Nearly all motorways and a few other roads with delusions of grandeur have numbered junctions. These are theoretically numbered consecutively starting at the hub end (see the numbering page for an explanation of this). There are some confusing exceptions to this, like the M27 which is in the 3-cone and has the high numbers at the London end. I conjecture that this worries me more than it worries most people.
However, sometimes a new junction is inserted on an existing motorway, or the motorway is extended in some way, so extra letters need to be used: an example is the M4, with its junctions 9A and 9B near Maidenhead. Some junctions are curiously missing - maybe they never got round to building them, even though there are obvious places to put them (like junction 6 of the M27, which should meet the A27). Sometimes gaps are left at the beginning, so that the motorway can be extended later, such as the London end of the M11. (Somehow I don't see The Authorities demolishing Liverpool to complete (start?) the M62, or demolishing south London to build the first six junctions of the M23.) However, there are cockups on this front: the M6 has a new junction before junction 1, which they can't number. What would they call it? Junction 0? Junction A? These names don't have the right ring to them, so the junction ends up without a name at all.
The M4 has two spur roads without road numbers, but with junction num bers at the far end:
whereas junctions 9A and 9B are on the A404(M) which leaves from the ridiculously named Junction 8/9, as does the miniscule A308(M) which is less than a mile long and doesn't have a junction number at the end.
The logic behind all these is clearly that any road which leads inescapably to a motorway must by definition be a motorway itself. (I haven't exhaustively tested this, but it certainly holds true for what must be the most jumped-up bit of the motorway network: the A6144(M) which as you have listed is a single carriageway (complete with bends) even though my 1:100,000 road map shows it as a chunky dual carriageway.
The reason for this is in fact due to when the motorway was originally constructed in the 60s as the Maidenhead Bypass. There is in fact a closed junction on the M4 just prior to 8/9 on the London side, if you look carefully you can actually see the closed slip road on the London bound carriageway. This was a junction with the A308 and was Junction 8. Junction 9 was then the now numbered 9A on the A404(M). When the M4 was extended to Reading and beyond in the early 70s, the old junction 8 was closed due to the fact the extension started very close to it: the point of the now junction 8/9. The A308(M) spur was built to maintain the old connection, and the old M4 section was renumbered the A423(M), which was renumbered again due to the M40 extension to Birmingham (the A423 being detrunked as a result of this and the A404 improvements). I do believe the numbering of the junction was done so confusion of the original junction disappearing was kept minimal!
My own personal favourite is J3 of the M1. The junction was actually built, and is still in use as the access to Scratchwood services, but the link road to the A1 through the Scratchwood Open Space must have been ditched long ago. Also, now that the junctions on the A1(M) are now numbered all the way to Gateshead, it will be interesting, if the various sections are ever joined up, to see if the Transport ministry has got its calculations right for the gaps that exist at the moment.
I know of two examples of ``secret motorways'' - roads which are legally special roads (the official term for motorways) but not shown as such on maps or having blue signs. One is the section of A55 from the Llanddulas junction (between Abergele and Old Colwyn) through Colwyn Bay and the Conwy Tunnel to the Conwy Morfa junction. The easternmost part of this (as far west as the junction for Rhos-on-Sea just west of Colwyn Bay town centre) has (narrowish) hard shoulders; further west there are lay-bys for emergency use only. If I remember correctly there are 2 sections of 50mph speed limit, one from somewhere west of Landdulas to west of the Rhos junction, and one through the Conwy tunnel and approaches.
Probably both the first speed limit and the motorway status are because it is an urban section with frequent junctions and the potential for heavy traffic generally and weaving movements in particular; maybe also because of other potential dangers associated with the sea and cliffs.
The speed limit through the tunnel is probably because of restricted sightlines in the tunnel, the potentially serious implications of any accident there, and substandard vertical curvature on the tunnel approaches.
The other ``secret motorway'' is the A1 from its junction with the A720 (Edinburgh City Bypass) to Haddington (with proposed extension to Dunbar). It is more rural in character, with better separated junctions and no hard shoulders, and the speed limit is 70mph. Part of it (Tranent to Haddington) was only opened last November; the rest (Old Craighall to Tranent) probably became a secret motorway at the same time. It had existed for some years as an all-purpose road.
[Another example of a secret motorway is part of the A12. See Matthew Youell's comments on the A102(M).]
You've observed that road numbers used to apply to ``routes'' (eg the A45 was the Felixstowe to Coventry road), and that over the course of time, bits of these roads become relatively less or more important than other bits. In the case of the A45, the Eastern part grew ever more important as Felixstowe grew as a container port, whilst the Western parts were gradually superseded by other roads - like M6 & M45. About 5 years ago, it was decided that the Eastern part, along with some new East- West roads in the Midlands, had become important enough to be worthy of a new and coherent number of its own, and A14 was born, as a road from Felixstowe to the M1/M6 junction. The old A45 from Coventry was then left as was as far as Rushden, and then downgraded to B645 from there to St Neots, and A428 from there to Cambridge. This is where the old route of A45 is subsumed under the new A14, which then continues to Felixstowe. It all makes sense so far, except that someone observed that a lot of traffic used to come south on the A1, then left onto A604 to ``cut the corner off'', before joining A45 Eastbound. Now, since A14 was the new name for the trunk road to Felixstowe, it had better apply to the route from the A1, too. So the old A604 was remumbered there, and now the A14 merges with the A14 at Huntingdon, to form the A14. (Click for map.).
But there's worse:there's a roundabout just to the North of Bury St Edmunds where the B1106 crosses the B1106. All four arms have the same number, and all extend for at least a couple of miles. They're even numbered like that on the signs - it's not a mapping error. (Click for map.)
Re: Steve Barnard's comments on the A14/A14 junction at Huntingdon: prior to the renumbering things were no better as the same junction joined the A604 with the A604. Incidentally, the section of the A14 from Alconbury to Godmanchester is the only bit that was formerly occupied by the ``old'' A14, which then headed south from Godmanchester to Royston, this section later being renumbered A1198.
There's another dodgy numbering scheme at Manea in Cambridgeshire: a T-junction where all three roads are the B1093. (Click for map.)
Paul Berry adds:Nice example, but there's nothing dodgy about it. It's merely a spur of the existing route number down to Manea a quarter of a mile south of the junction. Spurs are absolutely everywhere. Although I agree examples such as the A14 Huntingdon-Alconbury spur stick out like sore thumbs because they are major roads, the existence of a spur is nothing to get too excited about.
Chris Marshall points out:Chris Ward's find of a B-road going three ways from the same junction is far from unique! Four ways (as with the B1106) is a truly astounding discovery, but three simply means the road has a same-numbered spur. This happens all the time with B-roads where a short section of road is needed but doesn't justify its own number, so it gets the number of the ``parent'' B-road, forming a short spur. Examples follow...
B-roads that do this (to pick a few out of the air) include the B3357 in Devon (at Princetown). Have a look also at Somersham, Cambridgeshire (near to Manea) where the B1086 splits in two at the B1040. A-roads that do this include the A14 (yes, that colossal national trunk route) at Huntingdon, where the same-number spur heads north to the A1. The A15 does it at Peterborough too. In fact, if we're going to get pedantic about this (and yes, we are) the M4, M23, M25 and probably others all do the same. The M23 and M4 both have airport spurs which (including evidence on signs) show these short spurs are numbered M4 and M23. The M25 has one at Watford - connecting to the A41 - and another at Orpington - connecting to the A21. These aren't really that rare at all as you can see - though they are certainly confusing to find. It's rather reflective of the slightly sloppy numbering in this country!
I can think of two motorways that have the carriageways far apart (let's say more central gap than total tarmac, as a rule of thumb). The M6 between 38 and 39 in Cumbria (a minor road runs for a mile or more between the carriageways) and just north of 39. The M62 between 22 and 23 in West Yorkshire, where there's a farm between the carriageways. [This farm is mentioned in a John Shuttleworth song - Ed.] Any others?
Does anyone know what the circles and squares painted on motorway carriageways mean? How about those posts by the side of the road with numbers like ``109 3''? Let me know if you do!
I'm not entirely sure - but from personal observations, I believe it means that you would be 109.3 miles from one end of the motorway (it might be to the border of a specific county/area that looks after that particular stretch of road).
The numbers like 103 9 on ``milepost'' on motorways and some trunk roads are actually marking kilometres from the nominal start of the road: 103.9 km in the present example.
Your ``Carriageway Markings'' section asks the question about squares painted on the carriageway. I don't know what they're for but I have observed that they are exactly (according to my odometer) one mile apart. My guess was that they are for calibration purposes, possibly police car speed sensors. It could be that something automatically starts and stops on sensing the white mark.
The white signs on the road are for GATSO type timing - the police can time you between then and get an accurate estimate of your speed from some distance back. In the USA you often see these painted as light aeroplanes or helicopters to let you know they can and will time you from the air and radio the info to a roadblock ahead...
I believe we are talking about VASCAR here rather than GATSO.