Overlaps are traditionally used as Parent / Child (right turn overlaps), or as independent overlaps (timed overlaps or double clearance overlaps). See discussion in the
Blog Post on basic overlaps.
This Post starts talking about special uses of overlaps to do more with your traffic signal controller.
Think of an overlap as a tool that can be used to add flexibility to the rigid structure that your controller is chained to. In general, the controller must follow the ring and barrier structure. There is some flexibility in assigning time of day commands to cause the signal to have phase rotation (lead a protected left sometimes, lag a protected left at other times).
What if you wanted to:
- Lead and lag the protected left?
- Lead the protected left and include a queue dependent lagging protected left?
- Create a queue dependent early release on an offramp while in coordination?
- Create a queue dependent early release for a lagging protected left?
- Asymmetrically double cycle a minor intersection within a long parent cycle length?
and so forth.
Overlaps are the way to do these types of operations. In future blog posts I will explain how to do the bulleted signal operations.
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| Older generation hard wiring of phases to load switches |
Older Generation TS1 Equipment
So what am I talking about? A little more explanation about overlaps is needed. In older generation equipment, each phase was tied to a specific load bay position. This was more typical for NEMA TS1 equipment than 332 style equipment.
Generally, on the TS1 controllers running in TS1 cabinets, this was something that required significant rewiring of the cabinet to do differently.
In general, the controllers were set up to have phases 1 through 8 assigned to load switches 1 through 8, then the peds then the overlaps to the remaining 8 phases. If the signal needed something fancy, like a fifth ped, this created some interesting challenges for the engineer and technician to work out to make the conflict monitor work with the controller settings, and the cabinet wiring.
In the 1990's. traffic signal controller manufacturers started selling equipment that incorporated many of the TS2 functions into the same controllers that could be put in a NEMA TS1 or TS2 cabinet. An Econolite ASC/2 is an example of this. Eagle, Naztec, Peek and others had controllers that could be placed in either environment. This can create some confusion, where users get a lot of new features that they can use, while staying in a TS1 environment.
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| Newer generation allows software reprogramming to load switches |
Newer Generation TS2 and NTCIP equipment
Many of the TS2 controllers and all of the NTCIP controllers can be set up to allow any phase, overlap or pedestrian movement to be assigned to any load switch. While this is revolutionary within the NEMA world, it was not groundbreaking. Many of the 170 controllers could do this a long time before the NEMA world envisioned this ability. The 170 and NEMA TS2 controllers still generally had few overlaps available for use.
This can create a really interesting mess of programming for the signal techs to figure out, if this is not documented. In general, if you don't document what you are doing as an engineer, the field techs will not support it. In reality, even if you do document what you are doing as an engineer, the field techs may not support it, unless you make it really clear what you are trying to accomplish.
Using overlaps to keep things clear, and improve the operations
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| Sorting out the remapping using overlaps |
So why bother using overlaps to complicate things. Generally, traffic signals are very rigid in how they operate. The tools that exist such as conditional service and phase rotation can help, within limits. By using overlaps to run specific load switches - instead of the phase outputs - the traffic signal controller can be made more efficient and help with the processing of traffic.
In the example to the right, phase 1 and 9 are connected to overlap 1, which goes to load switch 1. This means that the controller can have phase 9 enabled, and added to the ring and barrier structure, which will allow the signal to do special things with phase 9.
One key to understanding this is that the pairs, or trios (in some cases) of phases that are routed through the overlaps to the load switches, are routed through an overlap with the same load switch the normal phase the phase would be routed through, allowing the monitor to be blind to this type of controller operations.
In other words, if you had three phases operating 7, 8 and 16 that are to be routed the normal operation of phase 8, these phases are routed to an overlap that is output to load switch 8.
IF you were to route 7, 8 and 16 to overlap 8, to load switch 6, you might have some problems.
Routing the phases to specific movements allows the signal to be modified without affecting how the MMU or CMU sees the inputs.
Following are examples of applications of how overlaps can be used to improve the signal operations. More detail will be provided in specific blog posts about how this works...
Example 1. 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.
By using an overlap to cover the protected left turn, while working through the detection parameters through alt tables, the signal can efficiently transition from lead to lag protected left turn without needing to go free, followed by offset seeking mode for several cycles. This means that your transition from lead to lag will be seamless... A big help if you this is being driven by a traffic responsive system.
Example 2. Special phasing coming out of railroad or emergency vehicle preemption
By using the overlaps and doubling up on the signal phasing on the overlaps, the signal can be forced to come out of preemption to specific phasing when the preemption phasing ends.
For example, if your preemption needs to force a call to a lagging left turn, but the lagging left turn is only to come up after preemption, Phase 1 can be assigned to the leading left turn with vehicle detection, and the normal timing parameters. Phase 9 can have no vehicle detection, and be in the ring and barrier structure as a lagging protected left, with special timing parameters (a special min green etc). The controller can be programmed to exit preempt to Phase 6 and 9, and phase 9 may be timed for a 20 second min green time. This would force the traffic signal to come out of preempt, flush a specific movement, then go back to normal operation.
This type of operation would allow the signal to serve a specific set of movements, then go on to the opposing movements, rather than go 1 and 6, then 2 and 6, then 3 and 8. The signal would go 6 and 9, then 3 and 8 for example.
Since phase 9 has no detection, the signal would never serve the movement, unless the signal was coming out of preempt.
Example 3. Cheap and dirty transit signal priority
For example, you wanted to allow a low priority preempt call to extend a green for a thru movement, but only when the bus was coming with the EVP low priority active, phase 2 and 10 could be programmed in the controller to overlap 2, which is programmed to load switch 2. Phase 10 may have 10 seconds of green time.
When the traffic signal receives a low priority preempt, the traffic signal controller has a logic statement that ties the low priority preempt to calling (and possibly extending) phase 10. Phase 10 would only come up after phase 2, if there was a bus with priority on.
The coord plans can be set up to accommodate the timing on phase 10, for the potential of a bus needing just a little more time to get through the intersection, but when no extra time is needed, the coord plan will move from 2 and 6 to the next phases across the barrier. With the use of fixed force offs, this bonus time could be provided to the side street if necessary.
Example 4. Cheap and dirty truck priority
If specific movement that had a lot of trucks, the phases could be assigned a special operation based on the presence of a truck.
For instance, for an intersection with a steep uphill grade, and heavy truck movements, a pair of phases can be assigned to the approach with the trucks. For this example, the uphill movement with the trucks is phase 4. Phase 4 is paired with phase 12, on overlap 4, to load switch 4.
With specific truck detection (pairs of loops with logic in the controller, video, radar etc) the traffic signal will detect large vehicles coming up the hill. The truck detection is assigned to extend phase 4, and call and extend phase 12.
In the applications we are putting in place, we are using a dopplar radar detection system (I am not advertising the product, but it is a Wavetronix HD detection system). The radar system will sense the vehicle, plus the speed of the vehicle. Depending on what the radar system sees - large vehicle 15 mph, vs. large vehicle at 25 mph etc - the radar system chooses to place a call on any one of the 4 contact closures on the detection card. The extension placed by the card for phase 4 and 12 will be longer if the truck is going slower, shorter if the truck is going faster.
This is not a foolproof method of truck detection, but it does allow for the signal to dynamically change its operation due to the presence of trucks - and decrease the possibility that a truck will need to stop on the hill.
Example 5. Queue dependent early release
In this form of operation, the traffic signal has advanced detection at an intersection, such as an offramp.
The traditional way of dealing with offramp queues is to place a queue sensor at the offramp, at a point that the offramp would get close to backing up onto the high speed facility. In the event that the queue of vehicles had backed up to the queue detector, the detector would place a call to the emergency vehicle preemption system, causing the ramp to dump onto the arterial street. This is not very elegant, because if the arterial street is busy, there may be nowhere for the traffic to go to.
The other ways that traffic signals are programmed to deal with the potential of a long queue on an offramp backing up onto the higher speed facility is to give very generous time to the offramp. So if the signal is running on a 120 second cycle length, the offramp may be given 50 seconds, just in case the offramp needs the time. This creates a problem, where the offramp provides random arrival traffic to the traffic signal, where the arterial street is more likely part of a coordinated system which has platoons arriving.
The long offramp split division timing creates a situation where the offramp may only need 20 seconds to flush the static queue of cars, but then the offramp is held for another 15 to 25 seconds by single cars exiting the ramp extending the signal while the arterial waits.
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| Queue Sensitive Early Release |
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| Queue Sensitive Early Release Phase Diagram Example |
It would be far more efficient to develop a queue of cars on the offramp, then serve the queue, and go back to the main street. The queue dependent early release timing parameters allows the signal to serve a normal timing to the for shorter queues, and when necessary, serve longer times for the offramp.
In general, this involves having stopbar detection, and advanced queueing detection, plus extension detection. Most of which probably already exists.
For example, this is being done at a T intersection where there is a lot of traffic coming off the T onto a major arterial. In this example, both phases 7 and 8 are tied to overlap 8, to load switch 8. The pedestrian call is tied to phase 7.
When a short queue of cars exists, only phase 8 is called and served at the normal time in the coord cycle. The radar detection provides extension to both phases 7 and 8.
When a queue of cars is on the detectors for phase 7, the signal ends the 2 and 6 coord phases early, and times phase 7, transitions to phase 8, then goes back to 2 and 6. Phase 7 may have 10, 15, or 20 seconds depending on what is programmed in the coord timing parameters.
If a ped call is placed, the ped comes up with phase 7.
For an offramp application, the stopbar detection would call phase 8. The early release queueing detection would be 250 to 300 feet from the stopbar detection, and call phase 7. All of the advanced detection would extend both phases 7 and 8. The queue detection for phase 7 would also extend both 7 and 8. Ideally, the queue detection for 7, and the stopbar detection for phase 8 would have NTCIP Queue detection enabled.
The split divisions would be set up for the timing needed to serve the short queue (phase 8) and the longer queue (phase 7). So for a 120 second cycle length, phase 8 would have maybe 25 seconds, and phase 7 would have 20 seconds of split division time. If a short queue exists, the traffic signal would serve the amount of time for the short queue, and give all of the bonus time to the arterial. If a long queue exists, the signal would terminate the arterial street 20 seconds early, and serve both 7 and 8.
Since this is a T offramp, there are no vehicles continuing straight off the offramp, all vehicle are turning right or left, the potential of needing extra extension for dealing with high speed vehicles is negated, and normal yellow and red timing can provide a safe and efficient transition from the offramp to the arterial.
Even better, using an advanced technology, like Wavetronix Advance detection (I am not advertising, but they have some great features) to have active dilemma zone monitoring will further improve the safety.
Example 6. Dynamic lagging protected left timing
This operation is very similar to the queue dependent early release, but it is used to solve another problem.
Sometimes a coord pattern really needs some signals to operate with leading protected left turns and other signals to operate with lagging protected left turns. The problem with lagging protected left turns is that a single car in the left turn pocket will cause the signal to start the green for the lagging left turn, and then hold it until the signal is ready to move to the other side of the barrier. This means that a single car, or a short queue of cars in the left turn pocket with the lagging protected left may hold the signal up for 30 or more seconds. This can cause frustration with the drivers as they wait without any reason to do so.
In this example, the phasing of the signal would have phase 7 be the protected left turn with a short queue, and phase 15 precedes phase 7 in the ring and barrier structure - where 15 and 7 were tied to overlap 7, to load switch 7. Phase 7 has the short queue detection, and phase 15 has the long queue detection. The split divisions would be set up with 20 seconds on the split division for phase 7, and 10 seconds for phase 15, the total split division for 15 then 7 would be 30 seconds.
In the event that a short queue was present at the time in the cycle to begin the phase 7 left turn, only 20 seconds of split division would be served. In the event that a longer queue was present for the phase 7 left turn, the traffic would start with 10 seconds of phase 15, followed by 20 seconds of phase 7. The drivers would see the signal go green at the start of phase 15, the signal would stay green while Phase 15 times yellow and red, then transitions into phase 7 green.
Example 7. Lead plus lag protected left turns
More detail will be provided for this type of operation, but essentially, the signal can be set up to provide a lead, and a lag protected left turn.
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| Queue sensitive protected lagging left turn - changed sensitivity by time of day |
This can be very powerful, especially if the signal is set up to terminate phase 2 as efficiently as possible, leaving the bonus time to phase 9. This can be done by making phase 6 the coord phase, and putting the signal into floating force offs. The sensitivity of the operation can be changed by the time of day plan, by calling different alt tables for detection parameters.
If the signal has very heavy northbound thru (phase 6) and very heavy northbound lefts (phases 1 and 9), the signal can provide the timing necessary for the southbound movement (phase 2), and give a great deal of time to the northbound left.
Example 8. Queue flushing through multiple signals with detection relay
This is a similar operation to the dynamic protected left turn operation. The twist is that the signals are hardwired so that queued vehicles on one upstream intersection is hardwired to a downstream intersection.
In this case, we are relaying the vehicle detection call in a 332 cabinet by hardwiring the outputs of the specific detector to a 242 card in an unused detection slot. The 242 card is hardwired to a 4cc/s cable that connects to a 242 card in another signal, and the outputs of that 242 card are wired to the input file of the second signal.
We have one of these going out to construction in the March of 2012... more to follow.
There are opportunities for this type of application in a TS2 cabinet using an EDI SSM662 card to provide the contact closure input to the TS2 cabinet.
Example 9. Active gridlock avoidance with double service
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| Active Gridlock Avoidance with Double Service |
This is being implemented with the lead plus lag protected left turns.
In many cases, poor designs of parking lots affect the operations of the traffic signals. In the example below, there is a very short stacking distance into the parking lot, and many times, the inbound traffic has nowhere to go, because of the yellow cars stacking into the first drive aisle.
In this case, there is a radar detector looking at the cars leaving the intersection, into the parking lot. The radar is capable of not only seeing a queue of slow cars, but knowing exactly where they are, and how many there are. When the radar detector sees a queue of slow moving cars on the inbound lanes of the entrance, the radar detector places a call to a normal detection channel.
The vehicle call does not extend the movement. Rather, the vehicle call is tied to a logic statement in the traffic signal controller, that translates the detection input to a force off for the specific ring that is feeding the entrance.
This is being implemented with the lead plus lag protected left turns, so that when this is enacted, the signal will come back and serve the movement a second time.
Example 10. Asymmetric Double Cycle
This takes a lot more explanation...
Asymmetric double cycle is being implemented at a few intersections today by me. It really works well. This is not adaptive. This is also not $50,000 to $70,000 per intersection
As a primer, look at the video. This is a T intersection running an Asymmetric double cycle on a 150 second cycle length, with 600 vehicle an hour coming off the side street.
On the upper side of the picture is a roundabout. There is only about 250 feet of storage before the signal backs up into the roundabout. At the upper right of the picture is a 600 spot park and ride, which pulses out 20 to 30 cars after each bus arrives at the park and ride.
Watch the video, and decide for your self if it looks like this signal is running on a 150 second cycle length.
