Number of limited access roads
Number of surface roads
Full in all directions
Estimated number in UK
First built in UK
The main problem with the cloverleaf interchange is that it causes conflicts between traffic entering and exiting the motorway on the sections between loop ramps. It's also very big. Redesigning the cloverleaf to eliminate those problematic weaving sections between loops means unravelling two of them and spinning the sliproads around the outside of the junction instead. What you get is a partially unrolled cloverleaf.
The resulting interchange probably isn't any smaller a normal cloverleaf, and in fact it may very well be bigger, because the new limitation on its size is no longer how big the loops are but how big the sliproads are that go around the outside of the loops. They can be truly enormous.
In some parts of the world these junctions have been created by adapting an existing cloverleaf junction to improve its capacity, but in the UK we have only ever built them from scratch, so in this country at least, this is a junction type in its own right rather than a variation created by incremental changes.
The first in the UK was at Winnersh, near Reading, at the junction of the M4 and A329(M), which opened in 1973. Since then we gained another in 1974 at the M6/M62 Croft Interchange, and finally near Denham at the M25/M40 interchange in 1985.
Why build one?
Supposing you wish to create a high capacity interchange between two motorways but there are some problems that mean you can't build that four level stack you've been dreaming of. Perhaps the site is very flat, so a high-rise junction would be intrusive, or perhaps only two of the four turning movements are expected to be very busy, while the other two are expected to be relatively lightly used. What you need is a junction that arranges traffic flows as neatly as a stack, but without all the fuss. You need a partially unrolled cloverleaf.
At Iver Heath, where the M25 meets the M40, the impact of a major motorway interchange on the local area was thought to be a particular concern. Wealthy people lived nearby and didn't want to see flyovers from the windows of the library in the south wing. By constructing a partially unrolled cloverleaf, with lots of open space inside for thick woodland, it was possible to build the whole junction below ground level, so from anywhere nearby it may as well not be there at all.
It's also possible to build these junctions very big indeed — to make the sliproads smooth and fast — and avoid land cost problems. Croft Interchange on the M6 does just that: it's vast (bigger, in fact, than the whole town centre of Warrington, which it serves) but most of the space it occupies is still productive farmland. Only the space needed for the roadways was used, and the open space within those big outer sliproads is made up of fields.
- No conflicting movements between entering and exiting traffic.
- Unrolled right turns are faster as a result of the wider curves.
- Can be used where minimal land is available in one corner of the junction: one corner only has a small left turn slip road to fit in.
- Is only built on two levels.
- Large land take which may mean a high cost.
- The pair of sliproads looping around the outside form a dual carriageway where traffic is on the "wrong" side of the road, which can be a bit scary if you've never seen it before.
- Ridiculously long name.
There aren't many variations on this theme. It's essentially a variation on established interchange designs already, being a development of the cloverleaf. The exception is the interchange between the A19 and A66 between Middlesbrough and Stockton on Tees, which would be one of these but for the fact that one of the loops is replaced with another direct sliproad. That puts it halfway between a partially unrolled cloverleaf and a four level stack.
With thanks to Stuart and Ian Bailey for information on this page.