A1 - M25 - A1081

South Mimms Interchange

Where is it?

M25 junction 23 and A1(M) junction 1. A vital northern radial route from London crosses the country's most important ring road, the M25.

What's wrong with it?

Another case of the hopelessly underpowered and inadequate. Two major roads cross, but when a junction has six arms you either need something devilishly complex and clever, or you need a big roundabout. The easy option is the big roundabout. You can see which of those two options was chosen here.

Not only is it a three-level stacked roundabout interchange, the multi-level interchange that brings despair and delays to important junctions nationwide, but it's also been blessed with that most fashionable of accessories, traffic lights at each entry point. There are anything up to five lanes on the roundabout's circulatory carriageway, multiple lanes on every approach, and vast gantry signs to point you to the right lane, but none of those things should fool you that this is a junction with plenty of capacity. For much of any normal weekday, it's slow going at best. A simple journey from the M25 anticlockwise to South Mimms Services takes the motorist through five sets of traffic lights, with another four on exiting the services to return to the motorway.

Why is it wrong?

The A1(M) terminates on the M25 here, but that means the A1 to the south has no motorway restrictions and therefore this junction must provide an exit for non-motorway traffic. Immediately a free-flow junction is made impossible since the A1081 now has to be there. But there was never any desire for this to be a free-flowing interchange - it was just designed in another era, a simpler time when the level of traffic we now take for granted was virtually unimaginable.

Indeed, it was actually designed - if you can believe this - for a third motorway connection to come barrelling in to the roundabout. Had the original plans for the Ringways come to fruition, giving London a series of concentric ring roads, this would have been a seven-way junction with another motorway called Ringway 3 leading away to the south-west and another arm to the roundabout. The people who designed it - clever people, you have to assume, with letters after their name, and a solid belief in planning highways based on empirical research, and probably wearing spectacles and white coats - looked at an array of options and thought this was best.

The short answer, then (and admittedly this could be the "why is it wrong?" answer for every single Bad Junction on this website) is that the junction's designer thought this would do, and it turns out it wouldn't. At all.

What would be better?

To make all journeys through this junction a fleeting moment of bliss, rather than a long and frustrating hold-up, would require a total rebuild to create a free-flowing interchange, relocation of the services to another site, and another junction somewhere else nearby to handle connections to the local road network. We may safely assume that isn't going to happen.

The best thing to do, then, is to provide smoother journeys for the busiest flows. Free-flowing left turns in the north-west and south-west corners are easy to provide; a single bridge and a length of new road would allow a free-flow turn in the south-east corner from M25 to A1 as well. A loop could be built from M25 westbound to A1 northbound, in the same way as the loop added to the M62/M57 interchange a few years back. That alone would take some of the load off the roundabout, freeing it up for the remaining traffic that would need to use it.

More ambitiously, a link between the A1(M) to the north and the M25 to the west could be provided through open land, cutting the corner significantly - but that's pie in the sky territory.

On the bright side, if you do find yourself stuck in the inevitable jams at South Mimms, you can always divert from the choked roundabout into South Mimms services, and rejoin the queues again later, after a cup of tea and a sandwich.


Right to reply

Road5914 10 October 2019

Some more fiddling to start soon on this junction, which rules out a proper scheme for a while.

Martin Britten-Jones 6 August 2020

I hate this interchange. Someone said it ok if you follow the markings on the road. At busy times these are always covered by traffic ahead waiting to clear. Making advance planning difficult.

Floyd 8 August 2020

I once used a similar roundabout and always ended up in the wrong lane, but that was when I was younger and had to speed everywhere. I soon realised that if I just took my time and read the road ahead it was fine. I think that's (often) the problem- people tearing everywhere (to only have to sit at the lights longer). Driving a 7.5 tonner made me a better driver for sure, because although it's not an artic, you still have to think differently compared to driving a car or van.

Tobye 9 August 2020

The problem stretches beyond the lack of lane discipline and sensible driving, although yes I do agree it is a small contributing factor to the problems that face this travesty of a junction. The ongoing 'improvement' scheme is dragging on, in doing so the already shocking lack of capacity has been made even worse by lane closures. HE have also started adding in additional ERA's on the M25 ALR section to the east of J23, which has reduced the clockwise entry slip to just one lane!

The problems with this interchange could be easily fixed with free-flow left turns, removal of the A1081 arm and the local access road from the Services. Increasing the visibility of directional signage on the roundabout, and increasing the amount of lanes for the busiest turns, such as A1(N) - M25(CW), which only has one designated lane presently! For such a major interchange, I am surprised it has been allowed to evolve into the travesty it has become today. The daily congestion and delays it generates surely has a greater economic impact than that of a proper, and thorough improvement and design overhaul.

Jonathan Wilson 28 October 2020

I am no expert but why not connect the A1081 (the local access road) to the A1 just to the south of the roundabout (using either traffic lights or another small roundabout). People who want the A1 south would just go south, people who want the A1(M) north would go straight through the roundabout with no traffic and only people heading on the M25 would need to use the roundabout (and everyone on that roundabout would have less traffic lights and less mess to navigate)

I don't think the A1 needs another roundabout so close to this one. How about re-routing the A1081 to the south (as you say), but along Trotters Bottom to the point where that road meets Galley Ln/Rowley Ln/Holmhill Ln and change THAT junction to a free-flowing one? There's plenty of space there. It would then be the junction that non-motorway traffic would have to exit from instead of the main roundabout to the north. Also get rid of the services and move them elsewhere. That would take a huge strain off. Looking at the Borehamwood junction to the south does suggest however, that there was a lot of lsd being used by the designers around the time these junctions were planned, so perhaps it's too late now really and we'll just have to live with the snafu's.

Iain. 24 November 2020

I have been driving SB on the A1(M) past this junction almost every working day for 8 years. I have seen the works progressing on the SB exit to the A1 and wonder what further circle of hell this will inflict on us. I don't use the roundabout enough to know what difference it may make to traffic on it, but I can say now that there are never any hold-ups, excessive traffic, or even just slow traffic SB on the A1 at this point, unless a bad day on the M1 coupled with an accident at Stirling Corner backs up traffic (and that is a real rarity). Form my admittedly limited knowledge they appear to have altered a piece of road that functions absolutely perfectly, pretty much always.
That's South of the junction. North of it, travelling SB past the slip road to the M25 can be downright dangerous. It's not entirely the road, although a double lane slip leading to the 4 lanes at the lights to the roundabout would help. With one lane, people hammer down the A1(M) and try to pull in at the last minute from Lane 1. Alternatively, people decide that the lane isn't moving fast enough, so pull back out into Lane 1, to gain a few 10's of meters.
If you don't know the road by the way, and travel down this part wondering why people are driving in the outside lane, completely avoiding Lane 1 next to the feed to the slip road, it's to lessen the huge accident potential. NB on the A1 approaching the Junction is exactly the same.

Sara21 16 May 2021

The new 'improvements' are awful! Lanes don't match up, traffic light phasing is wrong in places. Cars entering from M25 clockwise often cannot place themselves into the correct lane before the services exit because the phasing of the lights being on red for too long. So, this M25 clockwise traffic entering the roundabout, often has to move along in the A1 north filter lane to try and squeeze in to the services/M25 lane, blocking the traffic that wants to get on to the A1 North. The beeping and cussing that goes on is regular, yet half the time it is not the fault of the drivers blocking this lane. These vehicles blocking the A1 North filter lane then have to cut across to the lane that they need to be in. This can cause chaos during rush hour.

My other bugbear is that the A1 South/A1081/M25 anticlockwise lane, before the services, does not filter the traffic effectively.. Drivers in the know about this are now positioning themselves the M25 clockwise lane, because this filters you into the A1 South/A1081/M25 anticlockwise effectively, HOWEVER, the drivers who are correctly in the A1 South/A1081/M25 anticlockwise marked land now cannot always filter into the A1 South/A1081/M25 anticlockwise because the traffic flow on to the M25 clockwise lane is generally at a standstill. I feel sorry for nervous or inexperienced drivers. It's a multi million pound disaster! It's made it worse on this section of the roundabout.
Highways probably can't do anything about the filtering of lanes now - although they should - but they could amend the phasing of the 2 sets of lights at the services, so that the traffic there on the roundabout isn't stopped when vehicles from the M25 clockwise filter on, or when the traffic travelling on from the A1 North filters on and is stopped at the lights before the M25 clockwise.
I drive on this roundabout every day and, at around 4pm, it can still take me a good 15mins to get from the A1 south entrance around to A1081 exit.
I hope this all makes sense?? In a nutshell, what I intended to say was that the new 'improvements' have not really helped! In fact, they have made it worse in sections!
Rant over! Thanks!

Many years ago I used a roundabout in Birmingham that sounds like this one. If you 'played by the rules' and attempted to move over as you went around, you'd find that the locals and others in 'the know' were already in your lane as they'd started off in the wrong lane further back. (I do remember Northampton had some good ones though, where you couldn't go wrong).

Anonymous 20 July 2022

The roundabout improvements which were finished in 2020 brought some improvements but it still is very congested there. What would be a better solution is some loops connecting from the M25 to the A1(M) and a loop going from M25 east to A1 south.

What would be better is to remove all roundabouts on the A1 between the north circular road and south mimms

Nick K 31 July 2022

For something radical, use ANPR cameras on all lanes approaching and on the roundabout, programmed to detect lane followage. Don't penalise drivers for the first few transgressions of lane abuse, but maybe inform via email?
This would be really effective at keeping open lanes on the southbound A1(M), blocked by selfish queue-jumpers.
Furthermore, it would provided data into how improvements might be made.

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