Is that SPUI?

Yes it is

The advantages are:

1. You can use the widening in the central reservation you get at roundabouts without having to widen the central reservation especially for the u-turn facility.
2. Putting it at the roundabout usually means it is as far away as possible from the first LILO, which is desirable since having a right hand entry/exit for the U-turn can create weaving issues.
3. The mainline traffic will be moving more slowly when it has just negotiated a roundabout, and so there will be less of a speed differential with the U-turning traffic.

Though there is something to the disadvantage you mention, I think it is mitigated by no. 3. and by the fact that the version of the idea I'm defending includes a lane gain. This last point should make it safer than the example in Birmingham that Jeni linked to, which is a simple right hand give way.

Okay, it's a late reply, but I've not visited these pages for a while.

The double-dashed give way lines were once used in some Australian states. Definitely Victoria. I remember them well in the 1970s. I seem to recall the painted triangles too, but they were relatively rare.

This is where disappearing for a few weeks and not checking SABRE comes back to haunt me

There's a U-turn lane here on the A41 Chester "bypass". I think it exists as the A41 dual carriageway is a residential road with houses on both sides.

Where the two Chester bypasses merge, both Google and Live show a bizarre sliproad from the A41 turning left onto the A55. Anyone know what this is? I can't see it being a normal road - it's a right-hand exit, it has a very tight bend and it has no sliplane at either end, plus its functionality is duplicated by the A55/A51 junction nearby. My best guess is that it is the remains of a temporary sliproad used because the A51 junction was was partially closed for roadworks or something.

I took some photos of a roundabout in Klaipeda, Lithuania in September where you can make a U turn as you enter the roundabout. I apologise that the photos don't show the layout very well.

Looks distinctly like an emergency services slip - these exist in the centre of the Broughton Interchange on the M6, for example. I don't think it'll ever have been open to the public.

Correct. To the best of my knowledge this has always simply been a slip road for the emergency services. It’s never been open to the public as far as I'm aware. I have once seen an ambulance sitting at the end of it wanting to join the A55, but that's about it.

Similarly, a little further north when it becomes the M53, you'll find this odd little emergency services slip, joining the M53 northbound and M56 westbound carriageways. However, it's not quite in as good condition as the picture shows. It's slowly becoming overgrown in areas (the start of it on the M53 is in particularly bad condition) and it's very steep.

*** Late post to this subject!!

I think we're all thinking about this from a British perspective.

In Bangkok, Thailand, these u-turns are quite standard at Roundabouts. In Thailand most roads are dual carriageways and there are many minor roads which join on. Thailand usually uses a one way 'local road' system either side of dual carriage way and there are also various filter lanes that shuffles traffic around.

Things to keep in mind why Thailand uses the u-turn system:

1. Far higher volume of traffic than anywhere in the UK.
2. More cycles and motorcycles (so they don't have to navigate the roundabout)
3. De-regulated traffic (ie it's chaos)
4. Less traffic on the roundabout.

Click here and snoop about.

I had a quick look about, and all the examples of U-turns I saw were joining slip roads under GSJs. Compared to a u-turn between the two carriageways of an at-grade DC, this usually has the advantage of space - the two 'carriageways' (slips) typically have an entire D2+ between them. That reduces the tightness of the turn and increases stacking capacity.

There is in fact a related British example on the M8 (soon to be M74, probably), though here the space is bigger still since there are two D3Ms between them. This example, like some of the Thai ones, involves a diamond type interchange rather than a roundabout interchange - and indeed, there is as much reason for removing traffic from a signalized cross roads as there is from a roundabout.

As I mentioned, I really like the idea of u-turns at busy junctions, where there's the space, which in the UK usually means at flared roundabouts. However, the fact that they involve a right hand entry and exit probably prevents their widespread adoption in the UK - of course, when traffic is chaotic anyway, as in Thailand, this is less of a barrier.

There is a similar U turn lane at the Hanger Lane Gyratory for the benefit of local traffic from the A40 westbound.

Two U-turn facilities at the M62 J31 teardrop nowadays. They're a bit different to what we're talking about as they're essentially 'inside the roundabout', rather than bypassing it. Bird's eye here. Switch to Aerial and you'll see there used to be just one U-turn facility. Presumably on opening it was a simple dumbell, with no teardrop or special u-turn facilities.

The A411/A412 roundabout in Watford is just begging to have a pair of these installed (on the North and South sides). Might require strengthening the bridges (I've never understood why they're so wide anyway), but the infrastructure is essentially there.

Google Maps

I've just posted a photo showing similar markings being used in Santiago, Chile - with a UK style box junction thrown in for good measure...
http://www.uk-roads.org.uk/gallery/albums/userpics/11973/196172770_0916a12606_b.jpg