Number of limited access roads
Number of surface roads
Full in all directions
Estimated number in UK
First built in UK
Most countries with well-developed networks of limited access roads build a lot of diamond junctions for local access points. They are the simplest way of grade-separating a junction. Where extra capacity is needed, many countries — notably the USA and Germany, who employ countless diamonds — use traffic lights. But in the UK, most of our motorway junctions are too busy for a diamond to be the right choice. For that reason, and also because this is Britain and we have a predeliction for a certain type of junction, we add a roundabout into the equation.
The result is the most common type of grade-separated junction in the UK — there are more than 500 of them out there; perhaps many more than that. By building two bridges, not just one, and turning the whole area above or below the motorway into a giant roundabout, an incredibly versatile and efficient junction is created, capable of handling multiple approach roads, maintaining high throughput and still leaving room for some nice landscaping or flower beds.
The capacity of a roundabout interchange is such that they can also be used to connect two major roads together. In some places, where engineers in other countries would have built a free-flowing junction like a cloverleaf, we build one of these instead, and in doing so save ourselves a lot of money. They are easily understood, highly versatile and relatively inexpensive, which is why they have been the default choice for grade-separated junctions in the UK since the late 1950s.
Why build one?
The question should really ask why you wouldn't build one. The roundabout interchange has, for decades, been the starting point: if you don't need the capacity it provides, build something smaller; if it won't handle the traffic build something bigger. But start with a roundabout interchange and see if it fits. It's flexible enough that it probably will.
Two key reasons why it continues to be a favourite choice are that it allows multiple approach roads and there are lots of options for later upgrades. The first is important because many junctions are not as simple as a minor road crossing a major road: they are positioned where several roads converge, or on a site that is zoned for future development where new access roads may be added later. A roundabout interchange can have lots of roads plugged in to it, meaning it works well in all sorts of situations.
The second reason is important because it's hard to know exactly how well used a junction will be — so it might be wise to choose a layout that can easily be upgraded later. Roundabout interchanges can be signalised, they can have extra circulating lanes added, left-turn sliproads to bypass the junction, new roads cut through the middle (known as hamburgerisation if you're taking notes), and so on. That means that, with a roundabout interchange, you are unlikely to be installing something that requires costly replacement if your forecasts turn out to be a bit low. If you build it very big from the outset, you can leave room for an extra flyover, allowing future conversion to a three-level stacked roundabout.
Finally, you might choose to build one because you're not ready to build one yet. The roundabout interchange can start life as a normal roundabout, with extra space, and then have a flyover or underpass added later without difficulty. In the meantime it remains a perfectly useful junction. That's an option that doesn't exist with many free-flowing designs, but it's been well-exploited in the UK for decades: there are many grade-separated junctions that started life as a flat roundabout with extra space and became a roundabout interchange later when traffic levels required it.
- Relatively cheap to build: the second bridge is the only significant cost.
- Handles large volumes of traffic with ease.
- Adaptable to serve any number of approach roads and to fit available space.
- 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.
- Once upgrade options have been exhausted, can be difficult to replace with another junction type.
- Large roundabouts serving many roads can have high circulating speeds, making it difficult to join.
- Status as the default choice for grade-separated junctions means they are sometimes installed when another junction type would be better.
The simplest variations take away a few sliproads to make it limited access — making a half-roundabout interchange.
There are quite a few places where the roundabout is bisected by another road on the same level, forming a grade-separated hamburger junction. One of the earliest variations on this is M6 J23 at Haydock Park. At M62 J28 Tingley Interchange, the road through the roundabout is one way, and allows one specific movement to be carried out bypassing three quarters of the junction.
One final variation would be to have two roundabouts instead of one, but if you do that you've built a dumbbell interchange.
With thanks to Aleks for information on this page.