Easing the effects of phase rotation during coordination

In general, when a signal is coordinated, and you want to have the coord plan switch from leading to lagging protected lefts, the signal must go free for a cycle to enable the phase rotation, even though the cycle length and offset may be the same.

This is problematic since the controller will need to go into offset seeking mode for between 2 and 3 cycles to get back in step, when the signal transitions from one plan to another.  If the controller is running a 2 minute cycle that means that the signal is likely out of step to some extent for up to 6 minutes.  When you add needing to go free for a short while before changing your coord plan to allow the phase rotation to occur, this creates a rather long time to be out of sync with the other signals.

For example, given the phase sequence below, assume that phase 3 needs to be a leading protected left, some times of the day, and a lagging protected left during other times of the day.

Standard NEMA Phase Sequence Diagram

Every time the controller needs to shift from leading to lagging protected left, the time of day plan requires that the controller run in free operation for a short while followed by the specific action plan that allows the controller to shift the phase rotation.

Since the controller seeks to change its operation at the local zero of the cycle length, not the top of any minute, the Time of Day Plan will likely need to have the length of the free time be the number of minutes of the current cycle time, rounded down, plus one minute.  This is to make sure that the coordinator does not skip the free cycle action plan because the local zero occurs after the Time of Day’s free action plan, and after the Time of Day’s new coord plan.  Alternately, if you are really into math, you could calculate if each specific free cycle in the action plan will need to be 1 minute, 2 minutes or 3 minutes long and not be walked over the new coordinated action plan in the scheduler.  Good luck with that on a system wide timing program.

Since the controller needs to waste time it could be in coordination to go free, then go into offset seeking mode to do a phase rotation, why not use the power of your NTCIP controller to let it just switch plans, and inhibit or enable specific phases by time of day in your coordination plan?

For this example, Phase 3 needs to be able to be lead, or lagged by time of day.  Preferably, the lagging left would be inhibited during non-coordinated hours of operation.

This is done via overlap programming.  Instead of having phase 3 routed to load switch 3, phase 3 and phase 11 (the lagging phase corresponding to phase 3) are enabled through overlap 11, to load switch 3.  Generally, when I do this type of operation, I do not use overlaps 1 through 4 (also known as overlaps A through D), as these are commonly used as right turn overlaps.  Generally, overlaps 5 through 16 are rarely used.  For convention, I use the secondary phase number as the parent phase plus 8, and the overlap number to equal the parent phase plus 8 (hence phase 11 and overlap 11).

The new phase diagram looks like:

Modified NEMA Phase Sequence Diagram for Lead / Lag Protected Left Operation - By Time of Day

 The controller needs to be programmed to enable phase 11.  Phase 11 also needs timing values.  Special care needs to be provided to make sure that the timing values for yellows and reds match between phase 11 and phase 3.   The specific overlaps, compatibility tables, phase sequence operations etc need to be programmed to allow phase 3 and phase 11 to both be independently operate.  Specific care also needs to be given to make sure that the controller will not try to override the yellow and red times for the phases with the programmable overlap yellow and red times.

The Time of Day plan will call specific Action Plans that will enable phase 3 and omit phase 11, or omit phase 3 and enable phase 3.  

What about Cycle Fault and Cycle Fail for the controller operation?

Cycle Fault and Cycle Fail are functions of the controller that monitor if specific vehicle phases have calls that have not been served for some period of time.  The controller can be programmed to either go into all-red flash, or free, based on the lack of service of calls.  Generally, Cycle Fault is for coordinated operation, Cycle Fail is for free operation.

The way to deal with this is to program the detector tables, and use alternate detector tables.  The Action Plan must call either the normal detector table, or the alternate detector tables.  The NTCIP standards allow for multiple detector tables, which can be called by the Action Plan.

For example, the normal detector plan may have the detection for phase 3 be detector inputs 18 and 19.  The normal detector plan would have detector inputs 18 and 19 programmed with the proper call and extend features for phase 3.  The alternate detector plan would have the detector inputs 18 and 19 programmed with the proper call and extend features for phase 11.

Since the normal plan only affects phase 3, and the alternate detector plan only affects phase 11, the controller won’t experience Cycle Fault or Cycle Failure, as long as the Action Plans and Coordination Plans are properly programmed.

Documentation

Another extremely important portion of this operation is that it must be very well documented, and the technicians who are performing the maintenance must be aware of the special operation.

For example, if the loops in the phase 3 / phase 11 are cut, and the signal techs decide to put phase 3 in recall and turn off the failing detectors, the signal will only call phase 3, not phase 11.

It is important to understand how your particular controller deals with global vs. Action Plan settings.  All controllers have the ability to be put in ped recall, or various types of vehicle recall.  In some cases, the Action Plan, or the Coordination parameters may override the global settings.  So you may put a controller in MAX recall for the phase 2 main street green, but if the coord plan has the phase 2 main street has MIN recall for several of the coord plans, your MAX recall may become MIN recall.

It is important to understand how this works within the coord plans of your controller.  For the example of the cut loop for phase 3, you may need to go into each specific Action Plan and Coord Plan and set each specific plan to the specific recall you need for that set of active phases.

Conclusion

This offers a method to operate the signals in what could be a more efficient operation, if you have traffic signals that are in coordination and need phase rotation.  There are lots of things to make sure are programmed correctly.  If they are not, you will get twice per cycle phase operation, or maybe no phase operation by certain times of the day.