What makes a dual carriageway?

Published on 07 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).

I used that stretch of the A77, never again!

Many memories of that strange beast of the A56/ A556 going back to the 1970s ans80s.
Back then the four lanes started at the traffic lights for Dunham/Bowden, and the limit was fifty down the the M56 Roundabout. Memory is of the limit signs being 60 thereafter, two traffic light controlled junctions, a farm crossing with lights like the old motorway hazard lights and a bit of dual carriageway just before the M6 junction, followed by some miles of the 4 lane single until the Northwich bypass. Plus quite a few unclassified roads joining at T junctions where one did not even want to think about doing a right turn.

There seems to be a jinx on transport links to Manchester from the south making them run out of money for the last bit, which then has to be cobbled up as best as possible with the fumes in the kitty,, so for decades one could drive on the swish M6 motorway, and then have the oddity of the A556/A56 for the last few miles,while on the railway the overhead wires reached all the way but Stockport kept its mechanical signal boxes in the original 1960s rebuild and they were simply given a thorough overhaul during the rebuilding in the early 2000s and remain in use today.

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 ?

I doubt it. "Highway" is, like "carriageway", a term whose meaning is widely misunderstood. Legally a UK highway is simply any public road designed or maintained for general vehicular traffic - as opposed to a Byway which horse-drawn vehicles (and often motor vehicles) are legally permitted to use, but which are not always maintained for that purpose.
Ask most people to describe a "Highway" and they will likely describe something akin to a motorway, largely due to American practice. Its unlikely many would consider that a single-lane dual carriageway would be better described by the term "Divided Highway".

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.

Paul 21 August 2020

There's a short section of the A420 between Swindon and Shrivenham that becomes a single-lane dual carriageway, presumably to help traffic avoid hitting the central pier of the railway bridge under which it passes. There are 'keep left' signs either end but no "Dual carriageway ahead" signs. There is a 50 mph speed limit now but some time ago when I used to use it regularly it was NSL and at the time I always wondered if it was 70 mph for that stretch.

Mitchell 18 December 2020

Can anyone tell me the speed limit on the A442 South of Kidderminster before the 50MPH signs. The sign 100 M before the camera is round white sign with diagonal black line ( 70 mph) the road is two lanes either way separated by a grass strip and partial Armco the camera flashed as I went through at approx 55MPH. Half a mile further on the 50 MPH signs start .

If the sign is a circle with a black stripe, that indicates the National Speed Limit which is, as you say, 70mph. If the camera flashed it was either not working properly or it was triggered by another vehicle.

Not sure where you mean as the A442 there is only a short stretch signed at 40mph.

Floyd 22 December 2020

Years ago I regularly used the A556 from the M56 to M6. The camera(s) at certain points on there could differentiate between cars and hgv's. No end of HGV drivers were caught out as they mistakenly thought it was a dual (so 50mph for them) when in actual fact it was a 40mph zone as it WASN'T a dual (in places anyway), even though it had two lanes. Of course, professional drivers should have known this I suppose, but whether the foreign drivers would/should have is another matter.

Stewart Hill 9 January 2021

The a376, Exeter bound between Clyst st.george & clyst st.mary is always debated where I live . I believe it is a single lane dual carriageway because it has a national speed limit sign at the start and the 2 lanes narrow to 1 send there is a large grass verge between the opposing carriageways..
My old driving instructor 20 years ago told me that it is a single land dual carriageway but so many people say otherwise that I'm starting to doubt my knowledge. Anyone able to confirm this ?
Dropped pin
Near A376, Exeter EX3 0NT

Yes, it’s a dual carriageway. There is a physical divide between traffic in opposite directions, and that’s the only thing that matters. The national speed limit will be 70mph there. 

I think they recently rumbled that one, as now it has a 40MPH speed limit.......

john 17 February 2021

hi all, can someone tell me if they think Devonshire Place, Eastbourne, UK - is a Dual Carriageway or not? I don't think it is!

That's a really interesting edge case - Devonshire Place has parking bays down the middle dividing the two directions of traffic, so the road has a single paved surface but opposing flows are separated by the physical barrier of a line of parked cars.

The local authority seem to consider that it is a dual carriageway, and that the parking bays do not form part of either carriageway, because they've painted single yellow lines down the middle, adjacent to the bays, on both sides.

tricky but the street lights mean it is a 30 anyway so it doesn't matter so much.

Tim Clayton 30 December 2021

If a roundabout appears on a NSL (National Speed Limit) dual carriageway, it still has a 70 mph limit because traffic on any part of its carriageway is unidirectional, no?

Craig Morris 8 January 2022

St Mary’s Way in Chesham is another example of a four lane single carriageway road. However, the lanes are so narrow that it’s practically impossible to use all four lanes unless traffic is queuing.
I can only assume that it was formerly a three lane road that was altered in an attempt to add capacity, without widening the road

Originally St Mary's Way was northbound-only, with southbound traffic continuing to use the High Street making a gyratory system. I don't believe there was a third lane - there wouldn't have been any point in it.

My educated guess is that it was 2 generous (12-13ft) lanes when it was one-way, and a pavement on both sides, with the one on the park side removed to gain some extra width when it was made two-way, allowing the four (very narrow) 7.5ft lanes that are there today.

Farah 18 September 2022

What is the speed limit at the end of dual carriageway???

Charles Stuart 18 September 2022

There is or was an odd bit of road like the Welsh single-lane dual carriageway in Kent, off the A229 or A249. I used to drive on it in the mid 1990s because it led to a rather nice pub. Unfortunately, being the best part of 30 years ago, I cannot remember exactly where this was. Looking at the map doesn't jog my memory but I can say that I used to use the two A roads quite regularly.

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