N8/N25/N40 - Dunkettle Interchange [under construction]

hans aus dtschl

https://www.boards.ie/discussion/comment/120461704#Comment_120461704

Nobody is saying we shouldn't do Dunkettle. Some are saying that we should do far more!

In terms of crystal balls, we are almost all agreed that this project will not solve N40 problems. It doesn't need a genius to see. Most of us are also agreed that further N40 expansion will not solve N40 problems. We now likely need progress on N40 North and heavy investment in sustainable transport solutions.

niloc1951

From the latest news letter

A full closure of the N25 westbound will be required..................... We are currently targeting a closure from 7:30pm on Friday 21st April 2023 until approximately 9:00pm on Saturday 22nd April 2023. The main diversion route will be via the N40 and the southern ring road and this diversion route will be well signposted for the duration of the closure.

BTW, It's not the 'Southern Ring Road, it's the 'South Link Road' (N27)

Why not simply down the slip to the Dunkettle roundabout and back up the slip to the Glanmire roundabout?

hans aus dtschl

https://www.boards.ie/discussion/comment/120464984#Comment_120464984

It's the Glashaboy bridge that's getting treated I think? So there will be no Westbound connection over Glashaboy river whatsoever. I'm not fully sure what your proposed alternative routing is, but I can't see anything available other than a routing via Glanmire Bridge (L2998) which just wouldn't cope with the volumes of traffic.

Jayuu

https://www.boards.ie/discussion/comment/120461901#Comment_120461901

Yeah but there is equally no point in catastrophising either.

AugustusMinimus

https://www.boards.ie/discussion/comment/120466171#Comment_120466171

I don’t think I am. Seeing very significant tailbacks each morning from Bloomfield is just a realistic expectation.

We were told continually in the Bandon/Sarsfield flyover thread that there would be not additional tailbacks at the Douglas flyover. This was simply incorrect.

Jayuu

https://www.boards.ie/discussion/comment/120466186#Comment_120466186

So is your problem that the improved Dunkettle will increase traffic volumes?

AugustusMinimus

https://www.boards.ie/discussion/comment/120466218#Comment_120466218

There may be an increase in traffic levels due to people no longer avoiding the junction. This isn’t the crux of the argument though.

There are big queues back up both the M8 and N25. These traffic queues will simply move on to the next choke point which will be Bloomfield / Douglas Flyover. I think it’s a possibility that queues could reach back to the tunnel in the mornings, at least at the start. People will modify their behaviour to avoid traffic by leaving earlier / later or by taking alternative routes (less likely as there are very few if any alternative routes.

rounders

Isn't part of the solution to this the variable speed limits presuming people actually follow the speed limit. The variable message signs have already been installed, the use of variable speed limits just needs to be expanded from the M50 to the N40.

Drop the speed limit from 100 to 60 between Mahon and Kinsale Road Roundabout at peak time to slow the flow of traffic and stick a speed van on the road to ensure they are followed. Or better yet, a permanent speed camera on each gantry.

niloc1951

https://www.boards.ie/discussion/comment/120464984#Comment_120464984

My mistake, I thought the closure was for the finishing touches on the bridges over the Dunkettle roundabout, which were featured in the close-up photos of ongoing waterproofing.

hans aus dtschl

https://www.boards.ie/discussion/comment/120466186#Comment_120466186

I don't think you are either. And I think it's very counter-productive to hope that this project or another N40 one (and then just one more N40 project ad nauseum) will fix the issues we have. This is like the M50 all over again: at a certain point we are not "expanding" our way out of this. We need modal shift urgently and we likely need to sort out the Northside. If we're seriously talking about widening Douglas, then we're just wasting our time in my opinion.

KrisW1001

The lights at the current Dunkettle create an uneven distribution of traffic downstream of the interchange: the lights collect the traffic into closely-spaced platoons, with clear road between each platoon. This kind of distribution can cause congestion at lower levels than if the same number of cars were more equally spaced on the road. (A big cause of congestion delay is lane changing into small gaps; tightly packed groups of cars increase the chance of this happening)

Or, for the tl;dr - I think that traffic flow through Douglas flyover will actually improve after Dunkettle is complete, even if there’s more of it.

hans aus dtschl

https://www.boards.ie/discussion/comment/120469188#Comment_120469188

What you're saying makes sense, and you might well be right.

But I suspect the N28 project unfortunately negates any improvement and then some!

Let alone any user increase resulting from Dunkettle free-flow. I think we're not going to "more N40 South" our way out of N40's problems, realistically. And if I'm honest I don't think we should. If nothing else, the single point of failure on the Cork transport system is now absolutely enormous.

AugustusMinimus

https://www.boards.ie/discussion/comment/120469188#Comment_120469188

The theory behind what you’re saying makes sense.

My worry is that traffic coming through the tunnel will be as compact as it is now, except you will no longer have the gaps which are present at the moment due to light sequences.

We’ll have a constant flow in lane 1 from the N25 and lane 2 from M8 and city.

KrisW1001

It is not constant now, though. It only looks constant because delays at lights bunch up cars into long streams. If you look at traffic passing the point just after the junction before Dunkettle (so, for M8, just south of Glanmire junction; for N25, just east of the current Little Island/Glounthaune junction) and you’ll see what the real flow would look like with free-flow in place.

But again it’s worth bearing in mind that no road is designed for the absolute peak traffic it will ever experience. There will always be slower transit at the times that everyone is trying to use the same bit of road, and it is a really, really bad idea to try and “fix” this (ask Houston).

Jayuu

https://www.boards.ie/discussion/comment/120466259#Comment_120466259

I don't get this to be honest. Unless you are expeecting more traffic to arrive at these points then there's no reason to think that the problems downstream will change that vastly simply because it's more freeflow through Dunkettle. KrisW's assertion that the freeflow delivering traffic in a more uniform way may actually negate some of the issue is equally as valid.

As for people modifying their behaviour to leave earlier/later, well if they haven't done this already to avoid the existing congestion at Dunkettle or the M28 there's no real reason to think that they will suddenly adopt a new approach because of the changes to them. There are other factors involved in why people journey at certain times such as school runs, work start/end times that are outside their ability to change.

The point all of this makes though is that we really have no real way on knowing exactly how the effects of the improvements will impact on the downstream junctions until it happens. So let's just enjoy the improvement and then assess afterwards.

https://www.boards.ie/discussion/comment/120468473#Comment_120468473

I do agree that we can't simply engage in a program of constant expansion on the N40. Cork (like all of our major cities) requires more choice with regards to transport options and we also need better solutions in general around working life/times/locations in order to generate less car journies in the first place. However there may still be good reason to expand or reconfigure Douglas although looking at the existing junction I'm not sure how it could be adjusted given the tightness of the space there.

AugustusMinimus

https://www.boards.ie/discussion/comment/120470217#Comment_120470217

I’ll ask a simple question. If my logic is incorrect, then why did we immediately start to see traffic queuing going east over the Douglas Flyover when the Bandon Road and Sarsfield Road flyovers opened 10 years ago?

AugustusMinimus

https://www.boards.ie/discussion/comment/120470027#Comment_120470027

The key though is that the capacity / through put will increase with this junction improvement. Agree that the traffic won’t be as metered in the past. However, the increase in spacing out of cars will be marginal in my mind if at all. The gain in capacity will be due to cars filling the voids that were created by traffic light sequences.

Completely agree on your second point though. There is a limit on what can be done with the N40. It’s getting to the point of diminishing returns for additional spend. Other options will be focused on in future, mainly focusing on other modes of transport.

Chris_5339762

The pity with the N40 though is that, if you consider the M28 as an extra lane joining the N40W, then you have only a short 1 mile section over the flyover before it becomes 3 lane. Its such a short sighted bottleneck, albeit difficult to fix.

KrisW1001

The fix is not to widen N40, but instead to provide a proper east-west distributor road for the South-side of Cork (including public transport and active travel provisions), so that people won’t be drawn onto N40 when all they want is to get from one side of Douglas/Grange to the other.

hans aus dtschl

https://www.boards.ie/discussion/comment/120471714#Comment_120471714

I think what you're saying is probably the top "N40" priority alright.

Then a proper East-West corridor North of the City too. At present, Mayfield to Bishopstown (Tinkers Cross to CUH, CIT, ) is best done by Dunkettle and N40. Some traffic can be re-routed if the Northside is easier to cross, and it would help economic development of the Northside too.

Removing the single point of failure will be as important as improving the journey times as far as I'm concerned. And more traffic through Douglas is probably bad for local people's health too. So loads of reasons why not to go towards Douglas expansion, IMO.

Chris_5339762

https://www.boards.ie/discussion/comment/120471714#Comment_120471714

Good point, but ultimately neither M28 nor traffic taken from Dunkettle will use a distributor road. They'll stay on the "fast" road, not the distributor which will be functional but likely 50 or 60kmh with traffic lights. They certainly won't come off the M28.

We should probably all head to the N40 thread with this instead of cluttering up the Dunkettle thread.

KrisW1001

https://www.boards.ie/discussion/comment/120471830#Comment_120471830

The East/West corridor on the Northside will be the Cork Northern Distributor Road (which will run from Tivoli via Tinkers Cross, Dublin Hill, Fair Hill and Hollyhill to Carrigrohane road at the Lee Fields). There is supposed to be a Southern Distributor too, but it is much less well defined at present.

AugustusMinimus

No mention of Link F in todays update.

Embankment work has restarted on the N25 to M8 north slip I see.

Chris_5339762

Nothing sadly! From the cameras it looks like they are tying it in to the new bridge at the moment.

AugustusMinimus

https://www.boards.ie/discussion/comment/120477005#Comment_120477005

I think they’ll also line the inside of the loop with barrier. Outside is now largely complete.

AugustusMinimus

Suspect the loop could easily open this weekend.

Chris_5339762

https://www.boards.ie/discussion/comment/120487324#Comment_120487324

Yes actually, they've finished the bridge tie-in and tarred all that. Steamrollers going along there as of now sorting the tar. Final lining and all that to do only? This weekend maybe, but I'd certainly hope before end-April. I guess just sorting out traffic lights now.

hans aus dtschl

What does that do for the finished project? I assume it gets rid of one set of traffic lights (northbound N8, the lights on the West of the roundabout).

Does it also get rid of a second set of traffic lights though, the ones at the North side of the roundabout?

AugustusMinimus

https://www.boards.ie/discussion/comment/120488020#Comment_120488020

One set only where the N8 (City) down ramp onto the Roundabout was.

I think it will also result in part of the roundabout closing between the current M8 exit and M8 entrance. It won’t be required anymore.

AugustusMinimus

Metal fixings also being put on the exposed concrete beams alongside the M8 in preparation for the bridge deck pour.