The return of the 2+1 road

DumbBrunette

There's been a lot of discussion in the N26 thread about TII planning a 2+1 road for an upgrade of the Ballina to Foxford route, but it's not the only location where this once unwanted road cross section is making a comeback. I thought it was worth starting a separate thread as this change will have a big impact on all future non motorway road projects.

The N24 Cahir to Limerick Junction scheme is also planned as a 2+1, with the exception of the Tipperary town bypass which will be 2+2.

The question is why 2+1s are now being planned again, more than 20 years since the last ones were built and then deemed unsafe.

The Irish Times piece below from 2007 sets out why no more 2+1s would be built - the reasons being dangerous overtaking and the fact that 2+2s were only 10-15% more expensive and better future proofed.

https://www.irishtimes.com/life-and-style/motors/nra-decides-against-2-1-road-design-1.951308

One possible reason for the change is the national speed limit review, which says any new rural road with a speed limit over 80 km/h has to be a 'divided road', i.e. a 2+1 or 2+2. This is effectively a ban on the single carriageway with hard shoulders for big road projects, as these make no sense with a speed limit of 80.

See here - https://cdn.tii.ie/publications/GE-TBU-01045-01.pdf

Type 1 SC was not even considered for the new N26 upgrade, even though it has been the go-to road type for rural primary road upgrades (along with its predecessor road type, the wide single carriageway) for decades.

The open question for TII is why they now consider 2+1s to be sufficiently safe even though the NRA previously decided in 2007 that they were too dangerous to roll out more widely across the country?

cjpm

I’ve been told they work quite well in other countries. With the appropriate AADT.

The no 1 problem we have with them is that they were shoehorned onto existing wide single carriageway roads where the traffic volumes were way too high. Along with domestic and farm entrances along the route

The No.1 example of this is the N20 Cork to Mallow.

A constant stream of traffic for both morning and evening rush hours and fairly busy during the middle of the day too.

The 2 + 1 does work quite well between 10pm and 6am though 😭

Charles Babbage

The 2+1 design in Ireland had significant design compromises when compared to the Swedish original. It might work if done properly.

N2 sharp turn exit https://maps.app.goo.gl/KJ8gqoHAb7tueMrSA

Sweden slip road exit https://maps.app.goo.gl/WFDAP8R8Ribk43WV7

Jayuu

Agree that the pilot 2+1 schemes were really poor. I used the N24 one at Piltown/Fiddown quite a bit for a while and I hated it.

In particular the sharp exits and entrances like the one in the Castleblaney link above with not even a small hard shoulder to keep the mainline away from the junction were incredibly bad design.

It was unnerving if there was a car waiting to join the mainline and you were coming up on it because it felt like the car was right in your face. Have a look at the way the Swedish version merges. Much safer and less dangerous because cars can merge at a relative speed rather than from a complete stop.

The lack of any sort slip lane off combined with a sharp left meant you had to slow down quite a lot if you were exiting the mainline which also meant if anybody was behind you they were impacted as well if they weren't paying attention. A couple of times I had somebody almost rear-end me because I was slowing so much while they weren't.

I'm glad we are reconsidering them because I think they would work better than Type 1 for many roads in having no oncoming traffic and allowing for safe overtaking sections but they need to be implemented properly if we're literally going down this route again!

TheSunIsShining

https://www.boards.ie/discussion/comment/123448553#Comment_123448553

The Macaroon bypass has no merge lanes either. I've only driven it a few times but didn't like it at all.

hans aus dtschl

https://www.boards.ie/discussion/comment/123436229#Comment_123436229

Interesting. Do you know of anything official describing the idea behind the differences by any chance?

I wonder if part of the idea for our 90-degree side-road join was for vulnerable road users to have less junction to cross, as they used the main road. To which the obvious answer would be: add dedicated side-routes for VRU's.

I can only add to the chorus of people who hated the 2+1's here. And yes, they did seem to work reasonably well during low-traffic times. But are they better than 1+1 during low-traffic times I wonder?

KrisW1001

https://www.boards.ie/discussion/comment/123449722#Comment_123449722

With regard to your last question, they'd definitely be safer as they remove the chance of head-on collision from overtaking. You can't overtake in heavy traffic, but you can when it's quiet: a fair share of night-time fatalities are collisions caused by dangerous overtaking

From the perspective of the "above average drivers", I'm sure it would be irritating to be forced to wait behind a slower vehicle for a whole two kilometres, but again, it's worth it to protect other road users from their "skills".

If the existing schemes were shown to be more dangerous than a single carriageway, then they would have been removed after the pilots and converted back to wide single carriageways. The reason for dropping 2+1 wasn't that it was unsafe, but rather that 2+2, while a little more expensive per km, was even safer. I suspect that accident rates for the 2+1s are still significantly lower than for equivalent Type 1 Single, though.

As for merge lanes, it's my belief that unless you can make them at least 150 m long, you shouldn't have them at all. A merge lane prevents a driver having to stop, but that's based on the promise that they'll have time to assess the conditions before they have to join the mainline. If the lane is too short to do this, you're basically encouraging traffic to jump in without properly looking.

I think our 90 degree join is to force a stop and make joining traffic assess the conditions before pulling out.

Exit lane lengths are less of an issue: the leaving vehicle is always in view of following traffic.

As to why we're different to Sweden, maybe it was the Swedish road engineers who highlighted deficiencies of their own design. The Irish design wasn't just cribbed from Sweden, there was a process of exchange between NRA (as it was) and their Swedish counterparts. The thinking about what kind of road designs are safe has evolved over the decades as we've gathered more data. Some things that seem paradoxical actually work (e.g. making the merge lanes onto motorways narrower)

I think people are expecting to see "motorway" solutions just because there's a fence down the middle of the road. It's more useful to compare with the kind of junctions we have on 100 km/h single carriageway roads instead. A design that is safe on a single carriageway cannot become less safe on a 2+1.

Hailtothethief

Don't forget there's another piece of 2+1 road on the N71 from Innishannon to Bandon. It's got no barrier up the middle and it works reasonably well.

There are 2+1 climbing lanes on the M25 from New Ross to Waterford and they appear to work reasonably well.

I think more 2+1 should be put in, WHERE SUITABLE!

KrisW1001

https://www.boards.ie/discussion/comment/123475659#Comment_123475659

Actually, New Ross-Waterford is one of the least safe sections of N25. Not because of those lanes, but because of the reason they exist: the frequent dips in the road make overtaking very dangerous, and there’s a lot of heavy goods traffic here too.

This section of N25 is planned to be upgraded to 2+2, but the project keeps getting put down the priority list.

As an interim measure, It wouldn’t be a bad idea to add a “soft” divider (just those plastic rods would do) to the centreline on climbing/passing lanes on roads that are otherwise single carriageway. I reckon they wouldn’t last a month on the one outside Bandon.

Chris_5339762

https://www.boards.ie/discussion/comment/123475659#Comment_123475659

That section of N25 is absolutely lethal and sadly there has been a couple of fatal head-on collisions on it in the last few years. Its one road that desperately needs 2+1 removed and 2+2 put in throughout.

KrisW1001

https://www.boards.ie/discussion/comment/123481483#Comment_123481483

N25 has climbing lanes. Climbing lanes aren't a 2+1 road. Head-on collisions on a true 2+1 are almost impossible, because there's a hard barrier separating each direction of traffic.

That section of N25 is indeed planned to be replaced with a 2+2 ("N25 Waterford Glenmore" will find you the details), but the project was paused in the post-Covid cutbacks and has not restarted.

Hailtothethief

https://www.boards.ie/discussion/comment/123482757#Comment_123482757

https://www.boards.ie/discussion/comment/123481483#Comment_123481483

https://www.boards.ie/discussion/comment/123476934#Comment_123476934

I didn't realise that stretch of the M25 was that bad although in my defence, I'm not a regular user or it. Consider me suitably educated.