M25 junction 28 improvements

[jackal]

The consultation on this opens tomorrow. I imagine documentation will be uploaded soon thereafter, but the information so far is a £50m-£100m price tag and the RIS description:

upgrade of the junction between the M25 and the A12 in Essex, potentially including the provision of dedicated left-turn lanes and improvement of the gyratory system.

Edit: consultation now open: https://highwaysengland.citizenspace.co ... nction-28/

[M19]

You'd be wanting to see free flow right turn connectors for that sort of money.

[WHBM]

Worst junction on the M25. I've been jammed on the A12 London-bound for over an hour here because the roundabout is completely locked up, and backed up all directions. Apparently a regular peak hour event.

A12 London-bound to M25 southbound can't be easily dedicated left turned because the main road from Brentwood joins the roundabout between the two, unless the money pays for an overpass on this link.

[Gav]

If you look at this junction, and then think about what you have there already. M25 goes over the roundabout. The roundabout over the A12. If you look at google earth you can see that there is ground between the A12 and roundabout that is built up and same level as the roundabout, so you have room to have a carriageway in the inner space. Make those carriageways dip down to the A12 within the roundabout circle. No new bridges, but some clever traffic flow rerouting to remove congestion from the roundabout and seperate significant cross flows.

[jackal]

If you look at this junction, and then think about what you have there already. M25 goes over the roundabout. The roundabout over the A12. If you look at google earth you can see that there is ground between the A12 and roundabout that is built up and same level as the roundabout, so you have room to have a carriageway in the inner space. Make those carriageways dip down to the A12 within the roundabout circle. No new bridges, but some clever traffic flow rerouting to remove congestion from the roundabout and seperate significant cross flows.

Probably the space isn't there for the merges etc but I love this kind of idea. There's actually a weird junction in Kiev that is both a stackabout and a whirlpool/turbine As it is actually painted up it's really just a stackabout with the ramps joining on the inside, a bit like Great Barr. But if they just removed the weaving areas by separating flows with paint they would have a full whirlpool.

[jackal]

You'd be wanting to see free flow right turn connectors for that sort of money.

I agree. The M62 J6 scheme, for instance, added a freeflow left turn (including bridge, similar to what would be needed for A12wb->M25sb) AND freeflow right turn loop. All that for £31m. Okay, that scheme is a decade old now, but even accounting for inflation and the South East construction cost premium, something similar should be achievable for <£100m.

[qwertyK]

I think a pair of flyovers, one for the M25 South and one for the M25 North would be very effective. The traffic lights need replacing, they always break and are one of the main reason for gridlock.

[jackal]

I'm pleasantly surprised to find that all three options for consultation include the right turn loop! Clearly HE read my previous post this morning, realized the errors of their stacking-space-expanding ways, and quickly knocked up some freeflow solutions

https://highwaysengland.citizenspace.co ... ochure.pdf

The first option looks like a really efficient solution. The second and third ones certainly do the job but the land take is much greater (for the third one the first part of the loop also looks tight). But really any of them would be a massive improvement.

There's no new freeflow left turn, as the RIS had suggested there would be, but I'm not complaining given they found a way to deliver a better option for the same kind of money (£61.3m for the cheapest option). Turns out you can build freeflow for a decent price after all

[QuintA30]

An improvement is long overdue. But no provision for right turn from A12 London bound to M25 anti-clockwise would still be of concern. The last of the proposed M12s back in the 1990s was a cut-off of that corner between A12 and the M25 towards Junction 27.

Still, some change is better than none. I've had a reliable third-party tell me that the gridlock there has on more than one occasion been solved by an acquaintance leaving a mini-bus approaching from the A1023, directing traffic to leave space at the A12 west exit, to free up all the other stuck movements and then getting back on.

[Chris5156]

The first option looks like a really efficient solution. The second and third ones certainly do the job but the land take is much greater (for the third one the first part of the loop also looks tight). But really any of them would be a massive improvement.

The first option is more complex than it looks - it would involve a complex viaduct structure crossing part of the roundabout at the point where the northbound on-slip begins, and would then have to connect to the existing viaduct carrying the M25. It would be an unusual engineering job to bolt a new viaduct seamlessly onto an existing one, at an angle so as to provide a tapered exit, and I can't think of another scheme where that's been done in the UK.

We really do seem to be thinking a bit bigger with new road schemes these days!

[jackal]

The first option looks like a really efficient solution. The second and third ones certainly do the job but the land take is much greater (for the third one the first part of the loop also looks tight). But really any of them would be a massive improvement.

The first option is more complex than it looks - it would involve a complex viaduct structure crossing part of the roundabout at the point where the northbound on-slip begins, and would then have to connect to the existing viaduct carrying the M25. It would be an unusual engineering job to bolt a new viaduct seamlessly onto an existing one, at an angle so as to provide a tapered exit, and I can't think of another scheme where that's been done in the UK.

While complex in some respects, the 'bolting onto an existing viaduct' approach is simple in that the loop is thereby naturally elevated over the existing onslip. By contrast, the other two schemes require significant earthworks to get the onslip low enough for the loop to get over it. This probably explains why the first option is significantly cheaper (£61m vs. £72m vs. £78m).

[Glom]

It's about time we cracked the bolt on problem.

[WHBM]

It doesn't seem to do anything for the A12 westbound onto the roundabout problem, which by my experience is the worst issue here. A lot of knowledgeable traffic apparently diverts now from the Dartford Tunnels by the recently improved A13 and A130 to Chelmsford (it's freeflow between these two junctions), yet this is the flow relieved by the sole loop proposed for this junction.

[Chris5156]

It doesn't seem to do anything for the A12 westbound onto the roundabout problem, which by my experience is the worst issue here. A lot of knowledgeable traffic apparently diverts now from the Dartford Tunnels by the recently improved A13 and A130 to Chelmsford (it's freeflow between these two junctions), yet this is the flow relieved by the sole loop proposed for this junction.

It will help indirectly. There are two main, conflicting movements that overlap on the roundabout: A12 westbound to M25 northbound, and M25 northbound to A12 eastbound. By removing northbound-to-eastbound turns from the roundabout, the signal phasing can be adjusted to give far more time to traffic making the other movement. It won't be a revolutionary change but it will clearly help.

[c2R]

I'm not sure I understand why the public are being consulted on whether the loop is one or two lanes - surely that needs to be modelled? I also don't see that whether or not the loop is enormous or tighter is worth widely consulting on.

Presumably the single lane giant loop will be built as it offers the best cost benefit ratio.....

[jackal]

I'm not sure I understand why the public are being consulted on whether the loop is one or two lanes - surely that needs to be modelled? I also don't see that whether or not the loop is enormous or tighter is worth widely consulting on.

Presumably the single lane giant loop will be built as it offers the best cost benefit ratio.....

Capital cost is also often treated as a factor independently of BCR. You could either see this as illegitimate double-counting or as a sensible compromise with the reality of a limited infrastructure budget. Even though the first option has the worst BCR, it clearly does the job and would save £11m-£17m that could go towards some worthwhile scheme elsewhere.

It's also worth mentioning that the BCR is in some respects a first approximation of the public's views, and no replacement for them. For instance, the first option is 15-20% cheaper, with identical AM peak and offpeak time savings, and 97% of the PM peak time saving (2min20 vs. 2min25). So it has almost identical traffic benefits, and much less cost, meaning it looks like it should have the best BCR - but it actually has the worst. I suspect that this is because its BCR is significantly reduced by its impact on the local environment, i.e. from the fact that it goes between a bunch of buildings. One of the components of any BCR is monetized environmental cost, which is essentially a guess about the disvalue for affected people of this kind of environmental change. It's clearly much better to actually ask what these people think than just guess what they might think. If it turns out that the change is not as much of a problem for them as the model's guesswork suggested, it could be that the first option is the best one, and that the BCR estimate was actually inaccurate due to the overestimate of the cost of local environment impacts.

[jackal]

Hot off the press, my 'final' layout for J28, incorporating the first of the three consultation options. I've added a left turn (which would require a bridge) for A12wb->M25sb and a second loop for A12wb->M25nb, meaning that all M25<->A12E movements are freeflowed. I know a corner cut for A12wb->M25nb has been considered in the past but given the extra land take and long skew bridges this would be much more expensive than the additional loop.

[Truvelo]

The size of the loop depends on how precious Grove Farm is. The small loop will destroy it and separate the parcel of land used for storage vehicles from the main buildings, or what's left of the buildings. The big loop will not directly affect the farm but would leave it enclosed on all sides by slip roads.

My choice is for the 2 lane loop. There would be no 'bolt on' problem and it would avoid frustration where long single lane slip roads cause bunching when stuck being slower traffic. The widening of the M6 to M62 loop to two lanes throughout really made a difference in this respect.

[WHBM]

The left turn freeflow A12 westbound to M25 southbound is a challenge because not only does it have to clear the main A1023 road out of Brentwood but has to get merged into the M25 by the railway bridge just to the south of the junction. If you drive through this you can see the original design has already been very constrained in its vertical alignment by this, probably to the maximum allowable.

Mentioned above is that there can be a South-East construction cost premium. Most of this however comprises the much greater land acquisition costs, sometimes up to £1m per house on an urban scheme. Looking at this junction it's all agricultural land though.

[SouthWest Philip]

Hot off the press, my 'final' layout...
M25 J28.png

I'd be inclined to sacrifice the free flow for the A12 in favour of creating a more straightforward full trumpet to/from the anticlockwise M25. Run the A12 from the Romford direction along the line of the current westbound on-slip to join the roundabout. The addional traffic on the roundabout from the A12 to/from Romford would be many times less than that removed by free flowing from the M25 to/from Chelmsford.

With regards to free flowing between M25 clockwise and the A12 to/from Chelmsford I think the slip towards Dartford would need to pass under both the A1023 & the railway to be fully effective. All rather expensive, of course!