“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.
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:
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.
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”.
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.
“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.
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.
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