8-PHASE QUAD LEFT WITH DETECTOR REDIRECT
THIS SEQUENCE REDIRECTS DETECTORS TO PREVENT YELLOW-TRAP
When both straight-ahead phases are active, this sequence causes calls to left-turn phases on a given street to extend the straight-ahead phases instead of calling the left-turn phases. Yellow-trap cannot occur with this sequence. However, it will not work with any phase order except split lead.
How detector redirection works:
QUAD LEFT WITH REDIRECTED DETECTORS
The diagram on the right shows a quad split lead with left turns allowed only after the cross street phases are active.
QUAD LEFT-TURN PHASE ORDER
The quad left-turn sequence (quad split lead) normally operates in the following manner:
- Assuming traffic is waiting to use them, both left-turn phases on one street operate first.
- When one left-turn phase runs out of traffic, it ends with clearance periods, and the oncoming straight-ahead phase is released.
- When the other left-turn phase runs out of traffic, it also ends with clearance periods, and the oncoming straight-ahead phase is released. Now both straight-ahead phases are moving traffic.
- When both straight-ahead phases 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 above sequence can be omitted. For example, the street could start with one left turn and the adjacent straight-ahead movement, or even both straight-ahead movements.
- If one entire street has no traffic waiting, the straight-ahead phases can NOT terminate. Thus, the yellow-trap hazard can not occur.
The numbers on the diagram are phase numbers.
(1 5)
(1 6) ||||| (2 5)
(2 6)
(3 7)
(4 7) ||||| (3 8)
(4 8)
QUAD LEFT WITH REDIRECTION
- This is used with the 5-light protected/permissive left-turn display (see diagrams at right). A signal with all 3-light left-turn signals does not need this sequence unless some approaches don't have left turn phases.
- All signal faces are standard.
- The circular aspects of the 5-light signal display the same aspects as the adjacent straight-ahead signals.
- The wiring for this system is standardized.
- The MUTCD (Manual of Uniform Traffic Control Devices) prescribes this basic kind of display.
- With detector redirection, the protected/permissive version has protection against yellow-trap from phase skip
| Signal faces used with this method | |
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| Standard 3-light signal face for left turns that are always protected. Not used with this method: |
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| Standard 5-light signal face for left turns that are both permissive and protected: |
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| Standard 3-light signal for completely permissive left turns oncoming to either of the above displays: |
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| Standard signal for all straight-ahead indications, and left turns on streets without left-turn phases: |
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DETECTOR REDIRECTION
The phase order of a dual-ring controller is shown. The quad left-turn sequence and dual-ring controller are explained here.
In detector redirection, each left turn detector is redirected away from its left turn phase and toward the adjacent straight-ahead phase whenever the oncoming straight-ahead phase is active (green, yellow, or red clear).
The detector redirection causes the left turn detectors to extend the adjacent straight ahead green instead of calling the left turn phases.
Detector redirection prevents the timers from advancing out of the straight-ahead phases, unless a call is placed on the other street, and the controller actually cycles to a phase on the other street. Only then can the left turn phases be actuated again.
| PHASE TABLE | ||||
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| Timing Rings: | TR-1 (1 2 3 4) | TR-2 (5 6 7 8) | ||
| Concurrency: | CG-1 (1 2 5 6) | CG-2 (3 4 7 8) | ||
| Timing groups: | TG-1 (1 5) | TG-2 (2 6) | TG-3 (3 7) | TG-4 (4 8) |
| Overlap pairs: | (1 5) | (1 6) | (2 5) | (2 6) |
| Overlap pairs: | (3 7) | (3 8) | (4 7) | (4 8) |
| EXTRA OVERLAP PHASE TABLE | ||||
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| Overlap unit | Parent phases | Conditions | ||
| OV-A | - | |||
| OV-B | - | |||
| OV-C | - | |||
| OV-D | - | |||
| PHASE DIAGRAM | ||||||||
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Ring 2 |
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| TG-1 |  Phase 1 |
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| TG-2 |  Phase 2 |
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BARRIER | |
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BARRIER |
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| TG-3 |  Phase 3 |
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 Phase 7 |
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| TG-4 |  Phase 4 |
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BARRIER | |
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BARRIER | |
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INTERFACING THE INTERSECTION
DETECTOR REDIRECTION
| DETECTOR | ||||
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| 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 6 active | |
| 6 | 6 | - | ||
| 7 | 7 | 4 | phase 8 active | |
| 8 | 8 | - | ||
| SIGNAL FACE TABLE | ||||
|---|---|---|---|---|
| Face | Phase | Modifier | ||
| 1 | 1 | No red indication (phase 6 in same face) | ||
| 2 | 2 | |||
| 3 | 3 | No red indication (phase 8 in same face) | ||
| 4 | 4 | |||
| 5 | 5 | No red indication (phase 2 in same face) | ||
| 6 | 6 | |||
| 7 | 7 | No red indication (phase 4 in same face) | ||
| 8 | 8 | |||
| INTERSECTION DIAGRAM | ||||||||
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6 | 1(6) |  |
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|5|2| | |2| | |
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8 | |
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7(4) | |8| |3| |
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|7| |4| |
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4 | |
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|6| | |6|1| | |
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5(2) | 2 | |
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Detectors are blue. Signal faces are orange.
Redirected calls are in parentheses.
QUAD WITH DETECTOR REDIRECT
ADVANTAGES
- Maximizes time use at isolated intersection.
- Phase skip partially prevents wasting time at empty approaches.
- Conforms to driver expectation of phase sequence.
- Can be used at a simultaneous node in a progression system.
- Some signal controllers have settings included to implement this method.
- Can not yellow-trap.
QUAD WITH DETECTOR REDIRECT
DISADVANTAGES
- The order of phases can not be altered to make other sequences without causing yellow-trap.
- Can not be used away from a simultaneous node in a progression system.
- With no traffic on the cross street, the signal can let left turn traffic back up with heavy oncoming traffic.