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

Matt Dickins 15 January 2006

I love this roundabout use it every morning to go to work. They've even got the lights sorted so you can go all the way round without stopping. Only gripe is that the lanes don't match up as you go through each set of lights.

Adrian V Stokes in response to Matt Dickins 29 November 2017

In reply to by Matt Dickins

It might be possible to go all the way round without stopping but, if you enter the roundabout from the eastbound M25 (St Albans), then it is likely that you'll catch ALL the lights at red - creating huge delays and pollution. One great improvement would be the removal of traffic lights at the A1081 junction

Damian Kelly 15 January 2007

Another little foible of this junction is the route you take to South Mimms or to Welham Green. This involves driving through the services to a little 'hidden' roundabout.

Brian 15 February 2009

Why all the gripes about this junction? As long as each driver reads the road properly and follows the directional arrows which are clearly marked on the road, there is no problem. The problems start when those who do NOT read the road markings are in the wrong lane and cut across other lanes to get to their required lane. Usually the culprits are boy racers in flash cars with loud rubbishy music blaring out.

Peter 27 November 2009

I am with Brian on this. I use this junction regularly and all you have to do is follow the signs. I can think of many other junctions where the signs put you into the wrong lane but not this one.

Giles 2 February 2010

I hate the South Mimms roundabout! As a newish driver I use a satnav and I keep finding myself in the wrong lane, nearly going through red lights, feeling very confused. I would like to avoid it altogether but that seems more or less impossible for many journeys.

sara21 in response to Giles 16 May 2021

In reply to by Giles

You can't help but get in the wrong lane!! Don't blame yourself or the satnav ;) The filtering of the lanes on this roundabout is awful and, what makes it worse, is drivers who know about the awful filtering place themselves in the wrong lanes so as to filter themselves into the lane they want with greater ease. However, this makes it more difficult for the drivers who are in the 'correctly' marked lanes, who are trying to navigate the awful lane filtering system.
I feel your pain - I travel this roundabout every day and every day I get sooo frustrated!

Matt 26 March 2010

I use this junction a lot, since I live in Barnet just to the south. This means I usually come to the junction by way of the A1081, of which you complain; it leads into High Barnet town centre.

I think you're being harsh, here. This junction is my main gateway out of London to the country beyond, as it allows me fast access to the A1 and, via the M25, the rest of the motorway network. Without the A1081 link, a large chunk of north London suburbia would be denied easy access to the motorway system. I, and thousands of others, would have to drive through Totteridge or Arkley, on local roads, to reach the A1 at Edgware. There is no need for the A1(M) to continue beyond the M25; south of this point the A1 is a normal, if very busy, urban arterial road, with many access points and roundabouts. And so it should be.

This roundabout (junction 23, as I know it) may be large and busy and have more traffic lights than is ideal, but it is hardly a nightmare.

Roger 2 June 2010

Th A1081 has never been a cause of problems on this junction so shutting it off is a pointless suggestion! Also making the A1 south of here motorway would solve nothing. I use this daily and have done so for 10 years now. This junction regularly got grid-locked as selfish idiots ignored the lights and joined the back of queues, blocking entries and exits. Since the widening and remarking of lanes a couple of years though, this has problem has all but disappeared. Camera's to stop people jumping the lights would be good, especially on the set prior to joining the A1 northbound as many twats regularly have a long run up to them and jump them at red, endangering traffic exiting the M25 clockwise.

Peter 3 October 2010

Where there is a problem then it is often down to satnav users. The cars that weave from lane to lane usually have that telltale glowing screen above the dash. If you just look out of the window and follow the signs then there is no problem.

Steve 8 September 2011

I use this a lot, the only issue I have is the A1(M) southbound to M25 clockwise movement (one I do a couple of times a week) really needs more capacity as it often queues back to the A1(M) mainline, and results in people using the inside lane on the slip (signed for the Services) to cut in across the lane markings to jump the queue. However, the junction works pretty well- and the traffic volumes on the M25 often mean you are joining queuing traffic, so free-flow interchange from the A1(M) wouldn't really speed things up.

In addition to the roads you list, there are also a couple of unclassified local roads accessible via the Service Area approach road (one of these, to South Mimms village, can be a useful escape from the M25 onto the parallel B556). So, strictly speaking, the A1081 link is not essential as an exit for non-motorway traffic.

Though I still prefer to avoid using the roundabout here, its current guise has far better flows than formerly.

Ian Sergeant 15 February 2015

Your comment about moving the services is probably valid, but this would mean the necessity for two sets of service stations, one on the M25, one on the A1(M). I tend to agree with other posters that the A1081 isn't really a culprit - the London Borough of Barnet needs a way out to the A1(M), and this is by far the best one for the vast majority of its residents. By all means create a slip road on to the A1(M) north of South Mimms with a minor road continuing along the old A6 - but I'm not sure that is cost justified. The benefit to the junction would come from the removal of the services.

Patrickov 27 February 2015

Just a wild idea after reading the most recent comment... what about making the service's accesses on the M25 EB and A1(M) SB instead? That would eliminate the need of two services, as well as simplifying the junction. The two carriageways connected to the services would have to take some extra load though...

Clive 24 September 2015

I travelled South on the A1(M) then onto M25 heading East at peak time this morning and it was appalling. At the turn off there is only one feeder lane which had a huge queue of backed up traffic due to the traffic lights. Whilst it splits into 4 lanes eventually the single feeder lane is plainly inadequate at peak times.

Tobye 22 January 2016

I use this junction at least once a week, and every time there is always chaos of some degree, usually it begins on the A1(M) at J2, with queues even reaching back to the Hatfield Tunnel at times. I dread to think what it's like for the A1 northbound, but then they have an extra lane, which would be incredibly handy on the A1(M)! The clear issue here is that the sliproads, all of them in fact, do not have enough stacking space, with the most popular destinations poorly catered for! The whole gyratory should be 4 lanes, try and get rid of a few traffic lights (or at least upgrade to MOVA/SCOOT, so you don't sit at a red for 60 seconds unnecessarily), maybe even provide left hand filters, especially for the M25 (C-wise) to A1(M) (Nbound), surprisingly this is a very popular route! Investment has been made to the M25 above the junction, but as usual with infrastructure money, it failed to work its way down to where it was needed! We shall have to continue with this shambles I suppose, and continue the battle against confused drivers, and risk taking locals weaving like crazy!

Taylor 22 April 2017

This junction is just very confusing. Every time I attempt to join the M25 from the A1(M) southbound I get lost. I think instead of making a turn-off on the A1(M), make it come to a stop at the roundabout and reduce the space taken by the roundabout and it may be a bit easier.

James 23 January 2018

A dismal junction, especially confusing to those who don't use it regularly and don't know which lane they should be in. All right for those who do. I never go south towards it in the morning as the tailback for the exit ramp is often many hundreds of yards. Not so bad in the evening for those going west and then north on the A1, although the penny-pinching two lane layout north of the interchange frequently has traffic slowing to a crawl just from cars joining it from the sliproad.

Peter 17 February 2018

The main problem is the volume of traffic trying to access the roundabout from the A1M southbound. Once on the roundabout it is very well signed with lanes spiralling properly to take people off at their exits. The problem on the roundabouts is people who think that the direction signs are for decoration and end up making panic lane changes when they discover that the lanes are actually spiral to take you off at the signed exit.

Adam 21 September 2019

Looking at the Ringway 3 pages is strikes me one solution here is to take advantage of the earthworks on the M25 east of the junction where the lanes split apart and build the "GLC alternative route" which would take A1 to M25 east clear of the junction. How much pressure would that then take off the roundabout?

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