase is not unnecessarily gating the left arrow. Reducing the pedestrian walk time to the MUTCD minimum (based on crossing distance and walking speed) would free cycle time.
2. Recommend specific operational changes to reduce the left-turn spillback, and state what field data you would collect to verify the fix.
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Recommended changes:
- Convert the permissive left to protected-only during the peak, or implement a flashing yellow arrow (FYA) with a protected phase and a permissive phase. Given 1,150 veh/h opposing and platooning, protected-only during peak is the safer and more capacity-reliable choice.
- Serve the protected left twice per cycle (lead-lag or split phasing) if the ring structure permits, or shorten the cycle to increase the number of service opportunities. A 90 s cycle with one protected service per cycle limits left capacity.
- Recalculate pedestrian timing to the MUTCD minimum. For a crosswalk of, say, 18 m at 1.2 m/s walking speed, walk = 7 s plus a 4 s buffer, and clearance = crossing distance / 1.2 m/s. If the current 22 s is excessive, reducing it frees cycle time for the left arrow.
- Verify detector placement and call memory so the left-turn call is not dropped.
Field data to collect:
- Before/after left-turn queue length (measured at 15 s intervals).
- Cycle-by-cycle phase logs from the controller to confirm the left arrow is served twice per cycle.
- Gap acceptance counts for the permissive movement (if retained).
- Pedestrian counts to confirm the actuation rate and that the reduced timing does not strand pedestrians.
- Volume counts on the opposing through and the left turn to confirm the demand assumption.