N8/N25/N40 - Dunkettle Interchange [open to traffic]

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.
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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.

Borderline insane.

Half of the traffic will go through Glanmire instead clogging the place up again.

Hogzy wrote: »

They probably wont to be honest. Why would you go through an entire town to avoid 2 roundabouts.

Because the tailback at those roundabouts could be worse than going through Glanmire.

Never had the chance to go to the radisson, yesterday.

I can see a lot of the Dublin traffic destined for Mayfield/Tivoli/Lovers Walk (and of course industrial traffic of the Port of Cork) choosing to travel through Glanmire at peak hours rather than using the same miniature roundabout as the City-LI, Dublin-LI and Glanmire-LI traffic.

Also, unless I'm mistaken, Dublin traffic to the east (Midleton/Waterford) either has to go through the same miniature roundabout or cross over a slip road á la Bloomfield Interchange.

It's almost as though they forgot this was the primary route from the rest of the country to Cork.

The movements list shows that the following all go through the same miniature roundabout:

South-LI
South-Glounthaune
North-West
North - LI
North - Glounthaune
Glounthaune - South
Glounthaune - North
Glounthaune - West
Glounthaune - East
Glounthaune - LI

So in fact, of the 30 movements, 10 of them, or some 33% go through one miniature roundabout.

The same one the pedestrians and cyclists are expected to navigate, incidentally.

Genius.

The same amount (10) that go through the tunnel itself. I know usage numbers are the only important aspect, but it seems a lot of different routes are expected to use one small roundabout, critically including the North-West traffic.
Recipe for disaster.
Or more likely just getting traffic back into Glanmire.

Aidan1 wrote: »

The obvious practical reason for this design is, I assume, to keep costs down (by reducing the number of structures needed). Question to the crayonistas out there; what are your solutions?

the current plan if you look at it has one MASSIVE advantage is that theres minimal disruption during construction.

The existing east-west N25 flyover toward town remains in situ along with its 2 over bridges and they probably can do the other building work in stages to the side.

Any plan that requires the flyover to be ripped up or significantly altered means that during construction you have MORE traffic to manage, not less!

This project is not happening in a green field. Its a junction with 60,000 cars a day on it that will still need to be accommodated during construction.

I think the current layout is more than adequate:
- it links North cork and the rest of ireland to the employment centres in Little Island and to the cobh hinterland and beyond with freeflow.
- It also links the rest of Ireland with the south side of the tunnel by free flow, i.e. West Cork, the airport, Ballincollig etc
- It also links the residential areas south of the river, i.e. most of the city, suburbia like Ballincollig and Carraigaline with the industrial areas that people goto at rush hour like Little Island.

What it doesnt do is link north cork/ the rest of Ireland with Mayfield and the north inner city.
Are there really that many heading to those areas for working?

My knowledge of Corks Geography is that the main jobs centres are the south inner city, around the south ring (both in retail and industrial estates) and over in little island.
All those areas would presumably be where the majority of people are going to during rush hour from the Fermoy direction and those flows are freeflow.

And honestly, the traffic is only really bad during rush hour so this is probably not as big an issue as it superficially seems.

Hogzy wrote: »

To Be fair most of the South to Little Island traffic will probably just go to the second exit for little Island (near KFC).

The main Little Island junction is backed up most mornings due to the traffic lights, and a lot of traffic chooses to use the back road instead.

hans aus dtschl wrote: »

To those pointing to the fact that the Little Island traffic can access via the primary Little Island entrance, I point to the significant tailbacks on this route at peak times at present which result in tailbacks on the eastward motorway.

actually, when you look at it theres a bigger problem

Traffic from Dublin heading toward Midleton/Little Island must first merge with the traffic from the tunnel heading for Midleton/Little Island (not a big problem).
Within metres of that happening, you are then getting this traffic splitting for Little Island to the left and Midleton to the right.
At the same time the Dublin/Tunnel->middleton traffic is attempting to get out onto the N25 they are confronted with traffic coming OFF the N25 trying to cut across their path completely to get up on the ramp to Little Island.

If there is any blockage on the little island exit roundabout leading to a small tailback towards the mainline then the whole system will break down as Dublin/Tunnel->Midleton traffic will be blocked getting onto the N25 by a queue of traffic coming from town leaving the N25 snaking across their path to Little island.

Have a look at the diagram again. It doesnt work.
Two of the main flows, Tunnel->Middleton/Dublin->Middleton are in conflict with Town to Little Island.
It'll be carnage!
(A solution to all the criss crossing is to keep tunnel/Dublin-> Midleton traffic to the left and thread them under the roundabout but this isnt in the plan)

hans aus dtschl wrote: »

What's needed is a direct route north to south. Is it possible to build a proper north-south interface either below or up above the existing junction?

Unfortunatly not. The tunnel is too close to the junction. If they were to do that it would have to be at a VERY steep incline. There probably wouldnt be enough room for off ramps coming from the Tunnel and going east or west. Look on Google maps.

spacetweek wrote: »

One renumbering possibility that could be useful is to rename the South City Ring as N8 or M8, and the Dunkettle Junction->City road (currently N8) as a Regional Road. The N25 coming from the east would terminate at the junction.

What would the point of this be? I seriously doubt they will make the N25 a Motorway. There is no hard shoulder in the tunnel or on the Douglas Viaduct. It would be nice though. There are way to many tractors on the bloody road. Its dangerous. Mind you making a road a motorway doesnt stop the tractors coming onto it. Some farmers havnt a clue of the rules of the road.

Typewriter wrote: »

All looks good except north to west.

Good grief! I take it the people who do realise that this is the N8 National Primary Route that they propose to send on such a ridiculous wild goose chase? Conflicting with the pedestrian/cyclist route? And a bunch of other traffic routes?

Would it be possible to amend the plans to add a curved ramp from the route south over to the West? (See attachment)

WHy does there HAVE to be access to Glaunthaune from the junction anyway. Surely the Glaunthaune traffic could use the Dunkettle roundabout aswell as that road that "used to be a road".

This would keep the junction primarily for traffic switching between M8 and N25. It would also get rid of those 2 roundabouts.
Ill try draw out a map if i have time later.

Hogzy wrote: »

WHy does there HAVE to be access to Glaunthaune from the junction anyway. Surely the Glaunthaune traffic could use the Dunkettle roundabout aswell as that road that "used to be a road".

How would they get to the roundabout? Are you suggesting the slip road that goes from the west (Cork) to the north (Dublin) be two-way? That would be a barrel of fun - not.

How would traffic get from the M8 to Little Island?