What makes a dual carriageway?

Published: 7 November 2018

Road enthusiasts get a bit itchy about this. Two lanes each way - that's a dual carriageway, isn't it? To which the answer is: no, absolutely not.

Certainly lots of roads with two lanes each way are dual carriageways. And lots of dual carriageways have two lanes each way. But to be sure what constitutes a dual carriageway, we need to pick it apart a little bit. In a moment, we'll also take a short trip to Wales to see one of the UK's most bizarre dual carriageways, and perhaps see the true essence of dual carriagewayness.

Maybe the question we should start with is this one: what's a carriageway?

What's a carriageway?

Excellent question.

A carriageway is a paved surface for wheeled vehicles to travel on - literally, a way for carriages. A carriageway can be as wide or as narrow as necessary, and its width can be (but doesn't have to be) divided up by painted markings into multiple lanes. A lot of people think the "dual" in "dual carriageway" refers to two lanes side by side, but it doesn't.

So: a single carriageway road is a road made up of one paved surface. A dual carriageway road has two separate paved surfaces side-by-side, with some sort of physical divider or barrier between them.

What does it matter? Well, it becomes a bit more interesting when we look at a couple of examples. I promised you a trip to Wales a moment ago, and you will have one, but we're going to pop in to Birmingham first if that's OK. Let's go to Aston.

What's in Aston?

Why, the A38(M) Aston Expressway, of course.

The A38(M) Aston Expressway, with at least three lanes each way and sometimes more. Click to enlarge

The A38(M) Aston Expressway, with at least three lanes each way and sometimes more. Click to enlarge

The Aston Expressway is a tidal flow motorway, so it normally has three lanes each way, but electronic signals mounted over the road are used to operate four lanes one way and two the other during each rush hour. For that reason, it's a motorway without a central reservation.

The result is that the A38(M) has three lanes each way (or, sometimes, four one way and two the other), but it's still just a single carriageway road, because it has one unbroken paved surface.

Aston is home to the most extreme example of a single carriageway. To go the other way - and see a dual carriageway that looks nothing like a dual carriageway - we will make our long-awaited trip to Wales, and specifically to the village of Llywel.


The church at Llywel. Click to enlarge
The church at Llywel. Click to enlarge

Llywel. It's in the Brecon Beacons, lying just off the A40 between Sennybridge and Llandovery. It's at the foot of a steep hill, at the top of which is an expanse of moorland crossed by a military road that travels to the even more remote village of Tirabad.

The main thing in Llywel is a stone church. In fact, virtually the only thing in Llywel is a stone church, the rest of the village comprising two farms and about three houses.

Leading away from the church is a public road up the hill, which leads on to the military road, and - against all expectations - it's a dual carriageway.

The military road, built on relatively open moorland, is a conventional two-lane strip of tarmac, but the local road leading to it started life as a single-track country lane meandering its way between the fields. Whoever was given the job of widening it, presumably at the time the military road was built and in anticipation of it carrying lots of heavy army vehicles, decided not to make the existing road wider, but instead to put a new single-track lane alongside it. Perhaps that was thought easier, or perhaps it was done so that people wouldn't be tempted to try overtaking a platoon of slow army trucks grinding up the hill.

Whatever the reason, Llywel is home to the unusual spectacle of a single-lane dual carriageway stretching for over a mile up the hillside towards Tirabad.

Keep left, dual carriageway. There's no arguing with that. Click to enlarge

Keep left, dual carriageway. There's no arguing with that. Click to enlarge

It's indisputably a dual carriageway for three reasons.

  1. There are, physically, two road surfaces laid side by side with physical separation between them.
  2. There's a blue "keep left" sign at each end, which is the legal requirement for a dual carriageway to exist.
  3. The signs say "dual carriageway" on them in two languages.

Among other things, that means the speed limit on it is 70mph - as if such a thing were remotely possible here.

Let's take a short journey down the hill from the end of the military road back to Llywel.

All of this is the long way round to make the point that a dual carriageway is all about physical road surfaces and not about the number of lanes, and it's not nearly as prescriptive a term as some people often think.

That is why engineers and people writing about roads in a professional capacity will use more specific language, like describing a "two-lane dual carriageway" or "dual three-lane carriageways". It's why roads like the A556 west of the M6, that have two lanes each way but no physical barrier down the middle, are subject to the single carriageway speed limit of 60mph and not the dual carriageway limit of 70.

And it's why, if you go to the Brecon Beacons, you should definitely make a trip to Llywel.


George L 7 November 2018

So is the speed limit on the Aston Expressway 60, or does it being a motorway override that?

Good question! Motorways have their own speed limit regulations and they are always 70 unless signs say otherwise. On the Aston Expressway, though, it's academic because it has a 50 limit.

Gregg 8 November 2018

The road in Cheshire you're thinking of is now the B5569 and has been partially narrowed to one lane each way.

But even when it was part of the A556, the speed limit was 50, not 60, all the way from the M6 to the M56. I can't actually think of any 4-lane single carriageways that are at the national speed limit - they mostly tend to occur in urban or semi-urban environments with a 30/40/50 limit.

The road in Cheshire I'm thinking of is still there! The four-lane A556 between the M6 and M56 has been replaced with a new dual carriageway, and the old road narrowed down as you say. West of the M6, though, it's still there, and has a 60 limit.

I've amended the text of the article to make this a bit clearer.

Prior to 2005, the A77 had a very long stretch of four-lane single carriageway at NSL, but this has now been replaced by the M77 and the old road downgraded.

A short stretch of rural A6 between Penrith and Carlisle (just north of Low Hesket) is still four-lane single-carriageway, with national speed limit.

One memorable and notoriously nasty exception being the A77 between Glasgow and Fenwick (now bypassed by M77).

Harry 8 November 2018

Outside my old school there is a (technically) dual carriageway with appropriate signage, and being outside the school the speed limit is 20! Almost the opposite to the Llywel example.

Anonymous 8 November 2018

Would the term "divided highway" and "undivided highway" be more understandable ?

Jonny 8 November 2018

Would it be prudent to mention the 30mph and 20mph “dual carriageway” through residential streets near St Helier station in Surrey? It’s called Green Lane.

Toby Speight 9 November 2018

Dell of Inshes Road, Inverness is another road comprising dual one-lane carriageways. It's not anywhere near as long as the military road at Llywel, and is subject to a 30mph limit, but has obviously started as a single-track road and had another added alongside. My guess at the most likely reason here is wanting to keep the mature trees that lined the road (one row of which now forms the central reservation).

Terry Trumpets 11 November 2018

A very brief section of the A849 on the Isle of Mull is D1. When the road between Craignure and Salen was widened to S2, the existing bridge across the water near Pennygown and Glenforsa was not widened; rather, it was made one-way for westbound traffic only, and another bridge was built next to it to carry eastbound traffic.

Given that dual carriageways are rare enough in the highlands and isles, and most roads on Mull are S1, that was quite a surprise.

Thomas Walker 12 November 2018

A very interesting new one lane each way dual carriageway has just been built on the edge of Aylesbury here.

Martin B 13 November 2018

There's a similar, albeit much shorter single carriageway called Pilgrims Way just outside Eccles in Kent. I once had the pleasure of having to squeeze past a broken down vehicle in my 18T lorry along there!

Damian 22 November 2018

Sorry, published that anonymously and I imagine its what Martin B was referring to? Its got 40MPH limits now though.

Garry Brown 25 November 2018

So any stretch of D1 will have a 70 mph limit unless explicitly marked with a lower one?

Edward Joslin 25 November 2018

Interesting bit of dual carriageway on Mongleath Road in Falmouth, Cornwall.

Provides access to Penmere Manor Hotel, but only from one side of the carriageway!
Central reservation lined with very mature trees.
I believe the reason for making into a dual carriageway was the building of the new (1970's!) houses to the south west. Penmere Manor side was the original old single track road. This dual carriageway has 30mph and speed bumps along its entire length!

David Southwell 5 December 2018

I would also like to add about the A329(M) which stretches between Reading and Bracknell. It has a strange and interesting history filled with the hope of motorway mania in the 70s. It was intended to be part of the M30, a motorway never built, giving a bypass of the M25, stretching from the A1 all the way up to the M40. The A329(M) used to be two lanes along the whole way being a motorway by definition, but having that slowly clawed away from it with an end section near Reading being downgraded to the A3290. However now there is stretch between Winnersh and Wokingham which now after conversions to supposedly improve accessibility to the M4, it only has one lane going in each direction, however the speed is 70 and it is classified as a motorway. Despite only having in total 2 lanes in total on one stretch.

Wasn't the A329(M) supposed to be part of the M31? You might have confused yourself with a different motorway plan.

Martin R 9 December 2018

Then there is the A601(M) near Carnforth, a motorway spur from the M6 with a 70mph limit but it is a single lane single carriageway road (that is, one lane only in each direction with no physical separation). It has a mixture of blue motorway signs and red non-motorway signs, such as the "A601(M)" start of motorway sign adjacent to the "two-way traffic" sign seen here:

A601(M) google streetmap

Anonymous 10 March 2019

Haha! I use this ‘dual carriageway’ most mornings

Pat Kemp 11 April 2019

Nesscliffe bypass (A5)
4 lanes of traffic, 2 in each direction divided by a wire. I take it this is a dual carriageway?
There is no dual carriageway signage at either end of the stretch. 60 or 70 mph?

It has a physical divider between the two directions of traffic, so it's a dual carriageway.

Hannah 2 August 2019

I am studying for the theory test and this was most informative. Thanks for the trip to wales!

The number of carriageways is irrelevant with regard to speed limits. If there are street lights it’s 30mph UNLESS there are signs indicating otherwise. So you could have four lanes, separated by a central verge and it would still be 30mph if it had street lights. Take them away and it’s 70. Motorways are the only road type that have streetlights and a 70mph speed limit (unless signs reduce this). All limits refer to cars - other vehicles such as vans (weight dependent ) are reduced by 10mph on certain roads.

I don't see how you can say the number of carriageways is irrelevant. It's certainly irrelevant if the road has streetlighting or a specific local limit applies to it, but if an all-purpose road is derestricted, then the limit will be either 60 or 70mph and that will depend on whether it is single or dual carriageway.

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