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“Early-Return-to-Green”–What is the Big Deal?

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Just returned from the Transportation Research Board (TRB) 93rd Annual Meeting , a.k.a TRB.  Meandering through the jungles of  papers, presentations and committee meetings — this has been my 8th attendance to TRB.

I felt I did have gleaned some gems, meeting some really smart people and humbled by some of the brightest brains.  Handshakes,  beers and a little catching up with old friends.  Ah!  Never before had I reflected so much on the profession that I have devoted myself to.

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An interesting subject that I picked up from this TRB,  is an “issue” called “Early-Return-of-Green”, which is some sort of glitches coming out of Actuated Signal Control.   The following is an excerpt from FHWA website:

http://ops.fhwa.dot.gov/publications/fhwahop08024/chapter6.htm#6.7

One of the consequences of coordinated, actuated control is the potential for the coordinated phase to begin earlier than expected. This “early return to green” occurs when the sum total of the time required by the non-coordinated phases is less than the sum total of the vehicle splits coded for the phases. While this may reduce delay at the first intersection, it may increase system delay because of inefficient flow at downstream intersections or, most important, the critical intersection of the network. Figure 6-26 illustrates this within a time-space diagram, that vehicles in coordinated phases that begin early may be forced to stop at one or more downstream intersections until they fall within the “band” for that direction of travel. This can result in multiple stops for vehicles and a perception of poor signal timing.

6_26

There are also quite a few papers and references in the past years discussing this,  some recent ones trying to revisit the “issue” using the so-called “high-resolution event-based signal data”.

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To recap,  the “issue” is  nothing but that non-coordinated phases, with their actual green time being determined by vehicle actuations, may terminate early (gap-out)  without hitting the force-off points (which are defined by their splits).  The extra greens are then simply awarded to the coordinated phases.  As a result,  the coordinated phases start green earlier than the time that is defined by their splits.

This has been cited as a “BIG” problem that could damage arterial signal coordination in various publications.  Several serious methodologies, formulations and equations, or frameworks have been proposed trying to diagnose and “fix” the problem.

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To tell you a little secret about myself – I am a die-hard big fan of the late Henry Barnes,  known for the “Barnes Dances”.  One of my favorite quotes of Mr. Henry Barnes was

“In this business there are very few problems that can’t be solved with some yellow paint and a little bit of common sense.”

Without resorting to pretty LaTeX-typesetted equations  (as an ego-satisfying proof that I am reeeeeally good at the so-called analytical thinking to interprété simple things in a complicated way,  if nothing else 😛 😉 😛 ) —  my “little bit of common sense” now begins churning and cries out loud:

What is the Bigggggg DEAL?

Not a big deal, at all – and here is my chain of logic as a quick-and-dirty Traffic Engineer:

First,  granted,  for a coordinated arterial the initial formation, and  coherence of the formed platoon,  with some of the intersections having this “early-return-to-green”,  might be temporarily impacted/compromised – however, the designed band is still there; it doesn’t disappear, isn’t missing or is taken away by light of lightening whatever, it is SIMPLY BEING THERE.  This means, any vehicle arrivals show up within that band will still be able to clear the intersections without stopping (in most of the cases during off peak time).

Second,  as long as NOT ALL intersections have “early-return-to-green”,  there must be AT LEAST ONE intersection running according to the designed band,  where vehicles (that have arrived out of the extra-green time from the upstream intersections)  have to stop.  Contrary to someone’s belief that this is the “bad thing”,  my opinion is,  this is  GOOD instead,  as it provides THE  OPPORTUNITY for re-shaping the platoons. Stated otherwise,  the platoon might be impacted initially (see my first point), yet the platoon would be reorganized and reshaped sooner or later – the traffic system by nature is capable of self-organanizing by itself,  isn’t that beautiful?

Third, even if all intersections have “early-return-to-green”,  so what?   Those vehicles being stopped due to the out-of-band arrivals would  have to be stopped anyway, regardless “early-return-to-green” occurs or not.   “Early-return-to-green” will not significantly increase stops or total delay — because it simply re-distributes the stops at various internal intersections.  Or, put it more precisely,  “early-return-of-green”,  though seemingly disrupts the designed progression,  will NOT significantly increase number of stops, nor increase the total delay as compared to the normal situation.  It will not make things any worse, if not making them better.

The “issue” is even more trivia, if the arterial has a high platoon ratio (i.e., the percentage of through traffic volume all  the way from the first intersection to the last),  while the boundary intersections have high minor street demand thus eliminating “early-return-to-green” at the boundary intersections – this essentially results in the platoon formed at the boundary, progressing through the designed green band regardless of whether other intersections in-between having “early-return-to-green” or not.

Last,  it is worth noting that, early-return-to-green might help increase the arterial capacity by helping shared-lane turning vehicles,  largely due to the largess of extra green saved from minor phases.  And, that is the beauty of the actuated control.

Some papers proposed some methodologies, and showed that the new offsets proposed by the methodologies resulted in  less delays and number of stops.

Actually, to me, this proves nothing relevant to early-return-to-green.  It only proves that previous offsets are not optimal.  In my opinion, to verify whether early-return-to-green really screws things up, we would need to check — for the same demand pattern and the same offsets and everything else controlled — whether delay and/or number of stops increase with early-return-to-green vs. no early-return-to-green (i.e., put max recall on minor streets so the minor phases are always forced out). My telling is, no, it will not screw progression as some of the papers have assumed. At least, it will not make things worse, if not better.

In my conclusion,  “early-return-to-green” is mostly not a bad thing for the designed arterial progression.  It even has some advantages that were not explicitly noted before.  The benefits of the “early-return-to-green” comes from the benefits of actuated control, while,  at the worst  the potential impact on the designed coordination pattern is probably just the unjustified driver perception of “bad” signal timing, and at best the extra green saved from minor streets help increase intersection capacities, especially when the arterial direction has heavy volumes already.   It might also help shared turning vehicles.

So for “early-return-to-green”, is there really something calling for a serious fix?

I guess it depends, mostly not.  To me,  “early-return-to-green” is not a defect or curse of Actuated Control,  rather it is a welfare and benefit coming from Actuations.  Its impact on the arterial progression is probably not a serious “issue”  or a real problem,  albeit – it renders a good Don Quixotes‘ windmill for some type of exercises of the academic minds.

Agree? Disagree?

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Updated on Jan 21, 2014 – admittedly this is a compounding and convoluted problem  –   especially if all possible shock wave paths and nuances considered.  But that is my point and there is a simple engineering solution to obviate (well,  largely to some extent) the complexities:

1.  Explicitly, do NOT allow early-return-to-green at the boundary intersection(s) ( plural,  if both directions are considered).  This makes sure that the primary platoons are formed and will be progressing in the band width, as designed.

2. Secondary platoons can surely take advantage of the “early green”, to get out of the way for the primary platoon. If the secondary platoons have to stop (in front of the primary platoon), so be it, they have to stop anyway because…because that is what they called – i.e., “secondary”…., even without those early greens. This is exactly why I say, “early-return-to-green” will not make things any worse, if not better.

 

 

This post has already been read 1452 times!

5 Responses to “Early-Return-to-Green”–What is the Big Deal?

  • Hi, Wu. I agree with you idea. Just like you mention in the first reason, EROG doesn’t not change the planned green wave band. The planned platoon using the band is also not impacted. The vehicles arriving in the early green do not belong to the planned platoon. Thus, their stops do have noting to do with the planned green wave flow. From another view, if there is no EROG but some vehicles arrive and stop at the intersections during the red, they must influence the following platoons. Maybe some experiments need to support this. But, anyway, I think EROG is a kind of good thing in theory.

  • Well said.

    I just re-programmed a signal to purposely allow “early” green to let go of the new comers from the previous side street. Yes, they will most likely stop at the next downstream signal. With the change, we will have cleared the NB approach lanes, so that planned platoons will arrive without stopped vehicles ahead of them.

    Those early greeners may hate having to stop at the next red and complain about “bad” progression for the few new comers. However, importantly, this next red has been red, way before he gets his green, it’s not a surprise. We always try best to avoid sudden yellow red at the next light, after the green starts.

    This “bad” for early greener can also be compared to the alternative: “bad” green light for the crossing street without any vehicles using the green, if not early green.

    • Thank you – it is good to learn your creative practice to utilize the “early green”!

  • I chuckled when I read “big problem” as the description for early return to green. I practice we anticipate and look for early return to green to help open up the green band on some corridors. We operate an 800 signal system in the suburbs of Washington DC (no stranger to LOS F signals here) and try to take advantage of early green in our coordination offset programming. A side benefit is that it can help us when we are not in peark hour plans (we call that our “average” coordination programming, trying to get some kind of coordination in both directions) where the potentially longer green time can allow for a little more green band both ways.

    One disadvantage we do experience occurs when detection fails at an intersection. We anticipate the early return to green only to have the bad detection not allow that to happen which can foul up our best laid coordination plans! That can be significant in our “average” coordination periods.

  • Very interesting and compelling post. I wonder if allowing the coordinated movements to gap out (via detection) before their programmed force offs would help or hurt this situation? What I mean in NEMA terminology is that the coordinated phases would have their HOLDs released early, instead of going straight from the HOLD to the FORCE OFF state. This would actually allow early service of the minor movements, which, if they also had relatively light demand, might cause an even earlier return to green on the coordinated movements in the next cycle. I feel lik this may help in some cases, and would hurt others, but it may also prove helpful in that it allows minor movement drivers to proceed when they would normally be “stuck” looking at a red light, while the coordinated movement had a green signal with no approaching traffic.

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