Where is it?
M3 junction 10, and the place where A31 traffic — carrying Southampton-bound traffic from Guildford, Farnham and the A3 from London — joins the M3, and also where longer distance A31 traffic begins the long multiplex with the M3 and M27.
What's wrong with it?
It's pretty thoughtless. The eastern roundabout breaks up flow on the A31 and since a large amount of traffic comes from the north-western end of the A31, It would make more sense and be simpler to have put a sliproad on the south side of the junction to join the M3 rather than looping round from the north.
Why is it wrong?
It's a recent junction built when the M3 gap at Winchester was finally (and controversially) closed. Since the M3 no longer screamed to a halt on the A31, the A31 had to come to a (not so) screaming halt on the M3. To allow access from both directions, a pair of roundabouts were used to allow both directions (since the B-road is the main road south from Winchester) to reach the sliproads.
What would be better?
Remove the eastern roundabout and allow the A31 to flow straight through here, and add a new sliproad from the westbound A31 onto the southbound M3. Removal of the western roundabout would of course complicate things since it would stop the neat ending for the A31.
Right to reply
The suggestion you make about placing the A31/M3 southbound slip to the south of the site is unfortunately impossible, as the road is climbing amazingly steeply at this point, which would make the on-slip even steeper than it is at the moment, which in itself is ludicrous.
To be honest, the junction you highlight simply should not be there at all. The A31 comes to a stop one roundabout earlier, where the spitfire link goes up to the A34/M3 junction. This is fine for (what little) northbound traffic there is. The idiocy of the whole arrangement of the M3 between J11 and J9 is that the old road, to the west of St Catherines Hill (the alignment of which is still obvious on an aerial photo), should never have been removed. This should have been kept for A31 southbound traffic to then join the M3 south at J11. This would also have allowed traffic for Winchester south to exit at J11 and follow the old A31. J10 is not only a pointless waste of time, but the weaving that it causes on the northbound carriagway has to be seen to be believed. The problem is that traffic joining at J11 northbound does so at a snails pace (the uphill slip is again ridiculously steep) and so traffic on the M3 moves over one or maybe two lanes. At that point faster traffic from J11 pulls out, just as traffic for J10 is trying to move over 2 lanes... the entire 3 lanes of motorway becomes one big weave!
The main problem with this junction (along with the junction on the other side of the cutting) is the steep gradients on the sliproads. Coming off the motorway you really have to brake firmly to slow down, and joining the motorway you have to really rev the car before changing gear in order to join the motorway at a reasonable speed. Of course, there tends to be traffic in front, and these drivers aren't fussed about joining the motorway at 35mph!
The hills and existing roads here made it difficult to come up with a decent set of junctions, and I imagine that the budget was quite small after building the Twyford cutting. The real problem however is the A34. I am unaware of any plans, but I would hope that this junction will be improved soon, as has been done at the A34/M4 junction.
The gradients when joining the M3 at Twyford could be eased by building crawler lanes (I think there may just be room to do this without needing to cut more rock) as planned for Portsdown Hill on the M27. I think that this would help ease congestion all the way back to the southern end of the M3 in the mornings, and back to J9 in the evening. Not the best solution, but substantialy cheaper.
To be honest, the best solution would be to make junction 8 a proper junction so that you can join and leave the A303 from both directions. This would then also reduce the congestion on that part of the A34.
The onslip here has now become a 'ghost island' merge, with two lanes and adequate room for HGVs to speed up, so I think this should now be upgraded to 'Mundane Junctions'.