Signalized Merge

[Peter Freeman]

We've briefly discussed signalized merges before. Such an arrangement can save one bridge structure, for reasons of either economy or ease-of-fit. I encountered one recently in Denmark, turning off E20 into Copenhagen -
https://www.google.com.au/maps/place/Co ... ?entry=ttu
I believe it has been previously noted in Sabre, though I can't re-find that post. I'm sure the original poster remarked on it as being unusual - which it is, but not quite as unusual as we might think.

The reason for this one (2+2=3) is that a two-way split exists only about 400m downstream, providing insufficient time for safe lane selection. At first sight it looks like it's on a motorway, but actually neither direction downstream is motorway. Not that that should matter, in my opinion, if the arrangement provides adequate safety and capacity for the situation. The E20 and associated roads here are rather quiet.

'Split-following-merge' is the reason for all of the signalised merges that I know. I previously described one in Australia (Ferntree Gully Road) -
https://www.google.com.au/maps/@-37.902 ... ?entry=ttu
(GSV, since G.Maps satellite imagery has not caught up. Google Earth has - almost).
This modification immediately fixed a merging problem where a freeway off-ramp merged into a suburban D3 arterial. An explanation of why the modification worked is in my earlier post about it. The simple merge that was replaced looked like a classic correct design (2+1=3, lane add), but it had finally ceased to cope after many years of acceptable performance.

Another Australian one occurs at an exit from Brisbane's M2 -
https://www.google.com.au/maps/@-27.647 ... ?entry=ttu
(GSV, since G.Maps satellite imagery has not gaught up. Google Earth has).
Again, too little distance to a subsequent split. The signalised merge works just fine, thanks to generous lane provision (4 lanes and 3 lanes at the stop lines). The downstream road continues southwards for 5km as a motorway, then 10 more km as a semi-motorway.

The most usual place that a signalied merge is introduced is at a trumpet interchange that has a nearby roundabout or crossroad. Both off-ramps must then be signalized before they merge. A classic example is at M6J15. I'm sure there must be more -
https://www.google.com.au/maps/place/St ... ?entry=ttu

I think I've noticed, on Sabre, signalised merges sometimes being confused with ramp meters. While there is a superficial resemblence, the principles are quite different.

Downstream of the Danish example, after the split, is a related curiosity: a signalized, narrow-angle, no-turns crossover -
https://www.google.com.au/maps/place/Co ... ?entry=ttu
Again, Nordic pragmatism reigns. A flyover would theoretically be better, but could the cost be justified? The location is not motorway, so no principles are being offended, and capacity is more than sufficient.

Such 'no-turn crossovers' occur in the UK. One is on Heathrow Airport's Western Perimeter Road -
https://www.google.com.au/maps/place/Ho ... ?entry=ttu
I always notice it as I drive away in a hire-car. Another one, this time involving three carriageways, is on the M60 at J26 -
https://www.google.com.au/maps/place/Ma ... ?entry=ttu
Again, no problems are evident from this arrangement.

(edits: minor details added).

[Bryn666]

M60 J26 does suffer from visual confusion that can, in the right circumstances, imply you can turn left and end up on the M60 the wrong way. This has happened historically but since then the road markings and signs have been improved.

These sort of crossovers are not inherently bad, they just need careful design.

[wrinkly]

Probably mentioned before but here's a signalised merge on the Leeds IRR. Have to show an old date as more recent ones are affected by roadworks.

https://www.google.co.uk/maps/@53.79890 ... ?entry=ttu

[JammyDodge]

There is one at the north end of the Gateshead Highway (here)
Looking on Google Earth, it used to be a simple 1+1 merge until the early 2000s, when it was altered to the current setup

The other spot in the area that I believe could make use of either a signalised merge, or simple ramp metering is the exits for Coast Road off Central Motorway. Having even just a ramp meter on the northbound exit would be helpful in reducing the more dangerous queues that build on the southbound carriageway.

[Peter Freeman]

There is one at the north end of the Gateshead Highway (here)
Looking on Google Earth, it used to be a simple 1+1 merge until the early 2000s, when it was altered to the current setup.

Yes, that's a classic, good conversion. The original designer thought he was doing the right thing by trying to ensure 'free-flow' (no stopping required), but by the time the 1+1=2 merge actually became 2 lanes wide, you were sitting at the roundabout's give-way line (and quite likely in the wrong lane). There's a time and place for everything. Here - signalised merge!

The other spot in the area that I believe could make use of either a signalised merge, or simple ramp metering is the exits for Coast Road off Central Motorway. Having even just a ramp meter on the northbound exit would be helpful in reducing the more dangerous queues that build on the southbound carriageway.

I see the spot you're referring to: the approach (2+2=3) to Osborne Road crossroads, with bad congestion at both am and pm peak. There's a lot going on there! A 'ramp meter' on only one of the inputs would not do as good a job as a signalised merge. By signalising both inputs, with 2-phase turn-taking, there could be a degree of control over the relative lengths of nb and sb queues.

BTW, I assume that by 'simple ramp meter' you mean a UK-style platoon-release meter. Such a meter might marginally help here, but this situation is not one that ramp metering, either platoon-release or single-vehicle drip-feed, can solve.

Edited for clarity.

[JammyDodge]

There is one at the north end of the Gateshead Highway (here)
Looking on Google Earth, it used to be a simple 1+1 merge until the early 2000s, when it was altered to the current setup.

Yes, that's a classic, good conversion. The original designer thought he was doing the right thing by trying to ensure 'free-flow' (no stopping required), but by the time the 1+1=2 merge actually became 2 lanes wide, you were sitting at the roundabout's give-way line. There's a time and place for everything. Here - signalised merge!

The other spot in the area that I believe could make use of either a signalised merge, or simple ramp metering is the exits for Coast Road off Central Motorway. Having even just a ramp meter on the northbound exit would be helpful in reducing the more dangerous queues that build on the southbound carriageway.

I see the spot you're referring to: the approach (2+2=3) to Osborne Road crossroads, with bad congestion at both am and pm peak. There's a lot going on there! A 'ramp meter' on only one of the inputs would not do as good a job as a signalised merge. By signalising both inputs, with 2-phase turn-taking, there could be a degree of control over the relative lengths of nb and sb queues.

BTW, I assume that by 'simple ramp meter' you mean a UK-style platoon-release meter. Such a meter might marginally help here, but this situation is not one that ramp metering can solve.

Ay, if a signalised merge was to be implemented here, it would need to tie into a scheme to upgrade the Coast/Osbourne Junction
Something like this is what I keep coming to:

Basically:
Banning Right Turns at Jesmond Road/Osbourne Junctions
Making Portland and Osbourne Terraces one way
Adding in cycle routes (Yellow)
This would improve connectivity for cycles across a major road, while helping increase stacking space after the signalised merge onto Coast. Although the Southbound slip would need about 2/3rds of the green time, due to having only a 3rd of the safe stacking space

Other things as part of this I would do is make Sandyford Road Bus and Taxi only between the outlet onto Jesmond Road and the entrance for the Newcastle High School for Girls as well as widening the stretch between Sandyford Road and the Benton Bank underpass to provide bus lanes, as well as reconfiguring local roads make basically a signalised lilo at the Jesmond Road/Benton Bank junction. Something like this (Yellow is Pedestrian in this one):

[Peter Freeman]

Probably mentioned before but here's a signalised merge on the Leeds IRR. Have to show an old date as more recent ones are affected by roadworks.
https://www.google.co.uk/maps/@53.79890 ... ?entry=ttu

Yes. A classic. Looks like it's always been like that, so a brave decision by 'the Motorway City' back then.

[Bryn666]

Probably mentioned before but here's a signalised merge on the Leeds IRR. Have to show an old date as more recent ones are affected by roadworks.
https://www.google.co.uk/maps/@53.79890 ... ?entry=ttu

Yes. A classic. Looks like it's always been like that, so a brave decision by 'the Motorway City' back then.

More brought about by the scrapping of other plans - the westbound flyover before it was not original to the motorway and was done in the late 1980s if memory serves.

[Peter Freeman]

Something like this is what I keep coming to:

Yes, something like that. You've put much thought into it, but having no familiarity there I can't comment. It certainly would need more than white paint and extra signals. However, sometimes we must simply accept that city centres will be congested at peak. That area seems to flow well enough off-peak.

[Chris5156]

Probably mentioned before but here's a signalised merge on the Leeds IRR. Have to show an old date as more recent ones are affected by roadworks.
https://www.google.co.uk/maps/@53.79890 ... ?entry=ttu

Yes. A classic. Looks like it's always been like that, so a brave decision by 'the Motorway City' back then.

More brought about by the scrapping of other plans - the westbound flyover before it was not original to the motorway and was done in the late 1980s if memory serves.

May even have been early 90s. Before then all westbound traffic on the A64 passed through the signalised farrago at Burmantofts Street.

[jackal]

We might think of four cases of signalised merge:

1. Appropriately engineered road with appropriate signalisation.
2. Inappropriately engineered road with appropriate signalisation.
3. Appropriately engineered road with inappropriate signalisation.
4. Inappropriately engineered road with inappropriate signalisation.

Typical cases of type 1 are where there is simply not the space for a freeflow merge to have been constructed.

Type 2 is where the signals are required due to the unjustifiably substandard alignment (e.g., merge and diverge closer than they needed to be).

Type 3 would be where the road is actually engineered to a standard where it should be a freeflow merge, but signals have been added for no good reason. I've claimed this might be the case for A35 westbound merge at Redbridge Flyover, Southampton: viewtopic.php?p=1242142

Type 4 is logically a category but I haven't fully thought it through and don't have examples to hand. Maybe it covers a road so low standard even merge signalisation isn't enough?

In any case, signalised merge sceptics such as myself obviously have in mind types 2, 3 and maybe 4. The complaint with 2 is strictly speaking with the road alignment, but the signalised merge is nevertheless criticisable as a symptom of that problem, and perhaps even as its cause (if the alignment was designed for a signalised merge). By contrast, the complaint with 3 is strictly with the signals - if they were removed, the problem goes. There are grounds for holding that many signalised merges are of types 2 and 3 - hence their poor overall standing in my view.

But of course, even I accept there are some "good" signalised merges, i.e., type 1s. Furthermore, there are merges that I think should be signalised but aren't. Take a look at everything wrong with this M8 sliproad in central Glasgow: https://www.google.co.uk/maps/@55.86920 ... a=!3m1!1e3

[Gaz909909]

Here in Totton...

Redbridge Flyover
https://maps.app.goo.gl/S752nR9vgsBrEvSAA

[trickstat]

A well known one is just before the southbound Blackwall Tunnel.

[Keiji]

We've briefly discussed signalized merges before. Such an arrangement can save one bridge structure, for reasons of either economy or ease-of-fit. I encountered one recently in Denmark, turning off E20 into Copenhagen -
https://www.google.com.au/maps/place/Co ... ?entry=ttu
I believe it has been previously noted in Sabre, though I can't re-find that post. I'm sure the original poster remarked on it as being unusual - which it is, but not quite as unusual as we might think.

The reason for this one (2+2=3) is that a two-way split exists only about 400m downstream, providing insufficient time for safe lane selection. At first sight it looks like it's on a motorway, but actually neither direction downstream is motorway. Not that that should matter, in my opinion, if the arrangement provides adequate safety and capacity for the situation. The E20 and associated roads here are rather quiet.

That one looks most bizarre to me.

I would have thought you'd simply:

- On each two-lane approach, have the left lane for taking the left exit and the right lane for taking the right exit
- Have the two approaches join each other to form a 4-lane carriageway
- Advise (or mandate) lanes 1 and 4 to stay in lane
- Have markings to encourage traffic in lanes 2 and 3 to swap over (e.g. curved arrow plus destination staggered in each lane)

As is more-or-less done on the Coventry Ring Road as well as a couple other places in the UK that I can't remember.

Having said that, there's a bunch of other signals further ahead on that road, so it's not so troublesome to have this particular one as well.


Switch Island has an example that I really detest (not that I've ever driven there myself, but Switch Island shows up on Ashley Neal's youtube channel a lot, and when I saw one of his videos about this particular part of it I was horrified...)

https://maps.app.goo.gl/JQBAZNmrrcQUuw198

Two approaches, three lanes each, forming a six lane carriageway which then splits to three plus three again.

Markings on the right approach:
M57 | [A5036, A59, A5758] | A5036

Markings on gantry ahead:
M57 | M57 | A59 | A5036 | A5036 | A5036
The gantry won't even tell you where the A5758 lane is; there's a marking ahead that puts it in the A59 lane but that's not visible from the approach at all.

The expanse of tarmac between the merge and the split is pretty much a big free-for-all, with the leftmost and rightmost lanes given a lane marking but nothing else. That's a four-lane-wide area with no markings and a lot of traffic that just has to work out where it needs to be and somehow not result in any sideswipes with others from the same green.

If you think about just the destinations I've copied down above for a while, you might determine, like I just did, that lane 1 of approach could go to lanes 1 and 2, lane 2 of approach could go to lanes 3 and 4, and lane 3 of approach could go to lanes 5 and 6. Yet that marking that divvies off lane 6 contradicts that. The only safe combination I can see bearing in mind that marking, is for lane 1 => 1 & 2, lane 2 => 3, 4 & 5, and lane 3 => 6. I can't imagine how drivers are supposed to work that out on the fly.

The left approach is arguably even worse. The markings on that approach are:
M57 | [ALL DEST] | [A5036 ONLY]

Again bearing in mind the marking that keeps lane 1 as it is, this results in either lane 1 => 1, lane 2 => 2, 3 & 4, lane 3 => 5 & 6, or lane 1 => 1, lane 2 => 2, 3, 4 & 5, lane 3 => 6. It's absolutely impossible to determine where lane 5 is supposed to come from, so even with perfect planning, approach lanes 2 and 3 have to fight over it.

If I were to try and improve this one (without worrying about the rest of the Switch Island mess, that is) I'd probably have each approach be 4 lanes, split them into 2+2, have the inner 2 swap over via a signalised crossover, and then merge the pairs of carriageways again, with a marked weaving section on the left exit (as that splits again later) and a lane drop on the right exit (as all lanes there go to the same destination).

[Keiji]

Furthermore, there are merges that I think should be signalised but aren't. Take a look at everything wrong with this M8 sliproad in central Glasgow: https://www.google.co.uk/maps/@55.86920 ... a=!3m1!1e3

If there's a substantial amount of traffic on the approach with priority such that the drivers giving way get frustrated and start to emerge dangerously, then yes it needs signalisation.

Otherwise, it's fine IMO - I certainly wouldn't say "everything" is wrong with it at all.

I'm actually quite a fan of "Give way directly on to grade-separated dual carriageway" as opposed to a merge, where there is not space for a merge. It's much safer than a substandard merge* since traffic expects to have to come to a stop and wait, rather than finding themselves dumped on the main carriageway with a huge speed differential and not enough room to move over.

*My local example that I'll most frequently whinge about is Eagle Farm southbound on the A380, where traffic on the smooth 70mph main carriageway just has to be prepared for the silly amount of 20-30mph traffic that suddenly wants to merge, so good luck if you don't know the junction. This merge slightly earlier is even shorter, but at least it doesn't involve a loop, and far less traffic uses it. This give way between the two, by contrast, is much safer, although hardly anyone uses that. The recent A30 Temple section has several brand new give ways for minor roads joining the carriageway, where long safe merges could have been provided but would have been an unnecessary expense for the small amount of traffic using them, and this was IMO a very good choice by the designers.

[jackal]

Furthermore, there are merges that I think should be signalised but aren't. Take a look at everything wrong with this M8 sliproad in central Glasgow: https://www.google.co.uk/maps/@55.86920 ... a=!3m1!1e3

If there's a substantial amount of traffic on the approach with priority such that the drivers giving way get frustrated and start to emerge dangerously, then yes it needs signalisation.

Otherwise, it's fine IMO - I certainly wouldn't say "everything" is wrong with it at all.

I didn't say everything is wrong with it. But certainly quite a lot is - generally poor geometry, a simultaneous lane gain and give way, dilapidated signage, ambiguity as to whether you're under motorway conditions (there's no chopsticks if you come from Phoenix Road and the ADS is white). Some of this is clearer in GSV: https://www.google.co.uk/maps/@55.86926 ... ?entry=ttu From the satellite it seems there have been works in recent months so hopefully things have improved.

[Disgruntled Goat]

A414 off M1 J8 (https://maps.app.goo.gl/nXsxEixzGbiucsax6)

I'm interested in how the signals operate at the following roundabout if anyone local knows? There are only signals on the A414 East and Green Lane South entry arms and not on the circulatory, presumably these two just alternate for green time?

[Rob590]

The most usual place that a signalied merge is introduced is at a trumpet interchange that has a nearby roundabout or crossroad. Both off-ramps must then be signalized before they merge. A classic example is at M6J15. I'm sure there must be more -
https://www.google.com.au/maps/place/St ... ?entry=ttu

We might think of four cases of signalised merge:

3. Appropriately engineered road with inappropriate signalisation.

Type 3 would be where the road is actually engineered to a standard where it should be a freeflow merge, but signals have been added for no good reason. I've claimed this might be the case for A35 westbound merge at Redbridge Flyover, Southampton: viewtopic.php?p=1242142

There's one like this in Durham, which is unusual for being on an unclassified road and which might fit into your type 3 here. Presumably there were concerns about traffic backing-up on the slip road to the A690, but my experience is that the road is rarely really that busy.

[wallmeerkat]

Might this example in Belfast count? https://www.google.com/maps/@54.600105, ... ?entry=ttu

Albeit:

- Both the merges get a green light at the same time
- They only both go red so that the route from the immediate left can go green
- There is a lot of weaving, a lot of traffic from the left hand side wants to go right to head to the motorway. Traffic from the right hand side needs to head left otherwise end up on the route they just left

[Peter Freeman]

That one [Denmark E20 exit] looks most bizarre to me.

I would have thought you'd simply:

- On each two-lane approach, have the left lane for taking the left exit and the right lane for taking the right exit
- Have the two approaches join each other to form a 4-lane carriageway
- Advise (or mandate) lanes 1 and 4 to stay in lane
- Have markings to encourage traffic in lanes 2 and 3 to swap over (e.g. curved arrow plus destination staggered in each lane)

As is more-or-less done on the Coventry Ring Road as well as a couple other places in the UK that I can't remember.

I agree, that would also work well there - the merge-to-diverge length is not all that short, and that location is not short of space for a 4-lane carriageway. Also, the non-motorway (lower) speed limit could be imposed earlier, to help out.

As a matter of fact, I too have often envisioned that sort of disciplined lane-swapping down the centre of a carriageway, with the outer lanes undisturbed, in other settings. I described its potential usage in one of the LTC alternative routes. Oh, and I do see where it effectively occurs on the Coventry IRR, as you noted.

Having said that, there's a bunch of other signals further ahead on that road, so it's not so troublesome to have this particular one as well.

Agreed: the acceptability of signalised merges is far higher in the vicinity of other signalisations.