Redundant links at roundabout GSJs

[jackal]

Many roundabout-based GSJs duplicate movements. There are three main cases I'm aware of:

1. Dumbbell GSJs with four arms (countless examples). A classic dumbbell (two roundabouts joined by a link road) contains redundant links compared to a single elongated 'dog bone' roundabout. The redundancy is removed where there are five or more arms, or where slips are folded.
2. Two-bridge roundabout or dumbbell GSJs with three arms (e.g. M23 J9). There is no need for a roundabout here at all rather than a 'teardrop' a la M4 J19.
3. Three-level roundabout with four arms and freeflow links (e.g. Lofthouse). While the roundabout is still required, links to and from it can be removed as in the 'Northern Link' option for Simister Island:



Lofthouse is particularly galling as the unnecessary link from the roundabout to M62 westbound means there are traffic lights in the SW corner when a simple lane gain on the circulatory carriageway would have done the job if the extra link wasn't there.

I realise there are some unusual circumstances where such links are used (e.g. during closures or for U-turns) but these small benefits are surely outweighed by the impacts on capacity and safety. We do not, after all, habitually build redundancy and U-turn facilities into our network as some countries do. A more likely explanation for such layouts is that roundabouts with a full set of links are taken as the starting point and designers fail to notice their disadvantages in these particular cases.

[ChrisH]

There are also junctions where the roundabout itself is redundant and should be replaced with a trumpet: the two M3 junctions at Basingstoke, the M4/M32 junction, the M1 Luton junction etc.

Sometimes a roundabout is used for resilience purposes and access to maintenance compounds, for example the M4/M49 roundabout (which I have driven all the way around but that was with a sleeping baby in the car...)

[c2R]

The other thing about roundabout interchanges or dumbbells versus trumpets is that they slow traffic speeds on approach, nicely, while a free flowing junction needs to be massive to allow it to operate at high speeds, or have deceleration away from the main line - e.g. the old one at Hemel versus the new one.

A dumbbell takes up very little space indeed - although I do take the point that the dogbone variety is better as removing the additional movements reduces the opportunity to mix up what is going on, in priority terms, as well as the unused sections usually being full of miscellaneous detritus.

[owen b]

The other thing about roundabout interchanges or dumbbells versus trumpets is that they slow traffic speeds on approach, nicely, while a free flowing junction needs to be massive to allow it to operate at high speeds, or have deceleration away from the main line - e.g. the old one at Hemel versus the new one.

Yes. I think that would be the problem in the case of M1 J10 were it a trumpet or directional T instead of a roundabout GSJ, especially with it being in such close proximity to the west facing slips at (former) J10A with a lot of weaving.

There are also junctions where the roundabout itself is redundant and should be replaced with a trumpet: the two M3 junctions at Basingstoke, the M4/M32 junction, the M1 Luton junction etc.

Many years ago when I had a slightly more exuberant driving style I very nearly came a cropper at M1 J10 heading from the spur road towards the roundabout. There is never any traffic on the "redundant" part of the roundabout (between the exit to the spur road and the entrance from the spur road)...... until one day there was a car and I missed it by inches as I entered the roundabout without looking properly. Lesson learnt .

[Micro The Maniac]

There are also junctions where the roundabout itself is redundant and should be replaced with a trumpet: the two M3 junctions at Basingstoke

The residents of Dummar will disagree with you about J7...

But J6 could be much improved (a) by having a free-flow for spur(S)-->M3(E) and M3(E)-->spur(N)... although regular proposals for a services here keep such improvements in the pending tray.

[Bryn666]

The other thing about roundabout interchanges or dumbbells versus trumpets is that they slow traffic speeds on approach, nicely, while a free flowing junction needs to be massive to allow it to operate at high speeds, or have deceleration away from the main line - e.g. the old one at Hemel versus the new one.

A dumbbell takes up very little space indeed - although I do take the point that the dogbone variety is better as removing the additional movements reduces the opportunity to mix up what is going on, in priority terms, as well as the unused sections usually being full of miscellaneous detritus.

This is where we get freeflow design wrong - freeflow is about removing the need to stop, not maintaining 70mph.

Plenty of American and European freeflow Interchanges operate at lower speeds than the mainline and the world does not end.

[M4 Cardiff]

The other thing about roundabout interchanges or dumbbells versus trumpets is that they slow traffic speeds on approach, nicely, while a free flowing junction needs to be massive to allow it to operate at high speeds, or have deceleration away from the main line - e.g. the old one at Hemel versus the new one.

A dumbbell takes up very little space indeed - although I do take the point that the dogbone variety is better as removing the additional movements reduces the opportunity to mix up what is going on, in priority terms, as well as the unused sections usually being full of miscellaneous detritus.

This is where we get freeflow design wrong - freeflow is about removing the need to stop, not maintaining 70mph.

Plenty of American and European freeflow Interchanges operate at lower speeds than the mainline and the world does not end.

But is that where either we are too risk-averse or too many British drivers are just too stupid, that tight bends on freeflow are considered unsafe, even with a reduced speed limit?

On this point it interesting that the tight loop in M25 J16 M25sth to M40west is under NSL, however if there is any use of reduced speed limit on the main carriageway, the loop is automatically signed at 40, even if the congestion is on the mainline, not the loop or M40. This would appear that the loop has been subject to an updated risk assessment that suggests 40 should be the limit, but there would be a paperwork nightmare in permanently reducing the limit on the loop, so the lower limit is just signed when possible. It is subject to a permanent advisory 50 however.