We need to talk about Wisley

National Highways are spending a third of a billion pounds rebuilding one of the most congested junctions on the M25. Is it money well spent?

You might know Wisley better as M25 junction 10 - an interchange on the southwestern part of London’s orbital motorway, providing a connection to the A3, itself virtually a motorway leading from London to Guildford and Portsmouth.

Wisley Interchange is a three-level stacked roundabout, opened to traffic in December 1983 when the M25 had yet to become a victim of its own success. Today it connects one of London’s busiest radial routes to the UK’s busiest motorway, and has been unable to cope with rising traffic levels for years. The usual short-term fixes of signalisation and extra lanes have long since been exhausted.

A brand new Wisley Interchange in 1983, with very little traffic in sight. Click to enlarge

So, with serious money finally available for investment in the trunk road network, the junction was added to the list of schemes for RIS2, and in 2022 the bulldozers moved in to begin reconstructing Wisley Interchange and the surrounding sections of M25 and A3. The project has been in the news recently because it required a full weekend-long closure of part of the M25 for the first time in the motorway’s history, and more closures will follow.

A Surrey state of affairs

It would be hard to overstate the importance of this junction to traffic flows around London, the home counties, and indeed the whole national trunk road network. Now, the local economy is growing, housebuilding is picking up pace, and the adjacent airfield is about to be redeveloped to provide thousands of jobs - so demand is only going to intensify.

What’s more, Wisley Interchange is in Surrey, a county with the absolute minimum provision of local transport infrastructure. With few high capacity roads of its own, and little public transport beyond commuter services to Central London, Surrey relies heavily on the M25, M3 and A3 trunk roads passing through the county to link its towns and keep its economy moving.

In 2016, a study found that Wisley was among the busiest junctions on the trunk road network. Congestion, even outside the morning and evening peaks, was routinely backing up and blocking other nearby junctions on the A3. Worse still, the junction and the surrounding area have the highest recorded collision rate on the national trunk road network.

The usual trouble on the A3 northbound at Wisley. Click to enlarge

Being surrounded by Sites of Special Scientific Interest, and sitting within commuter belt Surrey, which is a development-hostile mix of low-density affluent suburbia and protected green space, there are no alternatives for the traffic that uses it, nor can any be built. It is a dangerous and dangerously overloaded junction that will only grow busier. It had to be fixed.

Enter National Highways and their £317m project to rebuild it. Hooray! Work is now under way, and the new junction will open to traffic in 2025.

That’s good, right?

Full circle

Let’s see what National Highways are getting for their £317m investment.

The A3 north and south of the interchange is being widened and rehabilitated, providing a fourth lane in both directions to create more weaving space between adjacent junctions and more capacity on the approaches to the interchange. Minor side turnings and property access are being eliminated, with new local access roads linking side roads and businesses to the interchanges at Ripley and Painshill. That will remove slow and dangerous turns on and off the A3 where traffic is heaviest.

An important change will be to link the Royal Horticultural Society’s hugely popular gardens at Wisley to Ripley Interchange, instead of having them accessed from the back of a lay-by on the northbound carriageway.

The roundabout now under construction at Wisley. North is to the right. Click to enlarge

At the interchange itself, new free-flowing left turn sliproads will provide non-stop turns bypassing the roundabout. And the roundabout itself is being widened and elongated, with new bridges over the M25 that will allow a mix of four and five lanes around the circulatory carriageway.

The end result will be a junction with more capacity, approaches with more capacity, safer access to the businesses and minor roads nearby, and a safer environment for through traffic too.

That’s good, right?

Here’s what you could have won

We forget sometimes, on this odd little roundabout-obsessed island, that roundabouts aren’t the only way to join two roads together. National Highways are spending £317m on a bigger roundabout, but that’s not the only thing they could have done.

In 2016, their consultants worked up dozens of design options, of which three final proposals were put to a 2017 public consultation.

One was the design they’re building now.

A second possibility was to leave the existing roundabout in place, and provide two new high-level bridges that would form right turns from the A3 to the M25. That would leave the roundabout handling only traffic exiting the M25, which would be adequate for years to come.

A rejected option for freeflow connections, and the first stage of a future four level stack junction. Click to enlarge

The design was consciously arranged to allow for another level to be added in future, for free-flowing right turns from the M25 to the A3, converting the junction to a completely free-flowing four level stack and removing the roundabout entirely. It was a pragmatic idea to provide a huge upgrade in stages, and even in its initial state would have made a lasting improvement.

A third possibility was to provide a cyclic interchange around the outside of the existing junction, so it could be built without interfering with existing traffic movements too much. At the end the roundabout could be removed, leaving a fully free-flowing interchange. It would have used up much more space and required the clearing of a large area of woodland, but would without a doubt have fixed Wisley Interchange for all time.

Another rejected design for a sprawling cyclic interchange. Click to enlarge

Both of those other options would have provided the sort of major free-flowing interchange that is found where the M25 meets other motorways - and the A3 may as well be a motorway. They are the sort of junction that almost any other country in the world would provide where two roads like this intersect. In fact, just to be clear: a free-flowing junction, without roundabouts or traffic lights, is something that many other countries would provide without hesitation even where far quieter roads intersect.

But we are not getting a free-flowing junction. We are not getting rid of the roundabout or the traffic lights. What National Highways decided to do instead was spend a third of a billion pounds to replace a signalised roundabout with a new signalised roundabout.

Maybe that was the right decision. But it’s a real shame that, at the end of three years of roadworks, we’re still going to be lumbered with a roundabout.

What’s wrong with roundabouts?

Well, nothing, really, except that a roundabout is a tool. Like all tools, it’s suited to some jobs more than others, and there are limits to what it can do. In some situations it will be exactly the right thing.

The junction of two incredibly busy roads is not the place for a roundabout, and almost anywhere else in the world, a roundabout would not be considered. But in the UK - for slightly arcane historical reasons - it is the default. From the outset, the Wisley Interchange roundabout was relieved by an underpass and a flyover, so that traffic continuing straight through on the M25 and A3 would avoid it. It only ever had to handle vehicles turning left or right from one road to another.

Red lights at Wisley Interchange's roundabout. Click to enlarge

Unfortunately, these two roads are so busy that those turning flows alone are more than enough to overwhelm it. Signalising the junction helped, but that’s really just a way of rationing demand for an inadequate resource.

The new design provides free-flow sliproads to remove left turns from the roundabout, so in future it will only handle the four right turn movements, and there will be either four or five circulating lanes, so that all right turning flows will have two or even three dedicated, continuous lanes through the junction. But the newly expanded roundabout will still have traffic lights because eventually those four right turn flows will overwhelm it again - and until then, it will be far too big and wide and unwieldy to ever operate safely without signalisation.

The reason it will eventually jam up again is simple. If you are leaving the A3 northbound and turning right onto the M25 eastbound, your journey through the roundabout will cross over the opposing right-turn flow (from A3 southbound to M25 westbound) not once but twice - two points where those traffic streams have to stop and take turns to use the same patch of tarmac.

Wheel meet again: opposing right turns needlessly cross twice. Click to enlarge

There’s actually no need for those two turns to cross at all. If their paths through the junction were arranged differently - as direct lines, instead of sharing parts of a big circle - they could pass by each other without crossing over even once.

On that same right turn journey, you will also share a quarter turn of the roundabout with right turners from M25 westbound to A3 northbound, who will appear from your right but will cross over to your left; you’ll then share another quarter turn with right turners from M25 eastbound to A3 southbound, who will do the same. Your right turn manoeuvre will have a total of four points of conflict, two of them with the same stream of vehicles.

On an ordinary roundabout all this is OK - the efficiencies offered to other flows of traffic make it worthwhile. But at the new Wisley there will be no other flows of traffic. There are only right turners, all getting in each others' way.

The aim is simply to get four flows of right-turning traffic to traverse a flat junction - but when traffic volumes are this large, a roundabout is an almost laughably inefficient way to go about it. The most efficient solution would be to have flows crossing the fewest number of times and for the crossovers to be as short as possible. You would end up with a sort of crossroads in the middle, which could be operated with just two signal stages.

A theoretical alternative to the roundabout where right turns avoid each other completely. The four-way junction in the middle could operate on just two signal stages. Click to enlarge

At Wisley that optimal layout isn’t physically possible - the junction already exists, so the supports for the A3’s flyover are in the way of building it. And in any case, you would still be crossing over huge traffic volumes at a set of signals. If you were serious about fixing it, you’d make a free-flowing connection, which is what the junction’s designers proposed in the other two options: a sliproad that takes you directly from one road to another, using bridges or underpasses to eliminate conflict points.

National Highways chose not to do that, which leaves us with right turns crossing each other on the level, and without replacing the existing A3 bridge structures (too expensive), they are stuck with more or less the type of junction they’ve already got. The only thing that remains is to add more lanes and traffic signals to the roundabout, bludgeoning extra capacity out of an inefficient design by brute force.

It will work, but it's an expensive way to get a very middling improvement.

The devil in the detail

The retention of the roundabout isn’t the only thing about the new Wisley Interchange that has the appearance of an elaborate own goal.

The roundabout will be approached by new, wider, longer sliproads, which is good - more capacity and more space for queues when it’s busy, without traffic stacking up on the A3 or M25. (You wouldn’t need any queueing space if you’d eliminated the roundabout and all its traffic lights, of course, but we’ve covered that.)

When you use the junction in future, the sliproad will have two or three lanes - but on three of the four approaches, they will all be for right turns. If you’re going left, on one of those new high speed free-flowing turns they’re building, you will need to take an exit on the left which will appear just before the traffic lights.

Scheme plans suggest the new left turns will branch off right turn lanes and may be easily blocked. Click to enlarge

In heavy traffic, of the kind that is common here, right turners queueing for the lights will occupy all the lanes, and the queue won’t have to extend back very far before it blocks off that left turn exit, meaning that left turners will be stuck waiting for the lights to change even though the traffic lights have been removed from their path at a cost of £317m.

Dedicating one of the lanes to left turners, until they’ve branched off, would mean they could pass through unimpeded even when the roundabout was congested - but that opportunity seems not to have been taken. We can’t see any reason for this not to have been done.

The struggle

We’re not saying that things won’t get better at Wisley. Things absolutely will get better, in every way. There will be more capacity than before, meaning fewer delays and more support for new homes and jobs nearby. The redesigned A3 to the north and south will be safer without its various side turnings and driveway entrances. These things will make a difference.

National Highways’ own analysis of the scheme suggests that, between now and 2037, the project will more than halve the delays currently experienced in peak hours; will reduce the number of accidents in this area of the trunk road network by about 10%; and will have a net economic benefit of £303m. That’s positive news.

The scheme will also offer benefits to pedestrians, cyclists and horse riders, by providing new routes that avoid the junction completely. At present their journeys through Wisley involve multiple crossings of sliproads and a journey through the inside of the roundabout.

The point is not that things won’t get better. The point is that they will only get so much better, and it wouldn’t be unreasonable to expect more for a third of a billion pounds.

There are, broadly, three reasons why this project’s bang-to-buck ratio is so unimpressive.

1. Treading lightly

The elephant in the room is the environment. Wisley Interchange is boxed in on all sides by protected woodland. Making any changes at all means expanding the highway boundary and cutting down trees. National Highways were acutely aware that their project stood a better chance of being built if it kept within the tightest possible area.

It’s not just about the protected woodland, either. The widened A3 pushes at the boundary of Battleston Wood, part of the gardens at RHS Wisley. Those who love Wisley and everything it stands for - and there are many, here in the NIMBY heartlands - fought hard against this project, and the campaign got plenty of national coverage because it was spearheaded by Alan Titchmarsh himself.

Just over the boundary fence from the A3 northbound, the Royal Horticultural Society's gardens at Battleston Wood. Click to enlarge

In evaluating which of the options to take forward, the widened roundabout won primarily because it used less space, required fewer trees to be felled, had the least impact on cultural heritage and caused the least visual intrusion.

The roundabout, in other words, is not the best design, but it is the pragmatic choice if you want to get something built while minimising opposition.

2. Playing the numbers

When it comes to projects like this, there is a problem with money. Inflation is high, obviously, and we live in a country where infrastructure costs are extraordinarily high - more so than in most comparable nations. That’s partly due to our eye-watering land values, but it’s also related to our stop-start, on-off relationship with major public projects, which means there is no consistent pipeline of works and so the civil engineering industry is forever battling an unpredictable boom and bust cycle.

Then there’s the way that Road Investment Strategy periods are structured. We are now in the tail end of RIS2, and all the projects due to start in this five year period, Wisley included, had a budget set in 2019. Wisley Interchange was allocated about £250m before anyone had even started working out what to build.

The exit to Wisley Interchange from the A3, as it was. What would £250m get you here? Click to enlarge

National Highways found some extra money to compensate for the rate of inflation, but in real terms the budget now is little different.

As a result, what can be built at Wisley did not have the primary focus of the needs of traffic, or of safety; it didn’t even have the primary focus of the environment. The number one job was to design something that could be built for the price that was already agreed.

3. We are institutionally bad at building junctions

This is probably a bigger subject for another time, but here in the UK we have some strange ideas about what makes a good motorway interchange. We like roundabouts, and we even like signalised roundabouts.

The rest of the world has spent decades developing a clear consensus on free-flowing layouts, and on innovative things like Single Point Urban Interchanges and Diverging Diamonds and other highly efficient solutions. Meanwhile, we carried on ploughing our circular furrow.

A diverging diamond interchange in Missouri. Innovative and highly efficient designs like this are becoming commonplace around the world - but are yet to be tried here. Click to enlarge

In other words, the rest of the world doesn’t build big multi-lane signalised roundabouts, because there is no compelling argument that they are better than any of the alternatives, and yet if you were to read the UK’s highway design manuals you’d think they were the solution to humanity’s ills.

So, when you have a heavily constrained site, and you have a fixed budget, and you have a pre-existing junction that’s a three-level stacked roundabout with traffic signals, and then you look at the design guidance and you find it favours signalised roundabouts… well, nothing else really stood a chance, did it?

We’ll be back

One final thought, regarding the lack of ambition at Wisley, is the future year against which its effects have been forecast. The study made in 2016, at which various designs were made and evaluated, chose 2037, which was then 21 years ahead.

When the project is complete and open for use, 2037 will be just 12 years in the future. Beyond that date, no attempt has been made to foresee the effect of the works. Nobody can say whether it will be just as congested as it is today, or whether it will still offer an improvement. Nobody can say whether rising traffic will have made the new layout an accident hotspot again. Maybe it will all be fine, but nobody knows. All bets are off.

Wisley Interchange from the M25. A brand new sliproad will replace this, leading to a brand new roundabout. Plus ça change. Click to enlarge

In other words, National Highways are spending £317m, and imposing three years of hugely disruptive roadworks, to put in place a scheme that they have only forecast to be adequate for twelve years.

Is it a good return on a third of a billion pounds? That’s up to you. In years to come, you can think about it, while you wait at the brand new traffic lights on the brand new roundabout.

Chris5156 Mon, 01/04/2024 - 11:50

Comments

Steven 1 April 2024

And of course, National Highways has also pulled exactly the same stunt as Wisley at M6 J10.

They're replaced a roundabout with an, erm, slightly larger and wider roundabout for an eye-watering amount of money. Now granted, M6 J10 is in the middle of the urban area where Wolverhampton meets Walsall, but still, it's just, well, couldn't we do better than this?

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Sources

National Highways: M25 junction 10.
Highways England (2016). Regional Investment Programme: M25 J10/A3 Wisley Interchange Improvements: Technical Appraisal Report 01 Nov 2016. [PDF]

Picture credits

Photograph of Wisley Interchange in 1983 is taken from Department of Transport (1986) The M25 Orbital Motorway.
Photograph of queues on A3 northbound is taken from an original by David Martin and used under this Creative Commons licence.
Diagrams of layout options taken forward for consultation are taken from Highways England (2016) Regional Investment Programme: M25 J10/A3 Wisley Interchange Improvements: Technical Appraisal Report.
Photograph of traffic signals at Wisley is taken from an original by Chris Reynolds and used under this Creative Commons licence.
Diagram of lane arrangement on the M25 anticlockwise exit sliproad is taken from Highways England Scheme Layout Plan.
Photograph of gardens at Battleston Wood is taken from an original by Colin Smith and used under this Creative Commons licence.
Photograph of northbound exit at Wisley is taken from an original by Alan Hunt and used under this Creative Commons licence.
Photograph of diverging diamond interchange is taken from an original by Missouri DOT and used under this Creative Commons licence.
Photograph of M25 approaching Wisley is taken from an original by Julian P Guffogg and used under this Creative Commons licence.

Routes M25 A3 Tags Highways England, South East, Junctions

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