It's probably more standard practice to start from London, but this article has been written in the opposite direction to make it more enjoyable! Historically, of course, the A4 started in Bath, running into London - the section from Avonmouth to Bath was originally part of the A36, but renumbered as the western end of the A4 in 1935.
Section 1: Avonmouth - Bath
The A4 starts out as a trunk road (the fairly new Avonmouth Relief Road) at a roundabout near Avonmouth Docks, where the only other numbered road is the A403 to Aust. Almost immediately there are two roundabouts which connect with a short spur of the M5 (meeting the M5 proper and the M49 at junction 18). After that, the road becomes the Portway, which runs as partly dual carriageway and partly S4 up the Avon Gorge. It is considered by some as one of the country's great drives and it's a very important link for Bristol - when it was closed a few years ago due to a burst water main there was congestion for miles around. It crosses the River Trym, runs beneath cliffs on the edge of Clifton Down and under the Clifton Suspension Bridge, where the view is breathtaking. Along here there's a section where the cliff edge was so dangerous that a special tunnel was erected to protect the road.
At the eastern end of the Portway there's a complex junction with the A3029 at the Cumberland Basin and the A4 runs round a one-way system at Hotwells to become Hotwell Road, which runs alongside the Floating Harbour - here you can see across to the SS Great Britain. Then it's Anchor Road in Canon's Marsh, which runs past the new "@Bristol" exhibition centre, and it arrives at the area traditionally known as the Centre, where traffic has been recently re-routed to go round a big concrete piazza. At this point there's actually a short break in the numbered route - what would once have been the A4, running along Baldwin Street, Bristol Bridge and Victoria Street, is now the B4053.
The A4 officially picks up again at the Temple Circus Gyratory, a fairly new junction replacing a notorious old single-lane flyover. It runs past Temple Meads Station, and round Bath Bridge roundabout , where it meets the A370 on a roundabout which spans the River Avon. The road then crosses the railway line and comes to a traffic-light controlled fork called the "Three Lamps" junction, where the A37 turns right.
Continuing west along Bath Road there's then a section past Arno's Vale Cemetery opposite which the A4320 heads north, and so up Brislington Hill to reach the junction with the A4174 outer ring road (southbound). The two roads then multiplex as far as the Hicks Gate roundabout, where the A4174 northbound departs once more. This is where the A4 becomes proper Dual Carriageway once more, at the start of the Keynsham bypass. This fairly old dual carriageway runs remarkably close to the centre of Keynsham - the railway station is actually beyond the bypass! The A4175 and B3116 now take the old A4 route through Keynsham.
At the end of the bypass the road reverts to single carriageway at another roundabout and passes through Saltford, with a section running close to the railway line and River Avon. The A39 joins at the Globe roundabout, then there's another short dual-carriageway section which takes you to a set of traffic lights at the western outskirts of Bath, where the road forks into two. This is a TOTSO; the A36 continues straight ahead as primary, while the A4 forks off to the left as non-primary (the signs attempt to disguise the fact that it's the A4). It heads over New Bridge and into the suburb of Newbridge, then through Lower Weston where it's joined by the A431, an alternative route from Bristol. It continues along Upper Bristol Road, with the A3604 crossing Windsor Bridge to link with the A36. Soon after, Royal Victoria Park appears on the left and the road heads into Bath's most famous area - the Georgian squares and crescents, many of which were designed by one of the John Wood's. The Elder Wood's Queen Square is first, now acting as a 'squareabout' and then left up Gay Street which continues to the Circus. The A4, however, turns right along George Street and through traffic lights at the bottom of Landsdown Hill to the Paragon. At the end of the road, a mini roundabout connects to Walcot Street, possibly still the A3039, and the A4 becomes London Road, which is soon joined by a spur from the A36, where it once again becomes a primary route.
Section 2: Bath - Newbury
It continues along the straight London Road through the eastern suburbs of Bath to Lambridge, where there's now a large signal-controlled roundabout with an underpass. This is the start of the Batheaston bypass. The road running north-south beneath the roundabout connects the A46 Swainswick bypass (which goes towards the M4) with the Batheaston bypass. This means that traffic coming from Bath wishing to continue on the A4 has to go three-quarters of the way round the roundabout, past an awful lot of traffic lights. It then runs down a slip road to join a useless dual carriageway with monstrous concrete bridges spanning the Avon. There was originally supposed to be a link road to the A36, but this was cancelled for political reasons - partly due to the toll bridge at Bathhampton, and so all the traffic just goes back over the Avon again to another roundabout at Bathford, where the dual carriageway ends. The A363 turns right on its way to Bradford on Avon, while the A4 continues straight on.
The road then soon crosses into Wiltshire, and continues via Box, where it meets the A365 and Corsham towards Chippenham. Just before Chippenham it crosses the A350, and becomes non-primary once more - indeed it stays non-primary from here until Slough, presumably because of the M4 running parallel. From the Bridge Roundabout in Chippenham, the A4 follows the route of the Avenue La Fleche, bypassing the town centre, and then Pewsham Way, another purpose-built route avoiding the eastern suburbs. It rejoins its original course on the eastern outskirts of the town, then passes through Derry Woods, where the A342 branches off, on its way to Calne. The northbound A3102 to Royal Wootton Bassett and Swindon leaves at a roundabout on the western fringes of Calne, and the southbound A3102 towards Melksham leaves at the centre. Passing via the intriguingly named Labour-in-Vain Hill, it continues eastwards to the roundabout at Beckhampton, not far from Avebury Stone Circles. Access to this site is provided via the northbound A4361 (formerly the A361) to Swindon, while the A361 with its original number proceeds southwards from the roundabout towards Devizes.
This section of the A4 is largely unmodified, with sensible speed limits, and typical open chalk downland scenery until reaching the Kennet Valley. It passes the chalk mound at Silbury Hill, part of the complex of Neolithic monuments in this area. After passing through the villages of West Kennett (junction with the B4003) and Fyfield, it enters the town of Marlborough through an archway forming part of Marlborough College. Taking a turn to the left as the A345 to Pewsey continues straight ahead, it then becomes what is reputedly the broadest High Street in England. The High Street's central strip is mainly used for car parking, but is occupied by market stalls on certain days. Eastbound and westbound traffic is separated by this strip, effectively creating two one-way streets.
The A4 yields to the primary A346 for a short distance after this - modern classification practices having no respect from the historic numbering system - and then continues out of the town into Savernake Forest. The stretch through the forest and beyond is reasonably straight, leading to the village of Froxfield and then over the Berkshire border to the town of Hungerford. This is an attractive little town, unfairly remembered for the tragic shooting incident in 1987 that brought it to public prominence. The junctions with the B4192 (formerly A419) from Swindon, the southbound A338 to Burbage, and the northbound A338 towards the M4 and Wantage all occur within a short distance.
Leaving Hungerford, the A4 runs parallel to the River Kennet, the West of England railway line and the Kennet and Avon Canal. Passing through the hamlet of Halfway, it proceeds on to the junction with the A34 Newbury bypass. Two roundabouts provide access to the A34 sliproads, which all lie to the north of the A4 - traffic heading north towards the M4 and Oxford leaves at the first roundabout, while traffic heading south towards Winchester and Southampton turns left at the second roundabout and then back over the A4. The road then passes into the western suburbs of Newbury.
After passing through the town centre, though, traffic needs to negotiate the original Newbury bypass - now numbered as the A339, it now connects Basingstoke to the south with the A34 to the north. The arrangement of roads is very curious here, with northbound A339 traffic joining the flyover from inside the roundabout, and there is no way of turning directly left onto it.
Section 3: Newbury - Reading
Coming out of Newbury the A4 passes through the town of Thatcham, still running parallel to the Kennet and Avon canal. At Aldermaston Wharf the A340 from Basingstoke joins from the south, and then the road becomes dual carriageway for a short distance. At the next roundabout the A340 leads northwards to Pangbourne, and the A4 once more diverts slightly from its traditional route onto the Theale bypass. The old A4 through Theale is no longer a through route owing to the construction of the M4, and so the A4 has a dual-carriageway section running slightly to the south. This terminates at a roundabout where another short section takes traffic to junction 12 of the M4 - the first time that the A4 has met its sister motorway coming eastwards, though their paths will cross several times from now on.
This marks the approach to Reading, and on the other side of the junction the so-called "Calcot Coachway" (in reality a few bus stops in a supermarket car park) provides an interchange between long-distance coach routes and local bus routes - one of only a handful of similar interchanges based around motorway junctions. The A4 alternates between dual and single carriageway here, and continues along Bath Road towards the city centre. However it's not actually possible to drive eastbound through Reading on the A4, thanks to the town's innovative and highly infuriating one-way system.
To the south of the town centre the A4 runs in a dead straight line as London Road for about a mile, but only westbound traffic has the privilege of using this short cut. Eastbound traffic has two choices after passing through the suburbs of Calcot and Southcote—either turning right into leafy Berkeley Avenue (the real A4) or continuing straight on down Castle Hill (the A4155). Either route eventually sees the driver sucked onto the infernal Inner Distribution Road, officially the A329. Not quite the road to hell, but the road to the Kings Road Gyratory, which is almost as bad.
Survivors must then endure a half mile crawl along Kings Road before rejoining the A4 proper at Cemetery Junction (watch for the amused smiles from traffic heading west). At this point the ordeal is almost over; just a short, traffic-light-punctuated journey through New Town before an escape route is offered in the form of the A3290 (previously the A329(M)) towards the M4. Often a tempting proposition, but by now the worst is over, the road widens and the driver is safely on their way eastwards towards Maidenhead.
Section 4: Reading - Heathrow
The A4 becomes surprisingly light through Reading's eastern suburbs, helped by being a dual carriageway. The A4 then becomes single for a short while and then dual again as it passes near to Sonning.
From Sonning to its junction with the A404(M), the A4 becomes essentially rural in character (well, rural for SE England). If the traffic clears, some of the bends can bring a smile to the face, but nothing to put on a "Top 10" list.
A fairly big roundabout marks its junction with the A404(M)) and the A404. Turn north here for High Wycombe and the M40 or south for the M4. The A4 (still single carriageway) becomes a suburban road, through to Maidenhead town centre, where it widens to a dual. You then have to negotiate 4 roundabouts before crossing the Thames. This bridge is always something of a bottleneck, as it is a single carriageway.
The trek to the outskirts of Slough from the Thames takes about 4 miles along a single carriageway road, passing through fields and suburban houses. This stretch can be a nightmare on Sunday afternoons, due to a car boot sale in one of the fields.
Another big roundabout marks entry into Slough. You can leave the A4 here for junction 7 of the M4. The A4 now widens into 2 lanes each way, though not strictly dual as there is no separating barrier. Instead of roundabouts, you now have to negotiate untold traffic lights to make your way into Slough town. Very frustrating. But at least you can admire the huge modern office buildings along the way, such as the HQ for Citroen, Fiat, Celltech, O2 etc. Also, see if you can spot a bargain car at the huge Trade Sales site, which straddles the A4.
Fans of The Office on BBC2 should be able to recognise the roundabout at the centre of Slough. Here the architecture gets pretty bad, with the Brunel Bus Station the worst offender. The A4 is now proper dual, although it only lasts for a couple of miles. As it hits the eastern Slough suburbs, it narrows and wends its way down to junction 5 of the M4 and Heathrow. Even if you are new to the area, you can easily get to Heathrow by following the planes as they descend to land.
Intersecting with the M4, the A4 passes underneath to the area of Colnbrook (recently now part of Slough). You can also admire the huge HQ building of Honda here. The A4 now skirts to the north of Colnbrook (known as the Colnbrook by-pass). This stretch is famous for one thing - the Spearmint Rhino lapdancing club. Don't become distracted by the roadside billboard which is there to advertise it!!
The A4 then crosses the M25 (which can induce smug feelings as you see the jams) and down to a huge traffic light junction. You can turn left here to see British Airway's new HQ or turn right to head to Terminal 4 (and in the future Terminal 5), Staines or indeed the M25. Straight ahead is to Terminals 1,2 & 3. I think this is where Slough officially ends.
Original Author(s): Balraj
Section 5: Heathrow - London
The A4 continues to an odd not-quite-roundabout at M4 junction 4A, which serves the spur to the M4 and the twin tunnels to and from Heathrow Airport. Then it's the dual-carriageway Bath Road, which after the A30 joins becomes the Great West Road, a monument to a great earlier era of road-building. From a point midway between Heathrow and Hounslow, as far as Knightsbridge, the A4 has been diverted. The original line is now the A3006 and A315. The present A4 bypasses Hounslow, and it's three lanes each way at this point. It skirts Osterley Park (which is bisected by the M4) and then comes to the famous section where it runs directly beneath the M4 - three lanes each way to the M4's two! There's an interesting up-and-down junction with the M4 at junction 2 (which has been described as an "ironing board" junction!), and then you hit the Chiswick roundabout, which meets the A406 North Circular and A205 South Circular Roads, as well as crossing the original line of the A4, now the A315. From here a slip road runs up to join the two lanes coming from the M4, which ends here, creating another three-lane dual carriageway.
I love the next section - I know it like the back of my hand. Initially it's called Cromwell Road, but it has various names along the way. There's a solitary set of traffic lights serving Grove Park. Coming the other way, after the traffic lights the centre lane back towards the M4 continues uninterrupted for over two hundred miles via the M4, M5, M42, M40 and A40 to the traffic lights at Gipsy Corner, about four miles away as the crow flies. Next comes the magnificent Hogarth roundabout (with the not-so-magnificent single-lane A316 flyover running above!). After this one lane goes down to Hammersmith, and the other two go up to the wonderful Hammersmith flyover (it's probably not wonderful in the rush hour, but I used to love it as a child). Then it's back down into Talgarth Road, where you pass several sets of traffic lights before a fairly steep bridge over the railway line and into Earl's Court. After the double junction with the A3220 (two one-way streets) it's back to single carriageway pas the South Kensington museums and Harrods to Knightsbridge. Here it rejoins its original route of the road, the A315 Kensington Gore. This junction is known locally as Scotch Corner, after the long-established (but recently renamed) Scotch House shop in the acute angle of the junction. Then you go beneath an underpass at Hyde Park Corner, which takes you into Piccadilly and the Congestion Charging zone.
As with all roads within the London inner ring road, no road numbers or through destinations are shown from here on. You run alongside Green Park, and on to Piccadilly Circus, then down the Haymarket (or along Pall Mall and up St James's Street if you're going the other way), and along Pall Mall East to Trafalgar Square. Then it's along the Strand (and round the Aldwych for eastbound traffic only) and into Fleet Street. The original A4 continued along Ludgate Hill and Cannon Street to the Monument, but the "ring of steel" around the City of London has put paid to all numbered roads there. So, a little disappointingly, the final section of the A4 now turns along Fetter Lane and New Fetter Lane to end on the A40 at Holborn Circus.