A smart (and rather eye-catching) way to provide free-flowing access in all directions between two intersecting routes. The Whirlpool is something of a halfway house: it contains much more structural content than the Cloverleaf, and therefore works out more expensive, but not nearly as much as the Four-Level Stack. For the added complexity, you get something that will flow nearly as well as a Stack, but is much tidier. It's relatively flat, so it isn't as visually obtrusive as four levels of bridges, and because you don't need miles of ramp leading up to high overpasses you can reduce its size considerably - depending on how tight you're willing to have the corners.
Where to Spot Them
The M25 has two where it meets the M3 and M11, the former a true Whirlpool and the latter more Octopus like. You can see a very constricted one (which causes serious traffic problems now) at M60/M62/M602. Scotland is more fond of the Octopus variant; there's one near Motherwell on the M74 and another that was never finished (but is mostly laid out) where the A823(M) meets the M90.
- Handles high volumes of traffic with ease.
- Intuitive to use - turn off, then fork left or right.
- Tidy merging and diverging to and from the mainlines.
- Only two levels high.
- Can be compressed onto very small sites.
- Smaller junctions can slow traffic with tight corners and reduce capacity.
- Large number of bridges adds to expense.
- Uses large amounts of land for a fast junction.
The main variation is to reduce the number of bridges involved by building what is sometimes called an Octopus (because it has eight arms). Each exit slip divides in two, as with the Whirlpool, but the right turns merge in straight away, causing one diverge and two merges for every through carriageway. This vastly reduces the number of bridges as the sliproads don't need to cross each other, but makes for less orderly merging and a few more problems for traffic on the mainlines.
With thanks to Phil Reynolds for information on this page.