M40 / A46 Longbridge Island

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S Parkinson
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Re: M40 / A46 Longbridge Island

Post by S Parkinson »

Jeni wrote:They have got the lanes all messed up on the M40 off slip. Who in their right mind will drive up to the traffic lights and wait to go to Coventry, when there is a freeflow just to their left!
>> Photo <<
My thoughts exactly. So I thought I'd email m40j15_bypass@highways.gov.uk to ask why this was, highlighting a potentially dangerous aspect to the current layout.
This is my email and their response:

--------------------

Team

I use Longbridge island on a regular basis and the improvements to traffic conditions on the roundabout since the work has been completed have been a revelation.

However I have a concern with the signposted lane markings at the top of the M40 southbound off slip road.

To have three lanes for A46 Coventry, two of which are via traffic signals, is surely a mistake given all traffic to Cov uses the new left turn slip lane.

Travelling from Birmingham to Warwick I have at various times used both the signed lane for the A429 Warwick (the centre lane) as well as the left hand lane of the three at the signals (as it 'feels like' the correct lane, whatever the markings may say, given the lack of Coventry traffic approaching the lights). In both situations there inevitably seems to be another driver alongside with the opposite idea, and we end up dangerously merging into the one lane for Warwick on the roundabout itself. In addition I've noticed an increasing tendency for drivers heading around the roundabout (ie. toward Cirencester) to use the centre lane at the top of the motorway slip road given the volume of traffic using the right hand lane only for both Cirencester and Stratford.

It would certainly surprise me to hear that the current arrangement isn't a mistake, but if not and it is actually meant to be like this I would be pleased to have the reasoning behind it explained.

Regards

SP

--------------------

Dear S

The contractors have advised me that the 3 lanes have been designed to A46 Coventry off the M40 because traffic figures (maybe not now but in the future) dictate that they are needed. This fact has been checked.
We have studied how traffic is using the junction and as you say drivers are not using the correct lanes.
We are of the opinion that the main cause is that regular users were still using the inside lane despite its destination changing and in time this will cease.
We have added some extra Warwick Only and left turn arrows to the road markings.
We have also added additional road markings further up the road to aid lane discipline.

Regards

MK
Project Support
Highways Agency

--------------------

I not entirely certain what the reference to drivers 'still using the inside lane' is referring to. If it is about drivers for Warwick using the left hand lane at the lights then the destination hasn't changed in this respect as prior to constuction of the Cov free flow slip the lane markings at the lights were the same as now (ie. both left and middle lanes to Cov with the centre lane shared with Warwick traffic).

Are there any other instances in the UK where in addition to a free flow slip road a destination is additionally lane marked via a set of traffic signals?
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Re: M40 / A46 Longbridge Island

Post by A307Patrick »

So many junctions on the 'new' M40 ie J8 to the M42 were/are wrong and need(ed) modification:

J9 (currently being discussed in another thread) is unsatisfactory and is having a few small things done to it;

J10 has been redone once already and many people are still not happy with it;

J15 has finally been done but, according to comments above, is still not perfect;

J17 had to be modified soon after opening.

(Also J8a was not built at the time of the extension but had to be added not long after the opening).

So, with the exception of J11, all junctions on this stretch of the M40 connecting with major routes have had to be modified and supposedly improved. Why is this? Did the planners severly underpredict traffic flows? Or was it the usual thing of trying to do it on the cheap whenever possible?
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Chris Bertram
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Re: M40 / A46 Longbridge Island

Post by Chris Bertram »

A307Patrick wrote:J17 had to be modified soon after opening.
Is that a trick question? There is no J17 on the M40.
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Re: M40 / A46 Longbridge Island

Post by si404 »

A307Patrick wrote:(Also J8a was not built at the time of the extension but had to be added not long after the opening).
It existed, but only as a half junction numbered as part of junction 8 - the services that were built needed south facing slips (though you can go via the A40/A418 junction on the spur, but that's just a bit of a long way around).

You can argue the same for the junctions with primary routes (1, 2, 4) on the other half of the motorway, plus the addition of junction 1a. OK it's quite a bit older than the northern half, but still.
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SouthWest Philip
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Re: M40 / A46 Longbridge Island

Post by SouthWest Philip »

Chris Bertram wrote:
A307Patrick wrote:J17 had to be modified soon after opening.
Is that a trick question? There is no J17 on the M40.
Perhaps he means jnc 3A of the M42? I'm sure I've seen some early maps that also showed this as M40 jnc 17. Originally the merge from the M40 to the M42(N) was by a single lane into what then became the outermost of three lanes on the M42 with two lanes of the M42 merging on the left. This was deemed dangerous and a more convention 2+2 into 3 layout was adopted. These days they'd just stick a 50mph limit on it, a la M3 jnc 2.
Chris Bertram wrote:(Also J8a was not built at the time of the extension but had to be added not long after the opening).
The north facing slips were built, and were deemed to be part of jnc 8, but south-facing slips weren't needed as there were no services on the junction at the time. Arguably the south-facing access slip still isn't necessary as services traffic could return to the M40 by turning left and left again to join the M40 spur from the Wheatley junction.

Edit - what Si said above!
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Re: M40 / A46 Longbridge Island

Post by roadtester »

Went this way today for the first time since the work was completed. If I use this junction, it's usually to cross the M40 while using the A46 on a long distance journey.

I can see that the new layout relieves pressure on the main roundabout on the M40, but from the point of view of A46 through traffic, it replaces a big roundabout with a small one and that's about it.

The whole thing is confusing and the opportunity fully to GS one of the most important junctions in the country has been missed at a time when the A46 is likely to grow in importance as a long-distance through route with the dualling works taking place further north.

Rarely in the history of British road-building has so much money been spent and so much earth been moved to so little useful effect.
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Re: M40 / A46 Longbridge Island

Post by A307Patrick »

Chris Bertram wrote:
A307Patrick wrote:J17 had to be modified soon after opening.
Is that a trick question? There is no J17 on the M40.
I meant the M42 junction. As South West Philip says, some maps definitely used to show it as J17. SWP also gives a good description of the original layout and what was done to improve it!
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Re: M40 / A46 Longbridge Island

Post by CrackersA361 »

Juast thought I'd add that J11 has been modified, it was quite a while back but they made the eastern side 4 lanes wide from the M40 southbound offslip, past the A361 and round to the A422 exit.

But yeah, it seems the M40 was underestimated in its importance to the motorway network, considering it connects London to Birmingham, via Oxford and at some point or another meets 4 main cross country DCs (A34, A40, A43, A46).

I have to agree about the improvements to J15 though, I sometimes use that route from Coventry back down to Banbury when I can't be asked to trawl back behind someone dawdling at 40 on the A423, and whilst the roundabout is much easier to negotiate, it won't be long before M40N > A46N traffic will start to clog the roundabout back up again.

What about a simple loop off the M40 to the north of the new main A46 carriageway?
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SouthWest Philip
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Re: M40 / A46 Longbridge Island

Post by SouthWest Philip »

CrackersA361 wrote:What about a simple loop off the M40 to the north of the new main A46 carriageway?
The M40(N) onslip would have to pass under the loop, otherwise you'd have appalling weaving problems. And you would also then need to rebuild the new merge on the A46 to the north of the junction. The through A46 movement (now also carrying M40(N) to A46(N) traffic) would need to become the mainline with the slip from the main roundabout joining from the left instead of the right.

And also, I think that your new loop would pass through the Site of Special Scientific Interest (Newt colony) that prevented them fully grade separating the A46 in the first place.
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Re: M40 / A46 Longbridge Island

Post by Truvelo »

I drew this a few years ago which included a loop and a C/D road to eliminate any potential weaving. With hindsight the loop would have been better for M40-A46(N) traffic like on Crackers' map rather than mine but the C/D road would be the same only that it would run along the southern side of the M40 instead.

It was bats that stopped the A46 being fully GSJ'd. Even so, the layout shown on my map would have avoided them.
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CrackersA361
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Re: M40 / A46 Longbridge Island

Post by CrackersA361 »

SouthWest Philip wrote:
CrackersA361 wrote:What about a simple loop off the M40 to the north of the new main A46 carriageway?
The M40(N) onslip would have to pass under the loop, otherwise you'd have appalling weaving problems. And you would also then need to rebuild the new merge on the A46 to the north of the junction. The through A46 movement (now also carrying M40(N) to A46(N) traffic) would need to become the mainline with the slip from the main roundabout joining from the left instead of the right.

And also, I think that your new loop would pass through the Site of Special Scientific Interest (Newt colony) that prevented them fully grade separating the A46 in the first place.
That's a fair point, I forgot about the onslip, but a C/D lane as in Truvelo's example might help solve that though admittedly an additional bridge to get the loop over/under the onslip would be the ideal solution. And then thats if the bats weren't there...

On second thoughts just completely discard my suggestion! Bloody bats... :P
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S Parkinson
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Re: M40 / A46 Longbridge Island

Post by S Parkinson »

S Parkinson wrote:
Jeni wrote:They have got the lanes all messed up on the M40 off slip. Who in their right mind will drive up to the traffic lights and wait to go to Coventry, when there is a freeflow just to their left!
>> Photo <<
My thoughts exactly. So I thought I'd email m40j15_bypass@highways.gov.uk to ask why this was, highlighting a potentially dangerous aspect to the current layout.
This is my email and their response:
Further to my post above I see that GSV now also shows the current layout.

The latest Right To Reply by blundelo on the CBRD bad junctions page for Longbridge also sums up the situation very well.

Regarding my own use of the junction, once I received the reply making clear that the layout wasn't a mistake I always use the centre lane at the top of the slip road as marked when travelling to Warwick however plenty of others (intentially or otherwise) still don't, resulting in regular merging conflicts (to get into the single lane for Warwick) on the roundabout itself. TBH it would be safer if the left hand lane approaching the traffic lights was just hatched out.
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Re: M40 / A46 Longbridge Island

Post by Richardf »

Sorry to bump an old thread, but I've been looking at this junction and I just don't really understand why the southern end of the 'bypass' isn't free flow like Chievley on the M4? Why bypass a roundabout, then basically replace it with another one!? Seems to me there is space for a free flow split to the south, or perhaps a simple gsj to serve the local B Road if nessesary.
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Re: M40 / A46 Longbridge Island

Post by pjr10th »

Richardf wrote: Sat Oct 24, 2020 22:57 Sorry to bump an old thread, but I've been looking at this junction and I just don't really understand why the southern end of the 'bypass' isn't free flow like Chievley on the M4? Why bypass a roundabout, then basically replace it with another one!? Seems to me there is space for a free flow split to the south, or perhaps a simple gsj to serve the local B Road if nessesary.
I would imagine it's similar to the Hardwick (I think) junction featured on roads.org.uk recently. The purpose is to take A46 through traffic off the M40 roundabout so (apart from Warwick S) all traffic is somehow interfacing with the motorway.

For the north junction it makes sense to freeflow it so to not slow traffic down, but on the Southbound there are too many roads to make a freeflow feasible (plus traffic accessing properties nearby would need to go to the motorway roundabout to turn around), while the roundabout doesn't mess traffic up too too much.
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Re: M40 / A46 Longbridge Island

Post by Bryn666 »

It was because of environmental constraints relating to protected species being in the way of proposed slip roads. The roundabout was built as a compromise on the assumption that most traffic was heading down to the M40 from Warwick and not into Stratford.
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Re: M40 / A46 Longbridge Island

Post by Richardf »

Bryn666 wrote: Sun Oct 25, 2020 00:05 It was because of environmental constraints relating to protected species being in the way of proposed slip roads. The roundabout was built as a compromise on the assumption that most traffic was heading down to the M40 from Warwick and not into Stratford.
This is a pity. And assumes traffic heading north to the M40 from Stratford and further afield isn't as important as that coming south from Coventry and Warwick to the M40, as it is forced to negotiate 2 roundabouts. It also interferes with the A46's apparent role as a long distance route.

I appreciate the conservation issue involved but I still wonder whether they could still have done better given those constraints. What was wrong with an A46 flyover on the main roundabout for example?
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Re: M40 / A46 Longbridge Island

Post by Alderpoint »

Richardf wrote: Sun Oct 25, 2020 10:32 What was wrong with an A46 flyover on the main roundabout for example?
The impact on Sherbourne village would have been unacceptable. The alignment chosen meant they could push the A46 further away from the village.

But I agree not putting in a roundabout was not the best solution.
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Re: M40 / A46 Longbridge Island

Post by ais523 »

Richardf wrote: Sat Oct 24, 2020 22:57 Sorry to bump an old thread, but I've been looking at this junction and I just don't really understand why the southern end of the 'bypass' isn't free flow like Chievley on the M4? Why bypass a roundabout, then basically replace it with another one!? Seems to me there is space for a free flow split to the south, or perhaps a simple gsj to serve the local B Road if nessesary.
One thing worth noting is that the connection between the M40 towards Birmingham and A46 towards Coventry is freeflow on this junction layout (and wouldn't be with a typical stackabout). I'm not sure how the traffic on that movement compares with the traffic on the A46's "straight on", but it wouldn't surprise me if they were comparable. (My usage of the junction in question is almost exclusively the Birmingham-Coventry movement. It makes an excellent alternative to the M42 and M6 when going between the M40 terminus and M6 J2.)

I'm often bothered by junction discussions on SABRE in which people focus primarily on the straight-on movements and neglect the turning movements; quite frequently, the turning movements are just as relevant, or even more so. (For example, the A47/A10/A149 junction has a lot of turning traffic, and its current design is good at handling the turning movements even though the A47's "straight on" is not freeflow and has a conflict eastbound.)
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Re: M40 / A46 Longbridge Island

Post by Alderpoint »

ais523 wrote: Sun Oct 25, 2020 12:07
One thing worth noting is that the connection between the M40 towards Birmingham and A46 towards Coventry is freeflow on this junction layout (and wouldn't be with a typical stackabout). I'm not sure how the traffic on that movement compares with the traffic on the A46's "straight on", but it wouldn't surprise me if they were comparable.
What is utterly ridiculous at this junction is if you are leaving the M40 southbound and want to head up the A46 towards Coventry then you have the choice of a free-flow lane (avoiding the lights) and TWO light controlled lanes as well (https://goo.gl/maps/yaciitrQzWK2LPWd7). Having used this junction hundreds/thousands of times since it's opened I can count on the toes of one foot the total number of vehicles I've seen heading this way.
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Re: M40 / A46 Longbridge Island

Post by Truvelo »

Alderpoint wrote: Sun Oct 25, 2020 16:34
ais523 wrote: Sun Oct 25, 2020 12:07
One thing worth noting is that the connection between the M40 towards Birmingham and A46 towards Coventry is freeflow on this junction layout (and wouldn't be with a typical stackabout). I'm not sure how the traffic on that movement compares with the traffic on the A46's "straight on", but it wouldn't surprise me if they were comparable.
What is utterly ridiculous at this junction is if you are leaving the M40 southbound and want to head up the A46 towards Coventry then you have the choice of a free-flow lane (avoiding the lights) and TWO light controlled lanes as well (https://goo.gl/maps/yaciitrQzWK2LPWd7). Having used this junction hundreds/thousands of times since it's opened I can count on the toes of one foot the total number of vehicles I've seen heading this way.
That is the movement I use the most so the current layout suits me perfectly. If the bats weren't a problem it would be possible to GSJ the small roundabout and have the A46 mainline freeflow as well.
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