Signalized Merge

[traffic-light-man]

When I talk about it being exit blocked, I'm referring to nothing being able to leave the merge locally, not there being nothing to stop them at the next site downstream. If there's nothing leaving the merge, then there's likely to be little effective green time at the merge at all.

Assuming there's coordination between the merge and the entry to the gyratory, coordinating the signals at the merge with those at the gyratory almost certainly will result in one of the two approaches running into a red downstream - that's inevitable, but useful overall. It's a tactic probably most often seen at signalised roundabouts.

The Switch Island end of the M58 is a good example of the same arrangement. At busy periods, it would appear that the M58 approach usually runs into a red at the Island, whereas the A59 generally runs into a green. It's complicated slightly by the fact there's three stages at the Island itself, but it by-and-large works and despite it looking a bit chaotic, wait times in the reservoir are quite low.

[Owain]

I think I had it in my head that large stop line-merge point distances could actually be more efficient. Surely giving traffic more space to speed up before the merge point means you can tighten up distances between alternate platoons, whereas if the line is right up at the merge point, you have to hold the other traffic sooner to allow the newly started traffic time to limp past the merge as it accelerates. I don't know if UK signal rules permit you to let traffic into the junction before conflicting traffic has fully cleared the conflict point, which is essentially what you'd have to do here to extract full advantage. But even if you can't, there's probably a sweet spot where you can minimise buffer time between the tail end of the last phase's traffic passing the conflict point and the next green.

You're also thinking of this issue as "How do I maximise flow through a signalised merge?". That's not the point here (though it might be interesting in itself, elsewhere). The objective of a signalised merge is (usually) to avoid causing a problem** in the lead-up to the NEXT traffic obstruction (which, in this instance, is the signal to enter the gyratory). If there is no next traffic obstruction, then a normal (UNsignalised) merge is preferable.

It is true that a signalised merge can perform the function of balancing the queue lengths on the two inputs. And there may be, but not usually, situations where signalising a merge might increase overall flow. But these two possibilities are secondary issues or side benefits.

** So what is the 'problem' we're trying to fix? If the queue for the gyratory reaches back to this unsignalised merge point, traffic squeezes together in a take-it-or-leave it fashion. So, vehicles merging from Gelderd Road will most likely be in the left lane, and those from the A643 will likely be in the right lane. Before reaching the roundabout, those wishing or needing to swap lanes have 200m in a solid queue in which to do it. Much of the swapping will occur at the last minute, just as the roundabout light turns green. This disorganised swapping and jostling reduces flow onto the roundabout. That's the flow we're trying to maximise.

On the other hand, if the merge is signalised (so it's no longer really a merge!), drivers can lane-select at these lights, in a leisurely and safe way. The 200m queue up to the gyratory now flows in an orderly fashion, with less lane swapping, and enters the roundabout smoothly and densely.

Indeed - the problem there is caused by the proximity of the merge to the gyratory.

For info, what tended to happen in practice before the revamp began, is that traffic on the A58 merge would split during peak periods, even though there were no road markings to provide distinct lanes. This is what it looked like before.

With local knowledge, drivers would organise themselves during heavy traffic, depending on where they wanted to go from the gyratory:

- lane 1 to go into lane 1 on the mainline, which splits at the gyratory to provide lanes for the B6154 or A647 gyratory exits, or A58(M) to/through the city

- lane 2 to go straight to lane 2 (which becomes lane 3 at the gyratory) on the mainline, for A58(M) to/through the city

So there are conflicting movements, as you point out, but the traffic exercised a high level of discipline in organising itself to let everybody get into the right place. It was still gridlocked though, due to traffic on the mainline not moving, because it was held by red lights up the road at the gyratory.

Assuming there's coordination between the merge and the entry to the gyratory, coordinating the signals at the merge with those at the gyratory almost certainly will result in one of the two approaches running into a red downstream - that's inevitable, but useful overall. It's a tactic probably most often seen at signalised roundabouts.

I'm assuming that the signals will indeed operate in coordination with those on the gyratory. And in combination with the increased capacity currently being added to the gyratory, I expect the addition of signals - as well as lane markings on the A58 merge - will make quite a difference.

The only caveat around here is that there are no traffic signal cameras, and at least three vehicles will jump a red; this can block a junction, even where there's a box that's supposed to prevent that from happening. This can be particularly bad!

In fact I'm now wondering if the distance between the stop lines and the A643/A58 merge could be in anticipation of light-jumping? It's surely cheaper than installing enforcement cameras...

[Peter Freeman]

Switch Island signalised merge is extensively discussed upthread, starting in Keiji's post on 18-09-2023. That signalised merge works well. I have driven through there, though not at peak.

I didn't relise that those M58/A59 merge signals are coordinated with the next ones along. I would guess though, that the signalised merge would work ok there even without that coordination.

[Peter Freeman]

Indeed - the problem there is caused by the proximity of the merge to the gyratory.

For info, what tended to happen in practice before the revamp began, is that traffic on the A58 merge would split during peak periods, even though there were no road markings to provide distinct lanes. This is what it looked like before.

With local knowledge, drivers would organise themselves during heavy traffic, depending on where they wanted to go from the gyratory:

- lane 1 to go into lane 1 on the mainline, which splits at the gyratory to provide lanes for the B6154 or A647 gyratory exits, or A58(M) to/through the city

- lane 2 to go straight to lane 2 (which becomes lane 3 at the gyratory) on the mainline, for A58(M) to/through the city

So there are conflicting movements, as you point out, but the traffic exercised a high level of discipline in organising itself to let everybody get into the right place.

That's interesting. In some ways, the signalised merge, if it has a couple of nice lane-assignment gantry signs, will formalise, especially for non-locals less familiar with dealing with that particular battleground, what already happens.

It was still gridlocked though, due to traffic on the mainline not moving, because it was held by red lights up the road at the gyratory.

Well, yes, the real culprit in all of this! And my belief that the signalised merge is the correct thing to do, is because there might still be a long queue even after the roundabout works.

I'm assuming that the signals will indeed operate in coordination with those on the gyratory.

I wonder. I actually don't think it's necessary. We'll see, when it's all finished and I can watch G.Maps traffic layer!

The only caveat around here is that there are no traffic signal cameras, and at least three vehicles will jump a red; this can block a junction, even where there's a box that's supposed to prevent that from happening. This can be particularly bad!

In fact I'm now wondering if the distance between the stop lines and the A643/A58 merge could be in anticipation of light-jumping? It's surely cheaper than installing enforcement cameras...

Yes, that could be the reason for what I considered too far.

[Owain]

Regarding the ongoing works on the Armley gyratory, I would have been providing photos of this, but it's now a hostile environment for pedestrians with a lengthy diversion via the footbridge near the A58/A643 merge that we've been discussing.

The last time I walked via the gyratory itself, I was almost run over by an Audi that was in turn avoiding a MINI that seemed confused by all the orange cones and adjusted markings...