8-PHASE QUAD LEAD-LAG WITH FLASHING YELLOW ARROWS
THIS MODIFIES THE DISPLAY OF THE QUAD LEAD-LAG LEFT TURN SEQUENCE TO PREVENT LAG TRAP
The quad lead-lag left-turn sequence has been in use for years, It is the way traffic signals are installed when signals are not in ideal progression locations. If permissive left-turns are used, the yellow-trap hazard occurs on every cycle. But a change in the way the signals are displayed can prevent the conflict, by delaying it until the oncoming traffic is also stopped.
How the 8-phase quad lead-lag sequence works:
THE QUAD LEAD-LAG LEFT-TURN SEQUENCE WITH FLASHING YELLOW ARROWS
The diagram on the right shows a quad lead-lag where left turns are allowed to filter through gaps in traffic when shown a circular green (orange broken lines). It also shows where flashing yellow arrows extend the permissive left turn until it can be safely terminated.
QUAD LEAD-LAG LEFT-TURN PHASE ORDER
The quad lead-lag left-turn sequence operates in the following manner:
- If traffic is waiting to use them, both phases on one leg operate first.
- When the left-turn phase runs out of traffic, it ends with clearance periods, and the oncoming straight-ahead phase is released.
- If the two-split lead-lag sequence is used, the straight-ahead phase can end first, starting the oncoming left-turn phase.
- When the other phase on the first leg runs out of traffic, it also ends with clearance periods, and the remaining oncoming phase is released. Now both phases on the opposite leg are moving traffic.
- When both phases on the second leg run out of traffic, they terminate with simultaneous clearance periods.
- The same sequence then occurs on the other street.
If some of the approaches or phases have no traffic on them, those phases can be skipped. This can cause the following effects:
- Parts of the sequence can be omitted. One street could start with both straight-ahead movements.
- If left turns are allowed to turn through gaps in oncoming traffic, this sequence normally causes yellow trap to occur every cycle. But the flashing yellow arrows modification eliminates the hazard, allowing the safe use of this sequence.
The numbers on the diagram are phase numbers.
(1 6)
(2 6) ||||| (1 5)*
(2 5)
(3 8)
(4 8) ||||| (3 7)*
(4 7)
* These displays omitted with single split lead-lag.
QUAD LEAD-LAG SYSTEM
- Special 5-light protected/permissive displays with flashing yellow arrows are used for left-turn displays (see diagrams at right).
- Special signal faces are used. The permissive aspects of the 5-light signal do NOT display the same aspects as the adjacent straight-ahead signals. Instead, they display the aspects of the oncoming straight-ahead signals. But the permissive aspects are darkened by the presence of the protected aspects by special circuitry.
- The wiring for this system is not standard. Extra circuits are needed on each mast arm or span wire for the permissive indications, since they are not the same circuits the adjacent straight-ahead signals use. And special signal driver circuits are needed. But no changes are needed in either the phase order or the detector connections.
- The 2009 MUTCD (Manual of Uniform Traffic Control Devices) was changed to allow this use of flashing yellow arrows.
- This is the best solution to a signal located away from the ideal crossing points of platoons in a progression system.
- This sequence is the most effective at preventing yellow-trap.
| Signal faces used with this method | |
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| Standard 3-light signal face for left turns that are always protected: |
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| Special 4-light signal face for left turns that are both permissive and protected: The top yellow arrow is for clearance, the bottom one flashes for the permissive turn. It has the same function as the face below, but clearance of the permissive turn is not as obvious. |
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| Special 5-light signal face for left turns that are both permissive and protected: The left side of the signal face is for the protected turn, the right side is for the permissive turn. The yellow arrow on the right flashes for the permissive turn. Not approved by MUTCD. |
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| Special 3-light signal for completely permissive left turns oncoming to any of the above displays: The lower yellow arrow flashes for the permissive turn. |
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| Standard signal for all straight-ahead indications, and left turns on streets without any left-turn phases: |
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FLASHING YELLOW ARROWS
The phase order of a quad lead-lag sequence is shown. The quad left-turn sequence and dual-ring controller are explained here.
In the flashing yellow arrows sequence, the standard lead-lag turn controller programming is used.
The flashing yellow arrows sequence changes only the signals displayed, not the controller sequence.
| PHASE TABLE | ||||
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| Timing Rings: | TR-1 (1 2 3 4) | TR-2 (6 5 8 7) | ||
| Concurrency: | CG-1 (1 2 5 6) | CG-2 (3 4 7 8) | ||
| Timing groups: | TG-1 (1 6) | TG-2 (2 5) | TG-3 (3 8) | TG-4 (4 7) |
| Overlap pairs: | (1 5) | (1 6) | (2 5) | (2 6) |
| Overlap pairs: | (3 7) | (3 8) | (4 7) | (4 8) |
| EXTRA OVERLAP PHASE TABLE | ||||
|---|---|---|---|---|
| Overlap unit | Parent phases | Conditions | ||
| OV-A | 1 2 | Dark when phase 1 is green or yellow | ||
| OV-B | 3 4 | Dark when phase 3 is green or yellow | ||
| OV-C | 5 6 | Dark when phase 5 is green or yellow | ||
| OV-D | 7 8 | Dark when phase 7 is green or yellow | ||
| A,B,C,D | - | Green is flashing yellow arrow | ||
| PHASE DIAGRAM | ||||||||
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Ring 2 |
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| TG-1 |  Phase 1 |
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OV-C |  Phase 6 |
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| TG-2 |  Phase 2 |
OV-A | |
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BARRIER | |
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BARRIER |
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| TG-3 |  Phase 3 |
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OV-D |  Phase 8 |
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| TG-4 |  Phase 4 |
OV-B | |
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BARRIER | |
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BARRIER | |
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INTERFACING THE INTERSECTION
FLASHING YELLOW ARROWS
| DETECTOR | ||||
|---|---|---|---|---|
| Detector | Normal phase |
Redirect phase | Redirect when | |
| 1 | 1 | 6 | Phase 2 active* | |
| 2 | 2 | - | ||
| 3 | 3 | 8 | Phase 4 active* | |
| 4 | 4 | - | ||
| 5 | 5 | 2 | Phase 1 active* | |
| 6 | 6 | - | ||
| 7 | 7 | 4 | Phase 3 active* | |
| 8 | 8 | - | ||
| * Do not redirect unless preventing simultaneous left turns. | ||||
| SIGNAL FACE TABLE | ||||
|---|---|---|---|---|
| Face | Phase | Modifier | ||
| 1 | 1 | No red indication (phase A in same face) | ||
| 2 | 2 | |||
| 3 | 3 | No red indication (phase B in same face) | ||
| 4 | 4 | |||
| 5 | 5 | No red indication (phase C in same face) | ||
| 6 | 6 | |||
| 7 | 7 | No red indication (phase D in same face) | ||
| 8 | 8 | |||
| A | OV-A | Dark when phase 1 is green or yellow | ||
| B | OV-B | Dark when phase 3 is green or yellow | ||
| C | OV-C | Dark when phase 5 is green or yellow | ||
| D | OV-D | Dark when phase 7 is green or yellow | ||
| INTERSECTION DIAGRAM | ||||||||
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C 6 | 1 |  |
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|5|C| | |2| | |
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D 8 |
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7 | |B| |3| |
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4 B |
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|6| | |A|1| | |
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5 | 2 A | |
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Detectors are blue. Signal faces are orange.
LEAD-LAG WITH FLASHING YELLOW ARROWS
ADVANTAGES
- Allows the safe use of phase skip or lag phases.
- Phase skip prevents wasting time at empty approaches.
- Can be used at a point offset from a platoon-crossing node in a progression system as is.
- No change in the phasing is needed to add flashing yellow arrows.
- The system works with all phase orders.
- The system works with all signal pre-emption schemes.
- The order of phases can be altered to make other sequences without causing yellow-trap.
- The system allows the use of phase orders that would not otherwise be considered.
- Meets federal standards.
- Can not yellow-trap if used properly.
LEAD-LAG WITH FLASHING YELLOW ARROWS
DISADVANTAGES
- Needs special signal display logic and extra signal circuits.