N3 - Clonee to M50

So the Option Selection Report for N3 M50 to Clonee has now been published. This is the justification for adding another traffic lane in the hope of solving congestion, this is one report which is well suited to its future role as a dust collector on a shelf, unfortunately it will be joining other reports which should be actioned.

About the only positive thing I can see there (and I haven't looked too closely) is the proposed closure of private entrances along the N3.

🙄We need 12 lane highways like the utopia in the US, right?

The idea that adding lanes to “fix” congestion is how you end up with 12 lane roads. You add more lanes, you get more congestion so you add more lanes. Each time you think this is the one that will fix it.

The M50 is already 8 lanes wide in parts and 12 if you consider the motorway and it’s ancillary roads around Sandyford. That €1bn that we spent a few years ago did very little to fix congestion.

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.

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?

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.

It won't happen though