A36 - A303

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Name
Deptford Interchange

Where is it?

The junction of two busy trunk roads in deepest Wiltshire, where the A36 pauses its journey from Southampton to Bath for a quick chat with the A303.

It was nominated by Taran Blake.

What's wrong with it?

Imagine a junction between two major roads in a rural part of England; two major roads that are both of a patchy standard with fits and starts of dual carriageway and layouts that evolved over decades of gradual change. Imagine how under-powered and underwhelming you'd expect the junction of those roads would be, given the British tendency to build terrible, congested interchanges involving multiple roundabouts. And now imagine hearing that, at some point in the past, someone had provided that junction with two underpasses - two of them - and a suite of free-flowing sliproads and loops and curves. Imagine the wondrous utopia you'd picture. Imagine how your hopes would rise. Imagine that.

Now imagine that they'd installed all those things in such a remarkably and inexplicably weird way that the junction still involved a signalised crossroads, bringing one of the through routes and five of the eight turning movements to a complete standstill.

And now imagine something else before Deptford Interchange gives you a headache.

Why is it wrong?

Decades have passed since this junction was created in its present form, and at this distance in time it is now very difficult to conceive of a situation in which a team of highly qualified professionals could look at a range of potential designs and then choose this one. Presumably the alternatives involved no connection between the two roads at all, or building brick walls across all the approaches, or maybe having the A303 and A36 descend slowly into a lake with crocodiles in it.

A laudable attempt was clearly made to allow non-stop movement between the two roads here, but it has been foiled by the fact that the junction is effectively sited within the hamlet of Deptford, and access needs to be maintained to the houses and buildings along the north side of the A36. No cost effective way of doing that was devised, so the access road meets the A36 at a signalised junction, and so does the off-slip from the eastbound A303 (which is itself the original path of the A303, explaining why it meets the A36 where it does). Once that concession is made, it doesn't matter how many free-flowing sliproads you have - a lot of traffic is going to have to stop at a red light.

What would be better?

In both directions, to the north west and the south east, the A36 rapidly deteriorates back into a single-carriageway road. Until the far-off day arrives when it is upgraded to something more significant, nothing will happen to improve this junction because there is no point, really. In fact, with that in mind, it's not quite clear why the free-flowing trumpet interchange was ever included in the design anyway.

Assuming you wanted to do something, though, the designers appear to have offered a way out of this mess. The link road around the south and east of the junction could have a trumpet junction installed at its western end, where it meets the A303, allowing the two roads to be connected by a link with free-flowing connections at both ends. There seems to be space to do this without much trouble.

That just leaves access to Deptford, which probably requires the closure of the direct sliproad from A36 southbound to A303 eastbound. Removing that provides space for a link under the A36, using the northern span of the existing bridge, which gives the village access to both sides of the A36 without having to cross it on the level, and allowing removal of the lights.

Until the Southampton to Bath expressway becomes a national transport priority, though, we'll probably have to make do with what we've got.

Routes
Region

Right to reply

SurfaceKrystal 9 May 2018

Additionally the at grade connection between Wylie high street and the southern link road is fairly unnecessary seeing as you can also access Wylye from the A303 via the next junction to the south west.

The Wylye turnoff (the pre-bypass A303) just turns the southern part of this junction just complicates things, turning what should be a freeflowing sliproad into a mess of give way signs. Luckily traffic on the A303 heading into Wylye is instructed by the signs to stay on until the next junction west.

I suppose its saving grace is that it's relatively quiet for an interchange between two trunk roads (and only four of the eight turning movements are actually useful, Warminster <=> Andover and Salisbury <=> Yeovil). I've never known congestion here to get bad, unlike the much more underbuilt A303/A350 junction. Before the junction was built this was a staggered crossroads with priority to the A36 and the A303 heading west plunged straight into the narrow roads of Wylye.

Floyd Pitt 11 May 2018

What is really astounding about the whole mess here is that looking at an old map from the 1950's, they had plenty of space to do a proper job instead of the usual British balls-up that costs way more in the long run due to traffic jams/accidents etc (eg like at Catthorpe). A total lack of foresight as usual.

David Collier 11 May 2018

I genuinely like this junction. The part on the north side of the A303 could be a model of how to lay out a cheap, compact junction with higher capacity than a standard roundabout. Admittedly the south side is overly-extravagant and idiosyncratic. But even if it looks a bit funny, there must be dozens of congested two-bridge roundabouts, all over the country, which would flow an awful lot better if they had been laid out like this instead.

Hey, I thought Bad Junctions was for shaming horribly congested interchanges, not just making fun of junctions because they're funny-lookin'!

This junction is honestly fine as is. The A303 is completely grade-separated here (thank heavens) and the A36 is hardly an expressway, there are worse bottlenecks it encounters between Warminster and Salisbury alone than a single traffic light. Based on what it's connecting, this probably "deserves" to be an ordinary roundabout/dumbbell/signalized diamond given its traffic, and it's better than that because it's got at least some free flowing links.

JG Morgan 15 May 2018

One of the oddities of this junction is the differential speed limits on the two sides of the A36.
In both directions the speed limit drops to (I think) 50 mph about half a mile before the traffic lights. It's then back to national speed limit immediately after the lights. So at any point on this short section of dual carriageway, there's a different speed limit on each carriageway.

If you're heading west on A36, the end of the dual carriageway is the only reference point for the unsigned turning into the secret village of Fisherton de la Mere. Don't blast away too fast from the lights, or you'll miss it.

Differential speed limits on the approach to a single junction (or hazard) aren't that uncommon. You have to drop to 50 some way in advance of the junction but there's no point in keeping the 50 beyond the junction, so that's what you end up with.

I just took a drive around Fisherton (on google street view). What a great place! I would love to live there.

Gary L 21 May 2018

I once worked on a signals improvement maintenance scheme here. The main reason you haven't got anything better, I believe, is that neither A303 or A36 are particularly busy at this location. Sure, it's a weird layout that could leave a lot to be desired, but by and large it does work. Much better than M4/M32/A4174, which really was a bugbear of mine during my time in Area 2.

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