7.2 Lead-Lag Left Turns, the Yellow Trap & Signal Overlaps
Key Takeaways
- Lead-lag left-turn sequencing is a progression tool — it lets the coordinated through band start at different points in each direction — but it is only safe when permissive left turns use a flashing yellow arrow rather than a circular green.
- A lagging left turn conflicts with the concurrent pedestrian movement it shares a barrier with, so pedestrian service has to be resolved explicitly rather than assumed.
- An overlap is a derived output driven by one or more parent phases; it holds green while any parent is green and must be programmed with its own clearance so it does not terminate before the parent it depends on.
- Right-turn overlaps are the most common overlap application and the most common source of field conflicts, because the overlap must drop whenever a conflicting pedestrian phase or U-turn movement is served.
7.2 Lead-Lag Left Turns, the Yellow Trap & Signal Overlaps
1. Lead-Lag Left-Turn Operations & The Yellow Trap
Left-turn phasing sequencing plays a critical role in progression bandwidth, intersection delay, and queue clearance.
Phasing Sequences: Dual-Lead vs. Lead-Lag
- Dual Leading Left Turns (Lead-Lead):
- Phases 1 and 5 initialize simultaneously.
- When Phase 1 gaps out, the controller transitions Ring 1 to Phase 2 while Ring 2 remains in Phase 5 (Pair 2+5).
- When Phase 5 gaps out, Ring 2 transitions to Phase 6, resulting in dual through movements (Pair 2+6).
- Limitation: Can restrict arterial coordination progression bands because both opposing through movements must wait for left-turn phases to complete.
- Lead-Lag Left Turns:
- One left turn leads while the opposing left turn lags behind the through movement.
- For example: Phase 1 leads concurrently with Phase 6 through traffic (Pair 1+6). When Phase 1 terminates, Phase 2 begins, yielding dual through traffic (Pair 2+6). Finally, Phase 6 terminates to allow Phase 5 to lag concurrently with Phase 2 (Pair 2+5).
- Operational Advantages:
- Arterial Progression: Maximizes continuous progression bands in both directions along coordinated corridors.
- Left-Turn Storage Bay Accommodation: Prevents left-turn queue spillback from blocking through lanes.
- Cycle Efficiency: Balances phase splits when left-turn demand is highly asymmetric between opposing directions.
+-----------------------------------------------------------------------------+
| LEAD-LAG PHASING TIME SEQUENCE |
+-----------------------------------------------------------------------------+
| RING 1: [ Phase 1: EB Left ] [ Phase 2: EB Through ] || |
| <--- Leading Left -> <--- Major Coordinated Through ---> || B |
| || A |
| RING 2: [ Phase 6: WB Through ] [ Phase 5: WB Left ] || R |
| <--- Major Coordinated Through ---> <--- Lagging Left -> || |
| |
| INTERVALS: Interval 1 (1 + 6) Interval 2 (2 + 6) Interval 3 (2 + 5)|
+-----------------------------------------------------------------------------+
The Deadly "Yellow Trap" Hazard
The Yellow Trap (or Left-Turn Trap) is a hazardous scenario that occurs when lead-lag phasing is implemented with traditional 5-section protected/permissive signal heads displaying a circular green (green ball) for permissive left turns:
+-----------------------------------------------------------------------------+
| THE MECHANICS OF THE YELLOW TRAP |
+-----------------------------------------------------------------------------+
| 1. DRIVER SITUATION: |
| Southbound driver waiting inside the intersection to turn left across |
| Northbound traffic during permissive circular green. |
| |
| 2. SIGNAL TRANSITION: |
| Southbound signal changes from CIRCULAR GREEN to CIRCULAR YELLOW |
| because Southbound through (Phase 2) is terminating to service a lagging |
| left turn (Phase 5). |
| |
| 3. FATAL ASSUMPTION: |
| The left-turning driver erroneously assumes opposing Northbound traffic |
| also has a yellow signal and is preparing to stop. |
| |
| 4. COLLISION DYNAMICS: |
| Driver accelerates into the turn on yellow to clear the intersection. |
| Northbound through traffic (Phase 6) STILL HAS SOLID GREEN! |
| Severe right-angle, high-speed impact occurs in the oncoming lanes. |
+-----------------------------------------------------------------------------+
The MUTCD Solution: Flashing Yellow Arrow (FYA)
The MUTCD 11th Edition (Sections 4F.01 through 4F.08) and 2009 MUTCD mandate the elimination of the circular green for permissive left turns in lead-lag operations, substituting the Flashing Yellow Arrow (FYA) display:
- The FYA head utilizes 4 separate sections: Solid Red Arrow, Solid Yellow Arrow, Flashing Yellow Arrow, and Solid Green Arrow.
- Because the FYA indication is optically and electrically decoupled from the adjacent through signal (driven via a dedicated load switch channel or overlap), the controller can maintain the Flashing Yellow Arrow display for the turning driver even while the adjacent through head transitions from green to yellow to red!
- The turning driver clearly understands that oncoming traffic has the right-of-way, eliminating the mistaken assumption of opposing clearance and eradicating the yellow trap.
Pedestrian Conflict Management During Lagging Lefts
When a lagging left-turn phase is active, the controller must actively protect pedestrians:
- Pedestrians crossing the street into which the lagging left turn is turning must NOT be released concurrently with the lagging vehicle movement.
- If a pedestrian call is serviced during the parallel through phase (Phase 2), the controller must either force the lagging left turn (Phase 5) to wait until the Pedestrian Clearance Interval (Flashing Don't Walk) has completely expired, or omit the lagging left turn for that cycle if minimum clearance cannot be satisfied.
2. Signal Overlaps: Programming, Parent Phases & Clearance Intervals
An Overlap is an auxiliary signal output driven by two or more parent phases to service traffic movements that can run safely across phase boundaries without requiring a dedicated ring.
Right-Turn Overlaps
The most prevalent application is a protected right-turn overlap:
- Consider an Eastbound right-turn movement on Phase 8 (minor street).
- When Phase 8 is green, the right turn flows freely.
- When Phase 8 terminates and the controller services Phase 2 (major street left turn from Southbound), vehicles turning left from Phase 2 enter the Westbound lanes, leaving the Northbound receiving lanes completely empty.
- Therefore, the Phase 8 right turn can continue running concurrently with Phase 2!
- Configuration: Overlap A is assigned Parent Phases 8 and 2.
Overlap Clearance Programming Rules
Technicians must strictly program overlap clearance timing parameters inside the controller:
- Trailing Green / Extension: When transitioning between parent phases (e.g., from Phase 8 to Phase 2), if the phases are continuous, the overlap green remains illuminated without interruption.
- Dedicated Yellow Change and Red Clearance: When both parent phases terminate and the controller transitions to a conflicting movement (e.g., transitioning from Phase 2 to Phase 3 cross-street left), Overlap A MUST time its own programmed Yellow Change Interval and Red Clearance Interval before the conflicting phase green is energized.
- Independent Conflict Monitoring: Modern NEMA TS2 MMU2 units monitor overlap channels independently (Channels 9–12 or 13–16). The conflict monitor must be programmed to treat the overlap channel as conflicting with any non-parent movement (such as U-turns or cross-street pedestrians).
U-Turn Versus Right-Turn Overlap Conflict (MUTCD Section 4F.09, Paragraph 4)
When a right-turn overlap runs concurrently with an opposing left-turn phase from which U-turns are permitted: $\text{U-Turn from the opposing left-turn phase} \iff \text{Right-Turn Overlap from the conflicting approach to the left}$ Both movements discharge into the same receiving lane. Section 4F.09, Paragraph 4 is an Option: if U-turns are permitted from the approach and a right-turn GREEN ARROW is simultaneously displayed to road users turning right from the conflicting approach to the left, road users making the U-turn may be advised of the operation by installing a U-TURN YIELD TO RIGHT TURN (R10-16) sign (see Section 2B.59).
Note what the MUTCD does and does not say here. It does not require the U-turn to be prohibited, and it does not name the NO U-TURN (R3-4) sign for this condition. Agencies commonly go further than the Option — prohibiting the U-turn outright, dropping the overlap during the opposing left, or installing a blank-out sign that activates with the overlap — but those are agency countermeasures, not MUTCD requirements. On an exam, the MUTCD answer is the R10-16 advisory sign.
A right-turn overlap is programmed with the adjacent through phase and the complementary left-turn phase as its parent phases. Which programming error most commonly produces a field conflict at this kind of installation?