Any detector can be assigned a SWITCH phase that allows that detector to EXTEND the switch phase when it is green. This is commonly used with protected/permitted left-turn signals. The left-turn detector calls and extends the protected left-turn indication and also EXTENDs the adjacent through (SWITCH) phase used to drive the permitted indication.

CALL REDIRECT allows one phase to call another phase when a programmed phase is green. The difference between REDIRECT phase calls and SWITCH detectors is that REDIRECT is used to CALL a phase when it is red while a SWITCH detector EXTENDS the switch phase when it is green.

Just remember.... CALL REDIRECT allows you to CALL phases when they are red, you can't EXTEND them when they are green - you need detector switching to do this.

CALL REDIRECT can be used to service a permitted indication in a protected/permitted left-turn display when there is no call on the adjacent through phase. For example, suppose there are protected/permitted left-turn displays on both cross street approaches. If calls exist on phases 7 and 4, then the cross street will move to phases 7 and 4 as shown below. However, there is a problem when a call is received on phase 3 while 7 and 4 are being serviced and there is no call on phase 8. The driver will not receive a permitted indication for phase 8 and will have to wait until phase 3 can be serviced again with phase 7. Left-turn drivers on phase 3 perceived this as a "skip" condition, especially when a single vehicle is serviced on phase 7 phase 8 is not serviced.

.............................................. calls received on 7 + 4

->> phase 7 + 4 serviced

.............................................. call received on 3 but not 8

->> phase 1 + 5 serviced
->> phase 2 + 6 serviced
->> phase 3 + 7 serviced (the left-turn driver calling phase 3 receives the protected left-turn indication)

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CALL REDIRECT prevents this dilemma as follows:

.............................................. calls received on 7 + 4

->> phase 7 + 4 serviced

.............................................. call received on 3 but not 8

* CALL REDIRECT on phase 4 redirects the call from phase 3 to phase 8

->> phase 4 + 8 is serviced (the left-turn driver calling phase 3 can turn permitted on when phase 8 is serviced)

->> phase 1 + 5 serviced
->> phase 2 + 6 serviced
->> phase 3 + 7 serviced