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...