N3 - Clonee to M50

KrisW1001

Longer slips add capacity. Junction design and spacing is the biggest factor in maintaining a road’s capacity. There’s a theoretical limit to trhoughput generally taken as 2000 cars per hour, per lane on a multi-lane divided road, but every time you come to a junction, you will reduce that throughput temporarily as traffic adjusts for leavers, passers-through and joiners.

If a car has to leap into the mainline from a short slip, anyone behind has to slow down. Give that joining traffic more space to build up speed, find a safe gap and merge in cleanly, and the new car will cause no disturbance to existing traffic. The same applies to cars leaving the mainline. If the exit slips are too short, then the queue to exit can extends into the mainline, and the leftmost lane of the mainline ends up being dragged down to whatever speed the exit junction can accept cars at. Longer (and wider) exit slips allow cars to leave the mainline at the same speed as surrounding cars.

J13 and J14 on M50 are what you have to do if your junctions are too close together to allow for acceptable merge and exit lane lengths. Here, the pair of junctions has been reconfigured to function as a single interchange. Doing this allows them to effectively share the same, longer length of sliproads, which gives traffic a lot more space to enter and leave. (The Bandon Road/Sarsfield Road junctions on Corks N40 are the same idea).

The reason that adding lanes for something like N3 won’t work is that through capacity isn’t the problem: junctions are. A series of closely-spaced juctions, onto 50km/h roads means that traffic never gets enough time to take advantage of an extra traffic lane, and also, a lot of traffic joins at one junction and leaves at the next, negating any advantage. The better solution is to separate the local and through traffic using a parallel access road, but there’s not enough space here to do so, as commercial premises are built right up to the road edge (same as with N7).

Closing the direct private entrances would do more for N3 than adding a lane each way.

spacetweek

At the Anne Graham NTA meeting last night, someone asked about this scheme. From the comments made it seems that 6-laning is out but the other elements might still happen. Closing side accesses and rebuilding junction 4 at Clonee.

Pale Red

https://www.boards.ie/discussion/comment/119297775#Comment_119297775

There is only one private entrance outbound (between j3 and j4) and a few before Parlickstown junction inbound. I don't think closing all of them would have any real impact on traffic flow as they are well away from traffic pinch points and lightly used. It's not like the approach to Killarney where there are private entrances aplenty.

I've looked at Fingal Co Co website and can find nothing since June 2022. Has nothing of significance happened to this proposal since or am I looking in the wrong place?

spacetweek

Seems to be dead in the water until there’s a non-Green minister in place.

cgcsb

https://www.boards.ie/discussion/comment/121832220#Comment_121832220

Will hardly matter the spending rules and policy will be the same, the state will also likely have already signed contracts for 10s of billions of euro for dart, bus connects and metrolink projects before next election so there'll be no spare cash for things like this.

spacetweek

Budgets are highly malleable and have nothing to do with fiscal space. They are entirely decided by political priorities. You can just borrow if you don’t have it.

If an incoming government decides that there should be more spending on roads then it’ll happen.

cgcsb

https://www.boards.ie/discussion/comment/121834351#Comment_121834351

It won't happen though

Pete_Cavan

The priority should be to add a bus lane, further general traffic lanes would be pointless.

marno21

https://www.boards.ie/discussion/comment/121835486#Comment_121835486

An outbound general traffic lane would be useful to remove traffic from the city more quickly. An inbound one won't achieve much as you say.

Pale Red

https://www.boards.ie/discussion/comment/121835486#Comment_121835486

There is an inbound bus lane, up to Snugboro Road on ramp where there is a lane gain. An outbound bus lane from j1 to j3 would help. I'm not sure if there is a short hard shoulder between j1 and j2 which forces traffic into mainline only to leave again after about 100m - it would be good to have that shoulder as an auxiliary lane between those junctions.

spacetweek

https://www.boards.ie/discussion/comment/121835688#Comment_121835688

Depends on the capacity of the N3/M50 junction. If it could handle 3 inbound lanes, they should be provided.

6 lanes+bus lanes is the way to go.