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

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  • Some more details: the new roundabout by Pfizer in Little Island is completely done and open. It's being used as the primary southside site access.

    The new Little Island flyover bridges are under construction. Pillars seem to be in place on both sides of the road. No mound of earth is yet in place yet for its associated slip roads and roundabouts (as of Sunday 15-August-21) on the south, but the mound of earth on the North is part-way done.

    You can just about make out the pillars and the mound of earth on the webcam:

    https://www.dunkettletraffic.ie/live/?id=2





  • More visible changes in the last few weeks.

    • The M8 South to N25 East slip is now mostly tarmaced. Bridge completed.
    • Substantial works done done for the slip tie into the M8.
    • The old slip from Glounthaune to M8 South is now open again. Substantial temporary barriers in place.
    • Concrete poured on the bridge for the new M8 South mainline. Still need to build up earthworks to match level of bridge on both sides.
    • Pillars nearly complete for the 4/4 Dumbell/Bridge east of the current interchange.
    • N25 West to M8 North slip is starting to take shape. Significant earthworks completed on it.




  • They're currently widening both sides of the existing M8 bridge over the L2998 too.

    Traffic underneath is one lane stop-go over the last few weeks.

    Generally no tailbacks.






  • They have new metal barriers up on the bridge on the M8 north section by the looks of it.

    Assume they’ll move traffic to the east onto the new bridge before long and then dig away the old M8 north section.





  • Yup, thats my thinking too. Maybe not initially? Perhaps they'll open M8(S) to N25(E), which also seems to be getting a lot of attention paid to it, to pull some traffic off the roundabout before switching to the big new bridge.



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  • That's exactly what I'm expecting also.

    And I suspect it will happen soon enough, too, given the looks of that new bridge.

    Meanwhile, Cork Co Co has also got approval for a new footpath/cycleway through from Bury's Bridge into the back of Eastgate, with a new dedicated Bus Gate.

    So buses will take the Westernmost N25 Little Island slip to access Little Island, via the new Dumbell Interchange. it's not perfect, but it's the most ambitious thing I've ever seen from them

    https://www.corkcoco.ie/en/planning/traffic-transport/statutory-processes





  • That is ambitious for Cork, let alone the country as a whole.

    A bus gate, and bus stops on dual carriageway slip roads is unheard of here. Glad they are thinking of doing all of this. Did they employ someone new?






  • This one is by Arup, and I haven't seen a design by them in a while, but I'm fairly sure they've been used before.

    It's like someone with a sustainable transport mindset has been magically parachuted in for this one. The bus gates in Eastgate, the removal of the "rat runs" at Harvey Norman, the kerb-separated cycle lanes, the raised table entrance crossings...OK it's not absolutely perfect, but there's some real ingenuity here. I was genuinely shocked at the new sustainable transport entrance to the West of Eastgate.

    I still am quite worried about that new Little Island West dumbell interchange though. I may stand corrected, but I just don't think it will cope with the Dublin+Glanmire+Glounthaune+Mahon traffic demands in the mornings. Dublin should be using it to get to Tivoli, Glanmire and Glounthaune using it to get to the tunnel, and Mahon using it to get to Little Island. It just looks like too many flows to me, and the weaving on the northern arm (Dublin+Tivoli) looks very short.

    https://www.dunkettle.ie/media/1334/traffic-movements_from-littleisland-glounthane.jpg





  • Looks good.

    It might seem strange, but the off-ramp is probably the best location for that bus-stop: lots of people exit the station there, and walk across to Little Island, so it could allow an easier transfer to the far side of Little Island for people coming in by rail. It’s good to see transport planners taking notice of desire-paths and using them, and better to see efforts being made to facilitate rail/bus interchanges.

    I spent about a year walking around there during lunch-breaks, and I can’t argue with any of those plans - this whole area was never particularly pedestrian-friendly (especially the circuitous access into EastGate Village by foot) and the new bus gate will improve connectivity without creating a rat-run.





  • Ive always worried about that Little Island "west" junction... for instance will N40E to N25E coming out of the tunnel in the evening get bogged down in traffic trying to join a potential queue up onto that junction. Quite how bad the weaving will be, or how long the merge lanes will be I don't know. None of the models have shown it well enough yet.The trouble is, what other option did they have? The whole interchange itself kind of blocks any attempt at further grade separation.



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  • Here are the movements onto N25 East. Pink blobs mark the end of merges. There’s a good amount of space there for N25 traffic to have a clear run at things before Little Island traffic tries to join. Remember, traffic joining (from the red path) must yield. Approaches to that junction could also be managed by traffic lights to control the rate of joining.

    There is no way onto N25 East for traffic that does not go through the lower dumbbell roundabout. From exit 1 of that roundabout that road diverges into to branches: one (left lanes) leads to N8 North, the other (right lanes) goes to N8 West. The second exit of that lower roundabout leads to the tunnel and N40 West only.

    I suspect the join between N40/N25E and the N8 North roads will be tapered in such a way that traffic coming from the tunnel will be allowed move right onto N8 North, but traffic that originated from the West has no way to cross over, and must continue onward to N8.






  • Where you say "the join between N40/N25E and the N8 North roads will be tapered in such a way that traffic coming from the tunnel will be allowed move right onto N8 North, but traffic that originated from the West has no way to cross over, and must continue onward to N8." that's exactly my concern area. It looks like there are three potential flows joining together at your left-hand pink dot. If that's cut to two, it's much more manageable.


    I have secondary concerns about this same junction: Glanmire/Glounthaune traffic will come through it too and I believe it will be quite busy. So I don't think North Cork traffic towards the city is going to choose the M8 when this upgrade is complete, I believe they'll still rat-run through Rathcooney etc. Ultimately, I think when this one's done, we'll still have a North-Cork to City problem.

    It's also the active transport route through the interchange! The only improvement I can think of would be an overhead route from M8 southbound through towards the Glashaboy river. It would need two new structures, and the one at the West would be quite complex.





  • Okay, this image is clearer. (source: HomePage_Sectors_SectorsListing_Geotechnical_ProjectProfile_Dunkettle.png (1000×459) (fehilytimoney.ie) )

    The slip heading toward M8 North is completely isolated from N40-to-N25 East. There is no opportunity for merging or weaving.

    The only place where weaving could occur is between point A (N8-South meets Tunnel-to-N25-East) and point B (diverge from Tunnel-to-N25-East for Little Island Junction). That’s going to have to be under a reduced speed limit, I think.

    Point C is where the Eastbound traffic from the tunnel meets the main N25.

    I’ve put traffic originating in Little Island in red. Only Little Island to N25 East touches any traffic flow that originated in the tunnel, at point D, and by that time, the eastbound traffic is well sorted.





  • As far as I can tell, the blue coming out of the tunnel will be 2 lanes, right along to C. Orange will be one lane. But I would like to see the proposed lane arrangements for sure, thats the sort of detail we haven't seen just yet.

    Of course, it might be somewhat self-sustaining, if traffic is bad at that dumbbell, people might just see it and go along to the Little Island junction.





  • Yes, blue is 2 lanes, Orange is one.

    From Point A to Point B is three lanes.

    Point B to C is five lanes, merging to three (the existing road is three lanes here already).

    The drawings here show the lane arrangements:


    .. it also has these nice renders of the final scheme, complete with lane-markings. This one shows the section between Point A and Point B on my drawing above:

    These are the original scheme drawings, last changed in 2012, but they are the same drawings as used for illustrating traffic movements on the current project scheme.





  • You've drawn it perfectly and confirmed my thinking also: it's point A to point B in your drawing that I'm most concerned about.

    My secondary concern is then the red traffic flow which crosses the bridge over the N25. It's drawn in red on the incorrect side of the bridge but that's immaterial, the point is that it will be Dublin, Cork, Mahon, Glanmire and Glounthaune traffic sharing that bridge lane and I expect it to be quite a heavy flow in the mornings.


    Combine these and you get: bridge backed up, slip lane backed up, tunnel backed up.


    This is just an observation, I haven't done any mathematics whatsoever and I hope I'm wrong about it!

    Chris_5339762, I don't think the tunnel traffic will be able to "see and avoid" if the weaving Dublin-origin traffic clogs point B above: the tunnel will cease to function Eastbound.


    I think the only way that this can happen is if traffic is leaving Little Island blocking the upper roundabout, if traffic on the new Little Island link road itself stops, or if there's a crash due to weaving between A and C above.





  • Look at the second montage that shows the layout between A and B. The M8 off-slip is slightly elevated, but there are good sight-lines for traffic already on the N40 off-slip to see what’s coming down that ramp. Also, and in fairness to the designers here, they have made sure to keep the extra lanes in place in order to reduce the effects of weaving: there are two destinations from here, but three lanes. Traffic from the Tunnel to N25 Waterford is pretty much out of this, and can just stay right and avoid everything. An 80 or 60 km/h limit through the interchange, white paint and good signage will solve a lot of these problems

    On my illustration, I’ve drawn the traffic flows from the Western side of Little Island, not into it, so the red line is on the proper side. ( On the original drawings of that scheme, which I linked to later, there is a red line along there, but it is a cycleway, not a traffic lane.)

    But remember that this junction is not the only access to Little Island - the existing Glounthaune/Little Island junction will remain further East. This junction only has to deal with the flows that would normally have used the Tunnel Roundabout to access the Western side of Little Island. For that, it will be fine.





  • Great work Kris! You're right, the third lane there will take away a LOT of the problem. I had thought it was two, and that was a major part of my concern!





  • The busyness of the interchange works is inversely proportional to the busyness of this forum.



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  • Interesting email this week. The beams for the new dumbbell interchange to the east of the present roundabout will be put in place during November.

    a lot if the work is also focusing now on the N8 east to N25 south movement which will hug the present flyover before looping to the left and joining the M8 mainline to go to the tunnel.



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