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

@Chris_5339762 There is always going to be a jam when you try to put more vehicles through a roadway than it can take. That happens every morning and evening at the JLT, so the designers have to decide how to manage this jam to cause the least disruption. That’s what they did, and while it’s not nice to be stuck in there, if you remember that the point of the new interchange was to prevent a jam on one arm blocking all others, you’ll at least know why it’s happening to you...

The wide diverge is necessary to sort Little Island and M8 traffic from tunnel traffic. At the point where the lanes branch off, the mainline is four lanes. The left two lanes go into the interchange, and the rightmost other two will go over it, here:

There’s two lanes here to allow any late arrivals to move left if they need Little Island. An auxiliary lane opens about 200 m down the road for Little Island traffic to leave.

Once Little Island traffic is gone, traffic from M8 needs to be sorted out from traffic that will use the tunnel. Again, one extra lane arrives here so that M8 leavers don’t interfere with tunnel-takers.

At this point, you could bring the remaining mainline down to one lane, as it has only one destination and that destination is only one lane wide (this lane runs directly into the left lane of the JLT), but if you do that, you lose capacity on the interchange itself, which could cause traffic to back up to a point where it prevents people from leaving for Little Island and M8 further up, which brings us back to the old problem of tunnel traffic blocking people wanting to get to M8 North or Little Island West. Keeping two lanes beyond here is a buffer: it isn’t a “jam generator” so much as jam isolator: it confines the delays to the traffic stream that will use the tunnel, or does so as much as is possible.

I agree with you that the end of that lane (below) is somewhat sub-optimal. A taper should have started further back, given the average Irish driver’s bloody-minded refusal to yield to anyone on their right, but it’s imperative that there aren’t three lanes trying to merge into the centre one here, as that situation is guaranteed to cause collisions.

I suspect the original design was to use bollards or some other kind of soft divider to prevent everyone trying to pile into the centre lane at the same time, but that may have been ruled out after the planning was submitted, and now that right-hand “lane” appears to be kept as a refuge for over-height trucks:

The tunnel should be widened (and it should have been built properly in the first place but this is Ireland). What will also be needed is new flyovers at the Dunkettle interchange from the widened tunnel on to the N25 in both directions and a rebuild of the shambolic N8 mainline through the junction.

We shouldn't accept "it'll do" because this is exactly the outcome that happens. The "new" Dunkettle interchange is redundant already because of small peripheral thinking.

Yes but first we need an underground rail system with at least 50 lines, connecting all the suburbs. Then we need cycleways connecting every individual house in the city. We'll get around to widening the tunnel out to 16 lanes then.

Most school runs are a diversion of an existing parental commute, and most are at the "home" end of that commute. The ones you notice, parents dropping children at urban secondary schools, should be better handled, but they have no real impact on pinch-points further back on the journey: that person would be driving that car through there anyway.

The big drop in traffic at this time of year is due to people taking summer holidays, something that a large share of commuters cannot do while their children are in school. And as this isn't the 1950s, one family holiday often takes two cars off the road...