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

The details of the CPO for this project were in the Examiner last Friday. At least something is happening on it anyway.

MYOB wrote: »

Sure it wasn't the CPO for the improvement works at the other roundabout (the one in towards the city on the Tivoli DC)? This isn't advanced enough for a CPO.

Actually it was neither . It's for a feeder road into the M8/N25 junction - the back road to Glanmire from the Ibis to the AIB in Glanmire. Looks like that road is being beefed up for future developments.

I posted about this over in the Cork forum already - didn't realise this was here

Acknowledging that asking for public opinion on the different proposals is a bit
ridiculous without details on current traffic volumes and predicted volumes,
construction costs and land available, I think they're all a bit over complicated. I
put my own proposal on the Cork forum, but looking at the posts here, I've re-done it
below.



In phase 1, I'd start with a ramp to join the road from Cork (Dunkettle roundabout) to
the Dublin road without passing through the tunnel interchange. I've drawn this as two


lanes that pretty much follow the path of the old road (pre-East Cork Parkway) there,
but I'm not sure it needs to be two lanes.

South west of the interchange I've extended the ramp from the tunnel interchange as far


as possible before hitting the railway bridge. I originally thought it wouldn't be
possible to build there, but several of the official proposals reclaim land there.
This is to make room for a new ramp later.

North east of the interchange I've drawn a new ramp to link the Dublin Road southbound
with the N25 eastbound, starting just after the bridge. This includes a new off ramp

to Glanmire East, though the new ramp will pass over the existing Glanmire East off

ramp.

South east of the interchange is where most of the work would need to be done. I
really can't see an easy way to get traffic from the Dublin Road to Cork city through
the interchange. My solution proposes a two lane ramp that starts south of the
interchange at the Little Island turn off, goes up over the existing road and all the
way out towards the PJ O'Hea Opel dealership to join up with the N25 there. I think it


has to go that far out to merge safely with the traffic.

Traffic in and out of Little Island at this point would have to be restricted or even
stopped during works. Afterwards traffic from Little Island to cork would have to use
a ramp up onto this new road to get to Cork city, while traffic to Little Island would
need a ramp down from it.



In phase 2, the existing exit from the tunnel interchange towards the city is closed to

allow works to begin on a new ramp. This two lane ramp is to bring traffic from the

N25 onto the Dublin Road north. Again I'm not sure if it needs to be two lanes. This

ramp would be tricky enough to put in. It's in several of the official proposals so I

presume the experts think it would be possible to safely merge traffic heading north,

before traffic heading west needs to exit.

A new flyover is also built in this phase to take traffic from the tunnel to the east,

to merge with the new ramp off the Dublin Road and subsequently the N25. At this point

traffic from the tunnel will be able to access Glanmire East again. I wouldn't have

thought there'd be room for a flyover here, but again it features in some of the

official proposals so I guess there is. There's also a ramp to bring traffic from the

tunnel back down to access Little Island East, but it's not needed at this stage of the

project.

The new flyover is two lanes, but I'd imagine the entrance during construction works

would be only one lane, with the left hand of the 3 lanes on the ramp down to the

interchange being closed to make room.

Traffic from Little Island East to the city would have to join with the N25 via the new

ramp.

You've hard a productive day Cheers for the drawings and explanations.

It's a good point. I never thought there would be room there so I left it out of my original drawings on the Cork forum, but I was planning something similar to the brown option. Now that I look at it, it might be better to reverse them. Take the blue lanes off closer to the tunnel, have the red lanes cross above them a second time and merge with the N8 northbound more or less under the existing bridges.

I said I'd give this one more go and see if I could make it any simpler, and I think I have with a new layout for the Dublin-Cork City/Dublin-Waterford roads. I tried to make some better drawings using Gimp, but failed miserably, so there's some paint ones below.

As before, the big considerations here are space, cost and space. I said space twice because it'd be so easy to do a simpler layout if there was more space for development. Also as before, I realise that I'm not really qualified to make good decisions on what land is suitable for building on or what traffic volumes are, but the council called for opinions, so these are mine.

I'm also acknowledging that when you add in junctions for Glanmire East and Little Island West, you're talking about a 6-way junction here, so that's going to add complexity and cost.

Here's some overviews :




Phase 1.

Phase 1 would begin the same as before; with re-opening that old section of road next to the train tracks all the way to Glanmire East. If we built a new on-ramp to the M8 from this road, and moved and/or reshaped the Glanmire East roundabout, we could take Cork City-Dublin and Cork City-Glanmire East traffic off the roundabout, with the minimum of cost and disruption. I've drawn this as a two lane on-ramp to the M8, but depending on cost and predicted traffic volumes, it could be scaled back to one lane.

Work could also be done concurrently on a new ramp to bring traffic from the tunnel northbound to the city. This would begin as soon as possible on the exit from the tunnel and end as close as possible to the bridge over the railway. This would be a one lane ramp to simplify merging with the N25 and make things safer. This land is quite marshy, but some of the official proposals show building here, so I presume it is possible.

I'd also build a new ramp south-east of the tunnel to take traffic from the N25 westbound into the tunnel southbound. This would go to the left of the existing ramp to the tunnel roundabout and merge with southbound traffic as close to the tunnel as possible. Because of traffic volumes, I think this has to be two lanes, despite the difficulty of merging them safely in such a short space. This also necessitates the closing of the access to Little Island West near the tunnel. Depending on the duration of works, hopefully it wouldn't inconvenience drivers for too long. We can also lose the roundabout at Little Island West now.


Phase 2.

Phase 2 is simply a new ramp to take traffic out of the tunnel northbound to the N25 eastbound. To accommodate traffic during construction could be tricky, but on completion it will begin just past the existing bridges, loop up over the M8, stay up, and merge with the N25 more or less where it does now. I wouldn't have thought there was space for it, but the red option has something similar. This is two lanes due to traffic volumes, though would probably start as one.


Phase 3.

Phase 3 is the heart of this design. In phase 3 we build a big new ramp to take traffic from the M8 southbound both east and west. We do this by bringing traffic from under the existing overpass up over our new tunnel access ramp and to the east. To the east this then splits so that traffic can join the N25 westbound before the existing flyover, and the N25 eastbound after the existing Glanmire East on-ramp. This is two lanes and would in the future be a vital traffic artery. Not only would this carry Dublin to Cork City traffic, but in the future Shannon to Rosslare traffic is planned to go north of the city and join the M8 southbound. This would be a very expensive ramp but it's importance cannot be underestimated.

This ramp has itself got an on-ramp and off-ramp for Little Island west. One to take traffic to Little Island west starts just east of the existing road, and one west of the existing road to bring traffic from Little Island onto the ramp and then to the N25 east or west. I've drawn these as pretty small but they would probably have to be bigger. These could also be on the other side of the ramp to create more space, but would then be merging with traffic from the right.

I'd also build a new ramp at this point to bring traffic from the N25 westbound to the M8 northbound during phase 3. This begins just past the existing flyover and loops down to join the M8 northbound just before the bridges. I believe the brown option has something similar. I think this would have to be one lane due to safety concerns, though as stated above, this would also have to carry Rosslare to Shannon traffic.

The last major ramp built in this phase is to take traffic from the city to the tunnel southbound (and Little Island). This starts just past the bridge under the M8 on the old road, goes up over the railroad tracks and joins the M8 southbound before the bridges. There's also a ramp from the yellow ramp from phase 2 which joins this ramp, to accommodate traffic from the tunnel northbound that wishes to go to Little Island west.

At this point we'd also start 'straightening' the tunnel roundabout so that it's basically two lanes straight through, with on-ramps and off-ramps on the left of each dual carraigeway. I didn't draw this because my crappy drawing is probably busy enough.


Phase 4

Phase 4, apart from sealing off the roundabout, is really just a minor and optional thing. In these plans there's no provision made for traffic from the M8 southbound to Glanmire East. I'm not sure what the volumes are, but if you didn't want to send this traffic through Glanmire or to the Little Island junction, you could build the green ramp shown to take traffic from the red ramp to the existing Glanmire east ramp.


And that's it. If you're wondering how you get from the N25 westbound to Glanmire east, you take the red ramp to the M8 northbound, then the yellow ramp to the existing Glanmire east access road. It's not great, but atm you have to go all the way around the roundabout, so with the new free flow system it'd probably take around the same time.

To get from the tunnel northbound to Little Island west is a bit complicated too. You have to take the yellow ramp, then the turn down onto the read ramp north of the interchange. You then turn off before the tunnel as before onto the main red ramp, and turn off that down onto the existing road. It'd probably take a bit longer than it does at the moment, but shouldn't make a massive difference if you count the time waiting at traffic lights.

So there you have it. With proper project planning delays could hopefully be kept to a minimum. A lot of what I've set out as separate phases could actually be built concurrently with a view to keeping road closures to a minimum.
.
The Glanmire roundabout I'd treat as a separate project. I may do a post on that in the next few days. I misread the bit about the 'new Little Island Interchange' before too, but there's traffic problems with the N25/Little Island junction too so I may do a post on that too.

I agree. In the current economic climate I'd love to do it as cheaply as possible, but doing half a job could cost more in the long term, and I'd be worried about traffic volumes in the future.

Jayuu wrote: »

/ N25 West > Tunnel /

If you look at the first picture in my first post above, I think a lot of what you propose is covered by the blue roads. The problem is, how do you propose to get traffic from the city to the tunnel southbound? It's the toughest road to situate.

I think you're also understimating the volumes involved in the clash between traffic out of the tunnel nortbound going either to Little Island or the N25 eastbound, and traffic on the M8 southbound to the tunnel and the city. I don't have data but from observation this seems to be the biggest traffic issue in the junction to me.

Your solution may also not be future proof - what about traffic from the N25 westbound to the M8 northbound? It's probably not a huge volume now, but in the future the proposed Rosslare-Shannon motorway is to go north of the city, so that traffic will also have to use this junction.

I think you'd be surprised how much some free-flow left turns would improve things, not so much in the amount of traffic taken off, but show much less interaction there would be.

Certainly, removing the city-Fermoy/Glounthaune traffic would make a big change, although it is likely to be the most expensive to do.

A straightforward left turn from Little Island to the tunnel might be difficult, due to the off ramp from the tunnel appraoch.

I can't see the M8-tunnel vertical alignment being changed - it is already very steep.

Blitzkrieger wrote: »

This is where my lack of knowledge of road building comes in - I would have thought that would be the cheapest part of the plan. I don't see how it would take that much work to re-open the old road to Glounthaune, and I'd have thought it'd be relatively simple to build a ramp from the old road onto the M8 northbound?

Sorry, I meant the most expensive of the four left turns.

To restore the Cork-Glounthaune road would mean putting bridge under the Glanmire Bypass.

I think it would be impractical for the limes of HGVs to manage the steepness of going from the Cork-Glounthaune road to the Glanmire Bypass - the bypass itself is already steep enough. This likely means putting a bridge on the slip road as well.

Am I reading this correctly? Does Dublin - Cork traffic have to go through the two roundabouts of a dumbell interchange?!

The Dunkettle interchange, no matter how you modify it, will never work properly. It is simply too close to the sub-sea entrance to the tunnel.

The M25 flyover is a disaster - one of the proposals at the time was for a cable stayed bridge, which would not have had supports, blocking the middle of the roundabout. But of course, it was turned down - by the same short-sighted people who scrapped the planned flyovers at the Kinsale road roundabout, as well as Sarsfields and Bandon Road roundabouts - the cost of construction these now has dwarfed the cost of building them the first day.

The N25 flyover at Dunkettle needs to be replaced with a single span cable-stayed bridge. The traffic to & from the tunnel could then run straight through northwards / southwards, with a proper grade separated junction between the M8 and M25, located to the north, where there is more space.

It would have to all worked out, but moving the junction northwards is the only real solution I can see.