Where is it?
North of Bristol, the point where two of the most important roads in South West England cross each other and, rather ambitiously, attempt to interact with the local road network at the same time. This is actually two junctions on the M5 that are stuck together — junction 15 for the M4 and junction 16 for the A38 at Filton.
It is one of the most-nominated Bad Junctions ever.
What's wrong with it?
In its component parts, nothing at all. It took me years to work up the courage to actually list this on the site because junction 15, the M4-M5 interchange, is a four level stack, one of the highest capacity junctions it is possible to build and one of only three of its type in the UK. We could do with far more of them, so it feels quite wrong to see it here.
The Maltese Cross at Almondsbury isn't the problem (though the motorways it connects could do with a bit of extra capacity). The trouble is that it has been glued on to another hapless junction with the A38, a major road into Bristol and now surrounded by shops, offices and car parks.
Linking the two overloaded junctions is a collector-distributor road setup that collects traffic for a brief scramble to change lanes before splitting up into sliproads for the next junction.
Why is it wrong?
In 1966, the year it opened, Almondsbury was the most incredibly complex and exciting junction the UK's road network had ever seen. It had a free-flowing interchange with four levels! It had four motorway carriageways running parallel to each other! It had it all.
Unfortunately what happened to Almondsbury is traffic. When there's not many people on the roads, all that lane changing on the outer linking carriageways is easy. Once it starts to get busy you have problems: traffic coming from the M4 westbound has to move right and right again on its way to the M5 southbound, while trying not to bang in to the M5 southbound traffic frantically moving left to reach the A38. The more vehicles there are, the harder it is to maneouvre, and the more everyone is in everyone else's lane.
The worst bit is that all of the roads here are predisposed to come to a shuddering halt in the summer when the holiday traffic is passing through, and any sort of traffic jam blocking those collector-distributor lanes will then spread onto all the surrounding sliproads and make the perilous lane-changing moves even more dangerous than before. Changing from a lane travelling at speed into a stationary one is not an easy trick to pull off.
What would be better?
This interchange is crying out for some investment. It was once the marvel of the motorway network and it doesn't deserve its bad reputation. I feel quite sorry for it.
The most obvious improvement is to get rid of those outer collector-distributor carriageways, and replace them with a braided junction. Separate sliproads would cross each other with flyovers to carry conflicting traffic movements without incident.
Right to reply
The interchange is at the moment getting a particularly bad name for itself, after an unidentified body has changed the phasing on the traffic lights at the J16 A38 roundabout. This is resulting in morning tailbacks across the collector-distributor road setup, with considerable hazard to lfe, limb, and vehicle. And in the evening, drivers struggle to leave Bristol via this route.
Coming from Wales on the M4 and joining the M5 southbound is so dangerous as you arrive blind on a flyover bend in the outside lane of the M4 joining the M5 south. If you are driving a lorry you cannot see in the left mirror what's coming. Even more dangerous if you survive the flyover and then want to come off on the A38.
I just don't see the problem with this junction. It's fine whenever I use it!
I live north of Bristol and work in Aztec west. It's worth noting that the M4 -> M5S has quite a long stretch for people to move lanes, with dedicated lanes for M5S and the A38 each way - quite easy to move across most of the time (for me coming from M5S especially) and the only issue is weight of traffic and the traffic lights at J16 which can cause queues but at least they move and there is usually time to filter in and for M4 traffic wanting the M5 to go around.
Joining the M5N from J16 however is a death trap - there is no separate lane for the M5N and half the drivers wanting the M4 seem to have no realisation that traffic might want to move across so goes at stupid speeds which force you to either miss the junction or pull out in front of them and chance it - so definitely better signage needed there.
And if there is traffic on the M4E, it is usually stationary and everyone has to sit in it while the M5N flows freely just a few meters to the right! M4E is queued all the way back from the M32 most evenings at the moment. And god forbid what's going to happen when the Christmas shoppers come back out in a couple of months...
In the meantime, to save lives whilst a permanent solution is found, drop the speed for the WHOE Junction mess to 40 mph at all times..... Lower to 30 mph when traffic particularly heavy and... most importantly, ENFORCE the speed limit.
Erm, this junction already has a variable speed limit with enforcement cameras.
To save lives? I bow to anyone with access to the relevant statistics but I don't recall there being too many fatals around here. Traffic hardly moves as it is; indeed that stretch of the M4 is already set to 40 at rush hour with HADECS3 enforcement.
What more would you like them to do??
First off, I agree with everyone who says that the weaving section over by the A38 part of the junction is dangerous. I've only ever used it once (doing M5 southbound to A38 southbound), had a near miss with a car that was doing M4 westbound to M5 southbound (and apparently unaware that cars might want to be crossing between roads in the opposite direction), and am planning to never use the junction in this direction again. So that's a 100% near-miss rate for me for that movement!
However, the weaving section isn't even the only terrible thing about this junction. What about the M4 westbound to M5 northbound movement? "That movement's just the same as it would be in a normal 4-level stack", you might say. But it isn't, it's so much worse than that.
The design principle behind a 4-level stack is that you decide whether to turn off first, putting you onto a slip road; then you decide which way to turn, and get routed round or over/under the junction as appropriate. Simple, no conflicts, no decisions for mainline traffic (other than whether to turn off); pretty much everything we look for in a junction.
Not so at Almondsbury. The westbound M4 usually has four lanes ("usually" because it uses dynamic hard shoulder running, so sometimes has only three!). The leftmost two lanes are dedicated lanes for the M5 southbound and A38. The fourth lane is for the M4 westbound. The third has to share between M5 northbound and M4 westbound traffic. In other words, the signage and lane allocation for the junction is designed as it would be for a cloverleaf, or for an octopus in the orientation where the two exits are separate; you expect you're going to go straight on past the diverge and then use another slip road a little later.
Instead, what happens is that the third lane splits into two (and this is a genuine, symmetrical split, rather than a slip road appearing that you can turn on to); and it splits only just before the junction. You have only a few seconds to figure out which side of the split you want to be in, otherwise you're going to run off the motorway. If you want to stay on the M4, you need to go right; to reach the M5 northbound, you need to go left. You're then on a three-lane sliproad for a little longer, until the sliproad splits again, with no "stay in lane" reminders, and you finally end up on the small single lane that takes you over the junction to the north.
All you need to do now is to merge onto the M5; simple, right? Of course not. As might be expected at a complex junction, you have signs letting you know how the merging works; first, a sign letting you know that traffic will merge in from your left; and then, contradicting this (after the traffic from your left has lane-gained to a second lane alongside yours), a small map showing the left-hand of the two lanes lane-gaining onto the M5 and the right-hand lane merging. This is a standard layout for a merge into a motorway (and the M5 itself gets a sign showing the same layout).
Which sign is correct? Actually, neither of them. What actually happens is that the lane divider in the middle of the slip road just disappears, and the whole thing narrows down to one lane. If you were driving alongside a vehicle in the lane to your left – something which is perfectly reasonable, and encouraged by the signage – you're going to need to merge at very short notice, or end up crashing into something. The only safe way to use the junction is, therefore, to merge together with the M4 eastbound to M5 northbound traffic into a single file, well before the sliproad narrows. (In practice, most drivers who are "in the know" seem to prefer the left-hand lane for this purpose, which implies that none of the signs are correct.)
So while the diverge needs an unusually fast reaction time to perform safely, the merge has a different problem; the layout is one that could be made safe with different signage (put up a "road narrows on right" sign, and some curved arrows to show that the right-hand lane of the sliproad disappears, and it should be possible to use safely); but if you try to follow the signs, you'll end up in a rather unsafe situation if you don't manage to merge immediately.
Perhaps this is why the lane markings at the diverge give you such little time to react: they're designed to make sure that you're only allowed to turn onto the M5 northbound if you have good enough reactions to survive the merge! Or perhaps it's just terrible design, like the south side of the junction is. It should be a lot cheaper to fix than the whole mess around the A38, though.
Once again, I really don't see the problem here, certainly not West to North in any case. There are more overhead signs than you can poke a stick at, as well as lane markings. Just get in the lane the signs tell you to! Yes, there is a brief section where (the lefthand) of the two Westbound M4 lanes and the one Northbound M5 use the same lane, that could be done better, but it really isn't a problem. You have plenty of time to get in the right lane.
''No "stay in lane" reminders, and you finally end up on the small single lane that takes you over the junction to the north.'' But there is a great big overhead sign saying ''M5 North''. Surely that suggests that you need to be in that lane and that lane only for M5 North?
Regarding the A38 part; It's certainly not ideal for sure, but when coming M5S to A38 you only have to move over one lane to get on the slip road to the roundabout (and same with M4W to M5S). You have about 600 metres to do this in. If you want to go North (ie right at the roundabout) on the 38 you don't have to do anything else. (If you want to go South on the 38 then you have to move over again, one lane to your left). In a car this should be an easy thing to do- it's just a matter of adjusting speed and slotting in, something a good driver can do easily. I can understand that trying it in 40 foot artic is annoying or even impossible on occassion, but even then if you couldn't get over into traffic on your left, just turn right at the roundabout, go round and come back in that way.
Failing this, how about sending all A38 southbound traffic off at the next junction instead?