The little island junction you decided to move has absolutely no effect on Dunkettle.
Its the Glaunthaune Junction thats the issue.
Light blue...N8/ M8
Dark blue... N25 Tunnel/ N8 City
Yellow....M8/ N25 East Cork Parkway
Black.....Little Island access roads. The black by the bottom centre of page could connect to Tivoli and the tunnel but to access M8 would require using road towards Tivoli and turning back at Dunkettle roundabout and following the slip road to M8. Basically to stop traffic crossing two three lanes.
Forgot to draw a slip of the green line heading east which would branch off and connect to the new roundabout...
Yeah - would be nice, but I'm guessing it'd be fantastically expensive to build and cause massive disruption. With the purple and others you're also looking at a lot of elevation changes in a short space.
@Chris - I don't think it'd be viable to lose the entrance/exit to Little Island there. It'd have a huge effect on traffic in Little Island. Glaunthaune would see less traffic, but there's plans to increas the traffic volume there too.
I was thinking about it this morning and I think I can make things simpler. My goal is to stick as close as possible to the preferred option, but I think the M8 traffic has to be made free flow. I think this can be done simply by introducing a contra flow on the bridge, changing the shape of some of the roads and moving the 'dumb bell' roundabouts a bit :
Green arrows hopefully illustrate traffic flow. Yellow indicates physical barriers to seperate contra-flow traffic.
Again, this solution does not take Little Island to Glounthaune traffic into account. I think this volume would be low, and could use alternative routes thoough the interchange to get there. If you wanted to take it into account, you could put another roundabout north of the junction :
I'm still not happy with the amount of roundabouts Little Island traffic (especially from the city) have to go through. It probably wouldn't be in the budget, but maybe sometime in the future you could add a couple of new flyovers and make the whole thing free-flow.
I'm also not happy with those two mini roundabouts in Glounthaune. I think you'd be better off either making the whole thing a traffic light controlled juntion, or make it one big irregularly shaped roundabout.
I think you could also improve it by altering the plans. I know in earlier posts I suggested bringing city to Little Island traffic along that road to Glounthaune, but I think it might be better to build a new ramp (light blue) and merge it with traffic out of the tunnel. This would add extra traffic into the conflict between eastbound to N25, Little Island and southbound M8 traffic, but I think it could be done safely, and would cut down on the amount of bridges you'd have to build.
Great idea. What you're proposing in effect is a U-turn lane. U-turn lanes are common in countries like Thailand and the United Arab Emirates. Here's an example.
I independently had this idea, but in my version the U-turn flyover is not incorporated into the Little Island junction. It would be either between Dunkettle and LI or east of LI:
Between Dunkettle and LI
Advantage: Short journey
Disadvantage: Space is tight, vehicle speed would have to be very low
East of LI:
Adv: Lots of room to build, so loops aren't so tight - vehicle speed is higher
Disadv: Long winded journey
I've mocked up a quick map.
The first design doesn't work as you give too short a distance for the bridge you add.
The second design makes some trips much longer than they should be: http://maps.google.ie/maps?saddr=Lower+Glanmire+Rd%2FN8&daddr=51.9030422,-8.3519439+to:R623&hl=en&sll=51.903931,-8.380251&sspn=0.026849,0.084543&geocode=FT4GGAMddNB__w%3BFUL6FwMdOY-A_ylrGEUfV5tESDGw1gv8pscAEw%3BFXjvFwMdAwmA_w&mra=dvme&mrsp=1&sz=14&via=1&t=m&z=14
Hogzy is right, I don't think there is any problem with the Little Island junction. If anything, the problem would be that it becomes over-subscribed, as it would be doing the work of two junctions.
Interesting layout. There is a risk, that without the ability to get from Little Island / Glounthaune to the city and tunnel, longer trisp would be necessary and some inappropriate journeys would be made, e.g. HGVs using the back road between Glounthaune and Glanmire. http://maps.google.ie/maps?saddr=R623&daddr=51.9176165,-8.4000669+to:Lower+Glanmire+Rd%2FN8&hl=en&sll=51.913356,-8.384886&sspn=0.026843,0.084543&geocode=FfoDGAMdeyOA_w%3BFTAzGAMdPtN__yk_MV-6HptESDGh2wn9pscAEw%3BFXYFGAMd-M5__w&mra=dme&mrsp=0&sz=14&via=1&t=m&z=14 and http://maps.google.ie/maps?saddr=R623&daddr=51.9176165,-8.4000669+to:Lower+Glanmire+Rd%2FN8&hl=en&ll=51.916665,-8.390636&spn=0.006115,0.042272&sll=51.913356,-8.384886&sspn=0.026843,0.084543&geocode=FfoDGAMdeyOA_w%3BFTAzGAMdPtN__yk_MV-6HptESDGh2wn9pscAEw%3BFXYFGAMd-M5__w&mra=dme&mrsp=0&sz=14&via=1&t=m&z=15&layer=c&cbll=51.916672,-8.390647&panoid=TMlr5Sux7trkOQIXGl27cA&cbp=11,202.16,,0,4.93
Your black line is rather impractical. The northern end would be immensely steep, there is no space for it and it would require a substantial tunnel - about 100 under the N25, similar to the railway one. http://binged.it/ArqCZq It would require a substantial section of the N25 to be reconstructed. If anything, one would make that an elevated section.
The pink bit should be able to use the existing bridges over the interchange roundabout, although minor reworking may be required to fit the blue bit under the east bridge.
Removing the original roundabout, while 'nice', may be inappropriate. Roundabouts are useful psychological markers to encourage motorists to change from motorway to urban driving.
Perhaps not the easiest to use with a truck.
To be practical, this would need to take a much wider curve. Note the 30km/h speed limit on the N3-M50 junction that is causing problems.
Where is that exactly?
OK,but at rush hour 30km/h is actually not bad.
The way I've drawn them, probably. In some ways I might have stuck too close to the original design. You could probably adjust the curvature on the approaches to the bridge on the east on both sides of the N25 with (hopefully) minimal impact to the design. Space is always going to be a concern, but I think you could get them to a point where you could have a 80kph limit. If you look at some of the mad cow interchange, 60kph could be a realistic goal too. I was trying to avoid it, but I think moving the bridge a little further east and changing the shape of it could help too, but might force a bit of re-jigging with the minor roads around there too. If I get a chance I might get the crayolas out again later and see if I can illustrate it better.
Not when there is a car doing 100-120km/h behind a truck that is obeying the 30km/h.
An Bord Planala or the construction company?
The two main problems I see with the design I've illustrated belown. The first is that with so much traffic from the north side of the interchange using the dumb-bell roundabouts, I'd worry that the traffic would back up all the way to point A. This would mean traffic blocking slip roads and causing hazards. But even if traffic didn't back up that far, where you get queues, you get queue skippers. Muppets driving down the outside as if they're going to the N25 then cutting in at the last minute would be a danger too.
I'd also worry that muppets would think the yellow route would be the path from the city to Little Island/Glounthaune. I'm guessing there'll be pletty of road markings to discourage it.
So I've had another go at drawing out a solution. Again, yellow indicates barriers between contra flow traffic and hopefully green arrows illustrate traffic flow. Some of the curves are not what I'd like them to be, but maybe somebody who knows what they're doing would be able to do better.
In this version I've moved the Glounthaune slip road to the other side of the "U-turn lane". This means we'd have to build another bridge but gives more room for the U-turn lane. The curve at the end of the U-turn lane to take traffic into Little Island is also a bit sharp - it probably could be moved or re-drawn to be smoother.
I've also given no provision for traffic from the N25 or Little Island to Glounthaune. At the moment this traffic uses the tunnel roundabout, going all the way around to get to Glounthaune. In my version something similar happens - traffic goes all the way around the interchange to get to Glounthaune. Alternative solutions for including these traffic flows can be seen in my earlier posts.
I would also propose building a new slip road to point B, rather than opening up the old road all the way to Glounthaune and bringing city-Little Island traffic that way. I think this would be cheaper and simpler than building all those bridges, and I think the extra traffic could safely merge between B and A.
Anyone considering emailing Jacobs with your own designs ?