The M3 runs from Sunbury more or less southwest to Southampton, but a better approximation is that it runs mostly west for the first half and mostly south for the second. The London end of the M3 is a continuation of the A316, which runs southwest out of central London. The motorway starts at Junction 1 (A308 to Sunbury and Kingston), three lanes each side. It passes under the M25 at J2, temporarily losing a lane in the interchange. (If you use it in the rush hour, you'll see why!) J3 is the A322, now reading Woking, Bracknell and Lightwater. At night you can see interesting lights in the sky around here - I'm not sure what they are. Presumably nothing to do with Birdworld, though. J4 (A331) sits exactly on the Surrey/Hampshire border. It used to be the A325 but they renumbered it for some reason . The exit is signposted to Guildford, Farnham, Camberley, Farnborough and Aldershot. Next we have J4A (A327, A3013) , labelled Farnborough (W) and Fleet. Pretty soon after that is Fleet Services, which is the only service station on the M3. (Maybe they need another one . "Twyford Gap" at J11 has a nice ring to it, I think!) It's in a pleasantly wooded area with the usual vastly overpriced food. And there's a footway over the motorway. The next junction is J5, marked (B3349) A287 to Hook. (Where did that spring from? It used to be Guildford and Farnham.) J6 (A339) goes to Basingstoke, Newbury, Alton, and, if you're going NE, Reading (A33). The roundabout is a pointlessly complicated system of motorway roundabouts. Hitch-hikers seem to have little or no concept of these and hang around the roundabouts anyway. Between J6 and J7 there is a remarkable feature  - an extra-wide hard shoulder which is basically a slip road from a gate to a farm!  I bet nobody ever uses it. J7 is the A30 to Basingstoke. The A30 runs almost parallel with the M3 about up to this point, and continues to Land's End. According to Kula Shaker, ``you can find your way home on the 303.'' If this applies to you, leave at J8, where the A303 splits off. It is restricted - you can only leave here going SW and join going NE. The A303 takes one of the lanes with it, splitting the road into two dual carriageways of two lanes each way. Going SW, look out for the sign "Andover 14, (Salisbury 29), Exeter 117". Going NE, the fast lane of the A303 joins quickly, and the other one joins much more slowly.
The next stretch, J8 to J9, is pretty boring, with a lot less traffic. (Think of it as a bridge between the two parts of the road.) It's two lanes all the way, except for a NE-bound bit just north of Winchester which is uphill and three lanes on one side. J9 is the A272 to Winchester (going SW) and the A34 to the Midlands and Newbury (yeah, like I want to go to the Midlands by going through Newbury!) going NE. The third lane comes back here, but in a bizarre fashion going SW which means that everyone tries to force their way into the middle lane. We are now in the infamous "Bar End to Compton" bit which was finished in late 1994 (apart from some curious "Work on Bridge Below" near J11 which seemed to take much longer). . J10 is restricted. It joins going up the hill into the Downs going SW; going NE, you leave here if you want the A31, or the new-fangled B3330 to Winchester city centre. There used to be some slogans on the bridges in the new bit. These ranged from the poignant (e.g. save England from the English) to the crap (e.g. get out and walk). The Authorities keep cleaning them off. Going SW we then go down a steep hill through the hole in the Downs to J11 (B3335 to Winchester, formerly Hockley Traffic Lights with an enormous traffic queue). For reasons which I don't understand, the exit has been mostly renumbered to "A3090" . The M3 goes over the railway line here (and another point near J13, although it's not so obvious from the road). The slip roads are a considerable distance apart, probably for this reason. Going NE, you are advised to take J11 for Winchester (S & W). J12 is the A335 to Eastleigh. J13 is also the A335 to Eastleigh, but is a more complicated junction because the M3 starts to split into M27(W) and M27(E). Joining at J13, you are forced into M27(W) - if you don't like this, you have to use J12. Immediately after J13 leaves (going SW), the motorway splits into two of two lanes each. Both of these have a J14 (A33 to Southampton) and the two sets of slip-roads merge. Going the "East" way is simpler because you get right of way. If you leave at J14, the M3 turns back into its old friend the A33, and continues (via Bassett Avenue and The Avenue) to Southampton Docks. If you join the motorway north-west bound from Southampton, it starts from the A33 (where there is a cheap petrol station) and immediately merges with the two parts of the M27 (from J4 of the M27, two lanes each) into a four lane road which loses its fourth lane to the A335 at J13.
The A3090 is the road from Winchester past Hursley (home of IBM Laboratories). This used to end just south-west of Hursley, as it met the A31 en route from Hockley toward Romsey.
The powers that be have, I gather, been trying to encourage people who plan on using the A31 below Romsey, toward the forest, where it meets the M27 at Cadnam, to use the M3/M27 instead. Ergo, you still get to the same point, only without having to go through Romsey. By re-numbering the road between Hursley and Chandlers Ford A3090 it helps this feint.
Of course, I could be completely wrong!
I mentioned IBM for one major reason. It's interesting to note that a whole series of IBM mainframe were numbered 3090 - the same number as the road going past their front door. (Although I don't know that Hursley was involved in the 3090 development.) Which came first or whether it's a coincidence... who knows?
M3 wide slip roads between J6 & J7. A service area was planned here when the motorway was built, and you can see both the diverge & merge flares in both directions, if you look. Presumably the service area was never built because the land was worth more for building houses on.
A new service area is currently [late 2000] under construction between J8 & J9, in Micheldever Forest.
The ``farm gate'' access is provided for the police to turn cars round from one carriageway to the other when conventional junctions are too far apart. There are ``police only'' accesses to a number of private overbridges over UK motorways - as an example, there is access to a bridge over M25 between J15 & J16 - neither of which allow a U-turn. I think there's also a gate somewhere between a couple of the links forming M25 J12, for the same purpose.
The old dual carriageway Winchester Bypass was, I'm told, one of the first two dual carriageway town bypasses in the country to be opened, just before the Second World War. (Stuffed if I can remember where the other one was.) [Andrew Tett writes that the other one was the A3 Kingston By-Pass, which was built in the early 1930s. Although this is not a motorway, its (rather narrow) three lanes apparently carry the most traffic of any of the non-motorway roads in the UK.] It was opened late Summer 39, and almost immediately closed again, to be used as a vehicle park for the duration. Incidentally, the bridge where the M3 passes under the B3404 is known as ``Spitfire Bridge.'' It's much wider now than it was, but there's been a bridge there since the bypass was built. I understand the plane that famously flew under it was actually a Hawker Tempest, though... (bigger and faster!)
If I recall correctly, the original Spitfire Bridge was a tall graceful three-arch brick viaduct, with the centre arch much wider to span the dual carriageway.
I suspect because of its size, it was really a concrete core with brick cladding (anyone know for sure?). Apparently the old railway viaduct still partly visible on the left when approaching Twyford Gap going North-East was this type of construction, only a couple of miles away.
I remember reading when the new Spitfire Bridge was being built that it was the longest single-span concrete bridge in Europe. I guess that record may have been taken elsewhere.