If you've read up on diamonds you'll know that they are the simplest way of grade-separating a junction. If you want to increase the capacity of a diamond, you can do a few things. America and Germany, who employ countless diamonds, use traffic lights. But here, we do the true British thing, and add a roundabout into the equation. The result is the most common type of grade-separated junction in Britain - there's more than 570 on the motorway network alone.
Where to Spot Them
If you haven't seen one yet, then you clearly haven't been paying attention. The very earliest ones still fully intact are to be found on the M1, and were built in 1959.
- Quite cheap to build: the second bridge is the only significant cost.
- Handles large volumes of traffic with ease.
- Can be upgraded fairly cheaply by widening the roundabout and approaches or adding traffic lights, or even adding a route across the roundabout.
- Extremely simple to navigate.
- Their high capacity often means they are required to handle unsuitably high levels of traffic.
The simplest variations take away a few sliproads to make it limited access - making a half-roundabout interchange.
There's quite a few places where the roundabout is bisected by another road on the same level, forming a sort of grade-separated hamburger junction. One of the earliest variations on this is M6/A580. On the M62/A650 junction, the road is one way, and allows one specific movement to be carried out bypassing three quarters of the roundabout.
In very rare cases, the sliproads get rearranged. On the M4 near Newport, the sliproads for each carriageway leave on the same side of the roundabout.
With thanks to Aleks for information on this page.