Signalized Merge

[Keiji]

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

Not bad... But why on earth does the leftmost lane end after the lights, instead of the rightmost..?

In fact, if they found a few spare pennies, they could put down the tiny amount of extra tarmac needed to run three lanes all the way to the roundabout, which has three lanes at the give way line after all...

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?

I'm not local, but my guess would be that traffic approaching from the west will be measured, and if it builds up too much (from traffic turning to/from Green Lane northbound), they would both go red to allow that traffic to clear. I'm interested to know what they actually do, though.

[Bryn666]

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

Not bad... But why on earth does the leftmost lane end after the lights, instead of the rightmost..?

In fact, if they found a few spare pennies, they could put down the tiny amount of extra tarmac needed to run three lanes all the way to the roundabout, which has three lanes at the give way line after all...

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?

I'm not local, but my guess would be that traffic approaching from the west will be measured, and if it builds up too much (from traffic turning to/from Green Lane northbound), they would both go red to allow that traffic to clear. I'm interested to know what they actually do, though.

That is a textbook example of highway engineers being obsessed with saturation flow through signals and not giving a toss what happens beyond the stop line, as long as there isn't a queue into it. Such layouts are never as efficient as the computer says because actual humans won't go into a lane they know they'll immediately be boxed into and struggle to get back out of. It's just pointless road space. Designing roads for the fantasy wet dream of self driving pods instead of for today's humans is why we have such a pitiful design manual and bad road layouts everywhere.

Still, when you try explaining to design wonks that there are human factors to consider... I think this is why some engineers get the hump when safety audits point this out.

[wallmeerkat]

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

Not bad... But why on earth does the leftmost lane end after the lights, instead of the rightmost..?

In fact, if they found a few spare pennies, they could put down the tiny amount of extra tarmac needed to run three lanes all the way to the roundabout, which has three lanes at the give way line after all...

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?

I'm not local, but my guess would be that traffic approaching from the west will be measured, and if it builds up too much (from traffic turning to/from Green Lane northbound), they would both go red to allow that traffic to clear. I'm interested to know what they actually do, though.

That is a textbook example of highway engineers being obsessed with saturation flow through signals and not giving a toss what happens beyond the stop line, as long as there isn't a queue into it. Such layouts are never as efficient as the computer says because actual humans won't go into a lane they know they'll immediately be boxed into and struggle to get back out of. It's just pointless road space. Designing roads for the fantasy wet dream of self driving pods instead of for today's humans is why we have such a pitiful design manual and bad road layouts everywhere.

Still, when you try explaining to design wonks that there are human factors to consider... I think this is why some engineers get the hump when safety audits point this out.

Indeed this junction was recently widened. https://www.google.com/maps/@54.5162694 ... ?entry=ttu I had thought that it may have been to add a bus lane, but no, ends in a pitiful merge https://www.google.com/maps/@54.5157773 ... ?entry=ttu

The sensible thing to do would've been to make the left lane left turning only, as there is a lot of traffic heads that way for industrial estates etc.

Then you have this example where there aren't even two lanes the other side of the junction, traffic has to try and awkwardly merge in the middle of the junction https://www.google.com/maps/@54.5851299 ... ?entry=ttu

[FleetlinePhil]

I assume the eastbound A58 in Halifax counts?

But probably not the westbound A58 on the other side of the same roundabout? I'm hoping somebody will tell me this was never a merge - I can't remember it not having signals.

Head west a mile or so, and this spot could do with being signalized in my view, even though I no longer have to join here in a bus three or four times a day - the joining road is the main exit from Halifax town centre heading west for the A58 or A646. Not much is going to move out for you because the right-hand lane becomes right-turn only at the lights in the distance. Maybe a signalized merge could be tied in with these lights so that through traffic on the A58 gets a clear run. Personally, if joining here with a signalized merge, I would see being stopped again at the next lights as a small price to pay for a safe exit from the town centre.

[swissferry]

I recall traffic lights with a very short green phase (maybe 5 - 10 seconds) in the 80's when exiting junction 15 of the M8 Eastbound heading North onto the A803. The junction went through various alterations during construction phases of the Springburn Expressway. I think the traffic lights were for a signalised merge and they existed for a number of years with a number of variations of road layout. All red time was negligible so I don't recall ever having to stop, you just slowed down on approach and speed up when lights went to green, producing something not too dissimilar to a merge in turn.

[Bryn666]

I assume the eastbound A58 in Halifax counts?

But probably not the westbound A58 on the other side of the same roundabout? I'm hoping somebody will tell me this was never a merge - I can't remember it not having signals.

Head west a mile or so, and this spot could do with being signalized in my view, even though I no longer have to join here in a bus three or four times a day - the joining road is the main exit from Halifax town centre heading west for the A58 or A646. Not much is going to move out for you because the right-hand lane becomes right-turn only at the lights in the distance. Maybe a signalized merge could be tied in with these lights so that through traffic on the A58 gets a clear run. Personally, if joining here with a signalized merge, I would see being stopped again at the next lights as a small price to pay for a safe exit from the town centre.

It opened as a give way in April 1973: https://www.sabre-roads.org.uk/wiki/ima ... y_1973.JPG

[traffic-light-man]

Switch Island has an example that I really detest

The Switch Island one would be quite nicely sorted simply by, on the M58 approach, sending lane 1 into lanes 1 and 2, lane 2 into lanes 3 and 4, and lane 3 into lanes 5 and 6. That's essentially how the traffic ends up sorting itself out anyway in practice, so it'd just be a case of making that more formal.

Some more of those 'intelligent' road studs wouldn't go a miss either, similarly with the four-lane left(ish) turn around the corner.

There's a new signalised merge on the A583 as part of the new PWDR in Preston, just in advance of the A582 roundabout, again to deal with the relatively short space between the two.

Another one that comes to mind is southbound on the A533 at the Halton Lea junction exit in Runcorn.

[Chris5156]

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.

I think that’s Jackal’s type 1 - the physical layout of the road was designed from the outset for signalisation, and it’s signalised. You couldn’t have a free flow merge there without substantially rebuilding it.

[Chris5156]

I recall traffic lights with a very short green phase (maybe 5 - 10 seconds) in the 80's when exiting junction 15 of the M8 Eastbound heading North onto the A803. The junction went through various alterations during construction phases of the Springburn Expressway. I think the traffic lights were for a signalised merge and they existed for a number of years with a number of variations of road layout. All red time was negligible so I don't recall ever having to stop, you just slowed down on approach and speed up when lights went to green, producing something not too dissimilar to a merge in turn.

This sounds like one of Glasgow’s early experiments with ramp metering rather than a signalised merge.

[FleetlinePhil]

I assume the eastbound A58 in Halifax counts?

But probably not the westbound A58 on the other side of the same roundabout? I'm hoping somebody will tell me this was never a merge - I can't remember it not having signals.

Head west a mile or so, and this spot could do with being signalized in my view, even though I no longer have to join here in a bus three or four times a day - the joining road is the main exit from Halifax town centre heading west for the A58 or A646. Not much is going to move out for you because the right-hand lane becomes right-turn only at the lights in the distance. Maybe a signalized merge could be tied in with these lights so that through traffic on the A58 gets a clear run. Personally, if joining here with a signalized merge, I would see being stopped again at the next lights as a small price to pay for a safe exit from the town centre.

It opened as a give way in April 1973: https://www.sabre-roads.org.uk/wiki/ima ... y_1973.JPG

Thanks, I knew I should have checked the Wiki , although I had remembered there wasn't anything directly on the A58 page.

That must have been a horrible sightline to the left, depending on what vehicle was being driven.

[darkcape]

Local example, where traffic from an arterial merges with traffic from the Ring road heading to the motorways
A5460/A563 Leicester
https://maps.app.goo.gl/QKKkKRk7yPpM4bYm8

In 2013 the inbound merge was also signalised as it was a pretty nasty short slip which you sometimes had to fight across four lanes to turn right at Fosse Park very quickly.

[Peter Freeman]

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

... 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.

First, it is a classic situation for a signalised merge, and is, I believe, the correct solution here (apart of course from ripping up the lot and starting again!). Your suggestions for modifying the signing on the L and R approach gantries may well help. However, does the video you mentioned actually show up a lot of confusion/swerving/etc?

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).

That could work, but it's a major modification, and would take up much more space (not that space is short there). What's wrong with it though, is that it's more of the never-ending tinkering that's brought Switch Island to the sorry state it's in.

Co-incidentally, I recently had the doubtful pleasure of driving through the mess, for my only time ever. I was on that very same approach (the R branch, coming from Maghull), and my passage, even at a fairly busy time, was easy. However, I was struck by how many signals I stopped at to negotiate that one intersection. I hope all the designers involved in reaching this abomination, including the latest only a couple of years ago, have since resigned out of embarrassment.

[Bryn666]

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

... 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.

First, it is a classic situation for a signalised merge, and is, I believe, the correct solution here (apart of course from ripping up the lot and starting again!). Your suggestions for modifying the signing on the L and R approach gantries may well help. However, does the video you mentioned actually show up a lot of confusion/swerving/etc?

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).

That could work, but it's a major modification, and would take up much more space (not that space is short there). What's wrong with it though, is that it's more of the never-ending tinkering that's brought Switch Island to the sorry state it's in.

Co-incidentally, I recently had the doubtful pleasure of driving through the mess, for my only time ever. I was on that very same approach (the R branch, coming from Maghull), and my passage, even at a fairly busy time, was easy. However, I was struck by how many signals I stopped at to negotiate that one intersection. I hope all the designers involved in reaching this abomination, including the latest only a couple of years ago, have since resigned out of embarrassment.

On the contrary, they've all been back patted for making the junction innovative and probably have plans to do more of it. Looking at how Haydock Island has been similarly continually messed with it seems someone in Area 10 loves insane signalisation schemes.

[Peter Freeman]

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.
...
But of course, even I accept there are some "good" signalised merges, i.e., type 1s.

I agree with the logic of your 4-way categorisation. However, I would split Type 1 into -
Type 1A - "where there is simply not the space for a freeflow merge to have been constructed", and
Type 1B - where there is sufficient space for a freeflow merge to have been constructed, generally requiring a bridge structure, but construction cost was reduced by adopting a signalised merge.

My examples at Denmark E20 and at AU Brisbane M2 are Type 1B.

The aspect of signalised merging that most interests me is whether Type 1B is justified, and, if so, then when. In my view, E20 is justified, because (a) it carries very low AADT, and (b) is on the approach to further inevitable signalisations downstream. In my view, M2 is also justified, because (a) the AADT is low-ish, (b) the road southwards degrades to a (high-quality) semi-motorway 5km downstream, and (c) drivers will not be taken by surprise as AU has a culture of accepting semi-motorways.

I supect that UK-based road users/enthusiasts will be reluctant to accept Type 1B owing to a purist interpretation of the term 'motorway' and its de-facto equivalent 'grade-separated HQDC'. Or, perhaps, it's acceptable on an HQDC, but not on a motorway ...?

BTW, a Type 1B inherently incorporates passive future-proofing for a freeflow conversion.

[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.
...
But of course, even I accept there are some "good" signalised merges, i.e., type 1s.

I agree with the logic of your 4-way categorisation. However, I would split Type 1 into -
Type 1A - "where there is simply not the space for a freeflow merge to have been constructed", and
Type 1B - where there is sufficient space for a freeflow merge to have been constructed, generally requiring a bridge structure, but construction cost was reduced by adopting a signalised merge.

My examples at Denmark E20 and at AU Brisbane M2 are Type 1B.

The aspect of signalised merging that most interests me is whether Type 1B is justified, and, if so, then when. In my view, E20 is justified, because (a) it carries very low AADT, and (b) is on the approach to further inevitable signalisations downstream. In my view, M2 is also justified, because (a) the AADT is low-ish, (b) the road southwards degrades to a (high-quality) semi-motorway 5km downstream, and (c) drivers will not be taken by surprise as AU has a culture of accepting semi-motorways.

I supect that UK-based road users/enthusiasts will be reluctant to accept Type 1B owing to a purist interpretation of the term 'motorway' and its de-facto equivalent 'grade-separated HQDC'. Or, perhaps, it's acceptable on an HQDC, but not on a motorway ...?

BTW, a Type 1B inherently incorporates passive future-proofing for a freeflow conversion.

I'm sceptical about the existence of a 1B category. The road typically has to be less wide for a freeflow lane gain than for a signalised merge because the flow is constant. So if anything the cost saving is on the side of freeflow.

[Peter Freeman]

I'm sceptical about the existence of a 1B category. The road typically has to be less wide for a freeflow lane gain than for a signalised merge because the flow is constant. So if anything the cost saving is on the side of freeflow.

These situations are not a choice of signalised merge versus a simple freeflow lane gain. A freeflow lane gain, without a bridge, is not viable for E20 or for M2, owing to the proximity of a downstream exit/diverge. The design decision is therefore between (a) build a bridge, or (b) widen and signalise, or (c) delete an exit.

Carriageway width, especially on a green-field or brown-field site, is not expensive. Assuming, for simplicity, a 1:1 duty cycle for the signals, only a factor of ~2 in extra width is required. Therefore I'm sure the no-bridge cost-saving is real.

Into which of your four categories then, do E20, M2 (and Switch Island) fit?

The Switch Island case that Keiji raised is an interesting one. Much as I dislike that whole intersection, signalised merge was a pragmatic solution for a location that had already been committed to be non-freeflow, under a constrained budget.

(Keiji's second idea for Switch Island, which employs a signalised carriageway swap, could be even better, and still avoid building a bridge. It could be a suitable alternative to a signalised merge at some other locations too).

[jackal]

So when you're talking about a bridge in the Danish case you mean a hypothetical bridge for braiding? That's such a far-fetched suggestion that I don't see how it could be relevant.

The freeflow arrangement would simply remove the lights and change markings. There is ample weaving space for a relatively low speed road.

[Peter Freeman]

I agree with the logic of your 4-way categorisation. However, I would split Type 1 into -
Type 1A - "where there is simply not the space for a freeflow merge to have been constructed", and
Type 1B - where there is sufficient space for a freeflow merge to have been constructed, generally requiring a bridge structure, but construction cost was reduced by adopting a signalised merge.

I'm sceptical about the existence of a 1B category.

The signalised merges at Denmark E20 and at AU Brisbane M2 and at Switch Island were deliberately built, despite there being sufficient space for a bridge (braiding). They 'work', though you might not like them (subject to your tolerance of signals on an otherwise-freeflow road *).

Type 1B must therefore exist, unless you believe all those examples fall into Type 2, 3 or 4.

* I call this type of road a 'semi-motorway', which is not the same as a sub-standard motorway.

[Peter Freeman]

So when you're talking about a bridge in the Danish case you mean a hypothetical bridge for braiding?

Yes. In all cases of a signalised merge 'saving' the cost of a bridge, the bridge would have been a 'braid' in the pure sense of eliminating a 'weave'.

That's such a far-fetched suggestion that I don't see how it could be relevant.

Far-fetched in the case of that E20 exit, yes, but not far-fetched at all in the case of Brisbane's M2. That's why I was surprised at first to find it. Switch Island: yes, most unlikely now, but could/should have been done at construction time. They are British 1970's motorways after all.

The freeflow arrangement would simply remove the lights and change markings. There is ample weaving space for a relatively low speed road.

At the E20 exit: yes. At Brisbane's M2: no - far too much weaving. At Switch Island: totally unworkable - two 3-lane carriageways converge then split to 7 lanes in <100m.

[Chris5156]

Switch Island: yes, most unlikely now, but could/should have been done at construction time. They are British 1970's motorways after all.

At the time it was built, braiding would have been out of the question on that particular approach to Switch Island because the road as built was meant to form the first stage of a much larger junction. It was not envisioned, at the time, that the length of road would ever need signalisation or that it would continue to be the only outlet for the M58.