From Roader's Digest: The SABRE Wiki
The A40 is the only 2-digit road to define a zonal boundary in its own right, as distinct from multiplexes like the A1/A41 and A1/A57, although the (3-digit) A900 in Edinburgh defines the Zone 1/9 boundary. As far as St Martin’s le Grand, where it meets the A1, the A40 used to define the boundary between zones 1 and 4.
Section 1: City of London - Marylebone
With the declassification of most roads in the City of London, the A40 now officially starts at St Martin's le Grand.
After Holborn Circus the A40 continues along Holborn, High Holborn and Oxford Street, to Marble Arch although only the upper classes, (in their taxis), and lower classes (in their buses) are allowed to use the Oxford Street section. The middle classes in their own cars take the A5204 (better known as Wigmore Street).
Like the A3/A24 in south London, the straight alignment of this road, all the way from the Bank to Shepherds Bush, made it ideal for building one of the earliest deep- level tube lines in London, the Central line, without sharp bends or risking undermining foundations. However, because the road is quite narrow, the tunnels in places are built above each other, rather than side by side. This is best seen at Chancery Lane station.
Until demotion of the A40(M) to A road status, the actual A40 continued from Marble Arch along Bayswater Road and Holland Park Avenue to Shepherd’s Bush, (the present A402), and then up Wood Lane past the BBC to join the present route at Western Avenue. Even further back in history, it continued from Shepherds Bush up the Uxbridge Road (the present A4020) through Ealing and Southall to Uxbridge. Now, it disappears at Marble Arch, presumably multiplexing with the A5 on the Edgware Road before somehow reappearing as the Marylebone flyover (which has no direct access from the Edgware Road).
Section 2: Marylebone - Denham
From the Marylebone corner the A40 rises up onto the Westway, a major piece of engineering which was completed circa 1969. It's one of the rare pieces of (ex-)motorway in the UK which travels for a fair distance on stilts, the M6 around Birmingham probably being the only longer section. There is also a maze of roads situated under the Westway here making up part of the A404. On your left travelling west you pass the Metropole hotel and then the Paddington on-slip joins from the left. This road leads from Lancaster Gate and the Bayswater Road (the old A40). This slip, and the equivalent off-slip, cross the GWR by large girder bridges, the on-slip joining the westbound carriageway being accessed through residential streets! The A40 continues, taking a gentle left hand bend where the Hammersmith and City line travels alongside, though on a slightly lower elevation. Travelling eastbound on this stretch, one can see a somewhat unusual (and almost certainly incorrect) sign halfway along the viaduct; an 'End of Red Route Clearway' sign ... on the middle of the Westway!??
The Westway ends as a flyover over the A3220 (formerly M41, part of the Motorway Box) and Wood Lane (A219: formerly A40), running into Western Avenue, which was originally to have been called the A403. This section is 3 lanes wide until you reach the next junction, Western Circus, a traffic light controlled junction which is snarled up almost constantly. The road now narrows back down to 2 lanes again. In the mid-seventies, the A40 was infested with roundabouts and traffic lights all the way to the motorway, and now this set, along with Gipsy Corner (A4000; Horn Lane) are all that remains. An abortive road widening project, with flyovers over the Western Circus and Gipsy Lane junctions, was stopped only after all the houses had been demolished -– the road now runs through a swathe twice as wide as it is.
We are climbing slightly on this section and we cross the GWR again at the top of the hill. We then drop again at the other side to more evidence of house clearance at the Gipsy Lane junction. The A40 now returns to its 3-lane status and makes the journey up towards Park Royal. Swinging to the right, we now encounter the Hanger Lane Gyratory system. The A40 definitely has the best of the junction as it passes right underneath. Despite this major flow being taken out of the way, the Gyratory system is still a major congestion headache for any other flow using it.Hanger Lane junction was remodelled in the 1980s, supposedly to cope with modern traffic levels, including the construction of the northbound limb of the gyratory system, making the tube station an island. Previously there was a simple crossroads at the junction, with the (1960s?) underpass passing underneath. People say this is a badly designed junction, but as with most old roads it wasn't built with the idea of today's traffic flows using it. The A40 dives under the junction in a 2-lane underpass. Past the Hanger Lane junction we return to 3 lanes and pass the Vanguard building on the right hand side, which on occasion has had a Harrier Jump Jet perched on its roof!!
The A40 now enters the visibly improved section up towards the M40. We then pass the B452 (ex B456) junction, where the A40 again dives under the junction. The old T junction can still be seen just before the large pub and petrol station on the left. We then go under one of the suburban railway lines and over the A4127 junction at Greenford. The next junction is the "Target Roundabout" for the A312, the junction is so named because of the pub which stood at the side; it's now a fast food drive-in!
The original route of the A40 carried straight on here and up to a traffic light controlled junction with the A437 at Hillingdon Circus. This is the most recently improved section. The road now swings off to the right and under the A437 and Metropolitan line under a unique double bridge: the road passes under the railway, which is in turn crossed by the A437. Hillingdon underground station was also rebuilt as part of the scheme.
The A40 rejoins its original alignment and starts the small climb to Swakeleys Junction at Uxbridge. We now go back downhill on the viaduct over the Colne Valley and Grand Union Canal, and on towards the M40 junction. This junction, where the old A40 (now the A4020) rejoins, is a "magic roundabout" style one, where you are allowed to go round the "wrong" way. The original line of the A40 can still be seen going straight through the middle.
Section 3: Denham - Oxford
After successfully navigating the junction, you then find yourself briefly on a dual carriageway courtesy of the A413, as this is the main route to Amersham. The first half mile or so is actually a multiplex with the A412 (a popular route for avoiding trouble on the M25, with which it runs parallel), which soon turns off towards Rickmansworth, and the A413 parts company soon after.
The road becomes single carriageway again shortly after this and crosses the M25 (there's no junction). We go through the small town of Gerrards Cross and journey on towards Beaconsfield. We encounter the A355 link road to the M40 junction 2 here. We venture on into the picturesque town centre and its famous model village. On the other side of Beaconsfield there is a section which gently bends to the right where the M40 passes very close to the road. This is the point of a temporary terminus of the M40 before it was completed to junction 2 and on to its end—the M40 ended at a roundabout here. Past here the road makes its way downhill towards High Wycombe. The next junction is with the M40 again (J3) and the A4094. There are good views of the Loudwater Viaduct on the M40 here.
The road carries on through the eastern suburbs of High Wycombe until we reach the next "magic roundabout" junction with the A404, which joins us from the Handy Cross junction of the M40 via a very steep hill! This next section of short dual carriageway is a bypass for the centre of High Wycombe. We then make our way up through the western suburbs, the road being fairly wide here. At the next roundabout, the A4010 junction, the road swings off to the left into the pretty village of West Wycombe.M40 yet again at junction 5 at the top of the Aston Hill cutting. This section of road is 3 lanes here as it makes its way down the steep hill.
We then encounter the prehistoric Icknield Way (B4009) at Aston Rowant. We carry on up through the villages of Postcombe and Tetsworth, the M40 always in view on our left hand side. We enter the small village of Milton Common and the Three Pigeons junction (named after the pub here) with the A329 and its junction with the M40 (J7). The A40 carries on, about a mile after the A329 junction the M40 used to end before the Birmingham extension was finally built after decades of talk—now this is the point where the M40 changes allegiance, forsaking the A40 for the A41 and A34. The A40 now heads down and to the right to the junction with the A418 Aylesbury road and loses its grand status, the A418 taking the number priority.
We follow the A418 past junction 8A of the M40 and the Oxford Services and then round the loop of the Sworford junction and back onto the A40 dual carriageway, which is the Wheatley bypass. This section, although an A road, had its own hard shoulders until the mid 90s when it was reconstructed. At the end of the Wheatley bypass there used to be the junction with the infamous "double number" B4027, but this is only available to eastbound traffic now as the gap has been closed. We now head up towards Oxford itself with the section with a bus lane, similar to the M4 bus lane. We join the ring road at Headington Roundabout.
Nowadays, the A40 turns right at Headington onto the Oxford Northern bypass. This section was completed in the 1930s as a single carriageway suicide lane road, and the second carriageway was added in the early 1970s. At Summertown roundabout we lose the dual carriageway once again and go on to Wolvercote roundabout, which is a junction with the A44 (which was the A43). This also carries the ring road off (to the right!) to the complex A34/A44 junction before crossing over the A40 at Wolvercote without a junction.
Section 4: Oxford - Cheltenham
The A40 passes over two stone arch bridges towards Cassington and Eynsham. The original line of the A40 joins just after Eynsham, and we journey towards Witney. The road bypasses the small hamlet of Barnard Gate on the right and then joins the dual carriageway Witney bypass, which was built in the mid 1970s. The first junction is with the B4022 which is the original route into the market town of Witney, which is famous for the old blanket factory which used to be there. The next junction is with the A415 which heads off towards Abingdon. From here we encounter the junction with the Brize Norton road. The huge air base can be seen off to the left. The last couple of miles of the Witney Bypass end at the roundabout with the B4047, the original line of the A40.
The next section through the Cotswolds to Cheltenham is largely unimproved, notable only for the Northleach roundabout (crossing the A429 Fosse Way) and the long winding three-lane section into Charlton Kings. We travel across the top of the Windrush valley with its views to our right, and soon enter the outskirts of Burford. The road would have once passed through the town but was bypassed long ago, probably during the 1920s or 30s. The long A361 crosses here on its long trek across southern Britain and the Cotswold Wildlife Park is a short drive down the A361 towards Lechlade. We now head off into the Cotswolds proper, passing the junction with the B4525 (ex A433) towards Bibury and Cirencester. We carry on, passing the remote "Inn for All Seasons" hotel and RAF Rissington. A few miles past here, at the point of two old derelict petrol stations, are some signs almost constantly covered which show the road being closed due to snow.
We now reach the start of the Northleach bypass, completed in 1984. Northleach is a picturesque Cotswold town which deserved its bypass long before it got it. The A429 Fosse Way crosses halfway. There is a short section of dual carriageway just before the drop down to the Frogmill junction with the A436. A40 through traffic to Gloucester can avoid Cheltenham by turning off onto the A436 (which has a short spur to cut the corner off).
The A40 now continues to Andoversford which is also bypassed, even though it's hard to tell. If you enter the village you can see the old line of the A40. From here we take the trip down the hill and past Dowdeswell Reservoir into Charlton Kings. We pass through here and towards Cheltenham itself. The A435 joins us from the left here. Cheltenham doesn't seem to have any direct routes through it, the A40 taking what must be 3 different ones! We travel out of the western side of Cheltenham past the GCHQ "listening post" building to our right. The road now bypasses Staverton and other villages via the Golden Valley bypass, which is a high-grade road with a 3-level junction with the M5 (J11).
Section 5: Cheltenham - Ross on Wye
Gloucestershire’s county town keeps its close neighbour Cheltenham in its place by denying it a proper bypass. Gloucester’s own bypass takes the A40, multiplexed with the A38 and A417, round the north of the city and then, (without the A38 which has struck off to Tewkesbury) across the Severn. The A417 then strikes off north to Ledbury and Leominster, and shortly after the A48 strikes off south to Chepstow and South Wales.
Between Gloucester and Ross through traffic is recommended to take the M5, M50 dogleg to avoid this unimproved section skirting the Forest of Dean. Ross is bypassed to the north, the A40 now meeting the A449 and multiplexing with it to Monmouth.
Section 6: Ross on Wye - Brecon
The present route is essentially an almost-motorway standard extension of the M50 to meet the M4 at Newport, passing along the Wye Valley past Symonds Yat and crossing the border into Wales just short of Monmouth. In Monmouth the headland between the Wye and the Monnow is pierced by a short tunnel. At Raglan the A40 and A449 part company, the A40 striking west to Abergavenny to meet the Heads of the Valleys Road (A465).
From Abergavenny the A40 strikes north west into the Brecon Beacons National Park, following the valley of the River Usk. In spite of the hilly surroundings, the road itself remains reasonably flat. Beyond Crickhowell the A479 forks to the right, heading for Builth Wells. Our road, however, continues on to Brecon, a Cathedral City in the heart of the National Park. We deviate from the River Usk, passing to the north of Buckland Hill through the village of Bwlch, before rejoining the river at Llansantffraed.
On the final approach to Brecon, the A40 becomes dual carriageway, meeting the main North Wales to South Wales artery (the A470) at a roundabout on the edge of town. The original A40 passed through the centre of Brecon but now, in tandem with the A470, the A40 follows the dual carriageway bypass to the south of the town.
If the weather is kind, you'll see that the road passes through the edge of the main part of the Brecon Beacons. More frequently, however, the cloud is such that whilst you get a feeling of being amongst the hills, you cannot see their true majesty, and you have to allow your imagination the chance to see what is there instead.
Section 7: Brecon - Fishguard
At the next roundabout, we say farewell to the A470, which passes through the heart of the Beacons on its way to the Valleys communities and to Cardiff beyond. The A40 continues its relentless push westward, once again following the Usk Valley. The road is very rural in character - tight bends, lots of peaks and troughs in the road, making overtaking difficult, if not impossible. Unlike the valleys of the South Wales coalfields to the south (which are littered with ribbon towns), this area is green - the fields are filled with sheep - the ground is too steep to do much else. Standing at the lay-by on this stretch, it is difficult to imagine that this is the same A40 that six lanes of traffic thunder along at Greenford in West London.
At Sennybridge there was an unusual feature for a largely unimproved rural road. There was a partially grade-separated junction with the A4067, which leads via the head of the Usk Valley into the Swansea Valley, and to Swansea itself. The Swansea to Brecon railway crossed the A40 at this location, but was closed under the Beeching closures. After the railway closed, the A4067 was built on the former line to take the traffic away from the village. When the A4067 reached the A40, the old railway bridge was used as a GSJ for eastbound traffic heading towards Brecon. This arrangement remained in place until the late 90's, when the old railway bridge needed extensive repairs to remain usable, so the decision was made to dismantle it and make it a normal surface junction.
We continue west through a much narrower valley to Llandovery, where we meet another north-south artery - the A483. Beyond the town, we follow the Towy valley, picking up the A482 at Llanwrda then through to Llandeilo. Having passed through the narrow confines of the Brecon Beacons, the road now enjoys a much broader outlook. The Towy Valley is wide and relatively flat along the bottom, allowing more freedom in choice of route for the people who first walked through this valley.
To the north east of Llandeilo a roundabout marks the point where the A483 leaves our route, passing through the town itself, then heading south to the end of the M4 at Pont Abraham. On the other hand, the A40 passes to the north of Llandeilo, passing the north side of the national Trust owned Dinefwr Park estate before continuing to Carmarthen.
The road network in Carmarthen is now unrecognisable from twenty years ago. Carmarthen was once a bottleneck as all traffic competed to pass round a gyratory near the railway station. For many years there was a lifting bridge on the west bound side of the gyratory, but that is no longer there. These days, a series of bypasses take the A40 to the south of the town, where the A40 meets the A48 again, a dual carriageway running as an extension of the M4.
This section of the A40 was dualled in the 1980s. Most of the upgrade was on line, but a new alignment was built around Bancyfelin, eliminating a very nasty S bend under the railway bridge. The A40 dual carriageway continues as far as St Clears, where it splits, the A477 going left for Pembroke (this is the original line of the A40), and the present A40 going right for Whitland, Narberth (where the original route of the A40, now the B4314 rejoins) and Haverfordwest. At St. Clears the new bypass eliminated yet another bottleneck of a narrow bridge.
From here to Haverfordwest, the road has been improved - there are many examples of new alignments, the biggest of which is the Whitland bypass - still only single carriageway, though. On the way, we pass Canaston Bridge, where we cross the eastern branch of the River Cleddau. Downstream, the river provides deep tidal water, which, at Milford Haven, has been exploited by the Oil companies in search of sites to build refineries. They are a blot on the landscape but they bring much needed employment to this part of South Wales.
At Haverfordwest the obvious straight ahead route is actually the A487 to St Davids. The A40 makes an abrupt turn northwards to carry the main traffic flow, which is to Fishguard for the ferry to Rosslare. A look at a map of Haverfordwest will show you the old alignments of the A40. Before the northeastern bypass was built, the road ran along the Fishguard Road, some of which is itself a newer alignment. Current Ordnance Survey maps still label Fishguard Road as A40, which suggests that Fishguard Road is now a spur of the A40.
Having spent the last 280 miles heading in a westerly direction we are now heading north along the Western Cleddau valley towards Fishguard. The last section, along the harbour side (The Parrog) is actually a multiplex with the A487. From here it's back along the A487 to St Davids or across the Irish Sea to Rosslare.
Original Author(s): Simon Davies, Tim & Jonathan