- Number of limited-access roads
- Number of surface roads
- Bridges required
- in all directions
- Estimated number in UK
- First built in UK
- ca. 1927
The most basic way to connect a road to a motorway or motorway-style road is a diamond interchange. It's quick, it's cheap, it's easy to build and use, and it demands very little land. As a layout, it hardly even needs designing: it's just what you get when you use a bridge to cross one road over another, and then provide the right number of sliproads to get on and off the major road. It gets its name from the diamond shape these four sliproads form.
The UK's earliest grade separated junction was a variant on this design: a junction on the A1 near Welwyn Garden City included a flyover and sliproads forming a folded diamond junction, and the bridge (which still survives) is dated 1927.
The downfall of the diamond interchange is its capacity. The four sliproads meet the minor road directly and they must be controlled either as simple priority junctions or with traffic lights. If you need to do anything else to accommodate the traffic — such as providing roundabouts, connecting other roads directly to them, or creating extra sliproads to bypass the junction — then you're starting to design something else.
All other types of motorway-to-road interchange are derived from this simple design.
Why build one?
On the motorway network, diamond junctions are surprisingly scarce. This is because we try to build motorways with relatively few access points, and so each junction typically meets a busy and important road and must handle a lot of traffic. That means that most motorway junctions need to be more substantial than a simple diamond, and its why the commonest form is actually the roundabout interchange.
Diamond junctions are more common on expressways and other busy A-roads, especially in rural areas, where the main road is typically not as busy as a motorway, the junctions are much more frequent, and there are more interchanges with minor roads. In those situations, diamonds are compact, simple, cheap and efficient.
The other situation where a diamond has real advantages is in an urban area. While it might be desirable to build a bigger junction with more capacity, in a city, it might be better to keep the footprint of the junction smaller by building four simple sliproads, built hard up against the sides of the main road, ending at a compact signalised junction. In these cases, the work to provide enough capacity might be worth the effort thanks to the way a diamond junction can be designed to take up very little space outside the width of the main road itself.
- Low construction costs, with just one bridge and very little land-take.
- Easy to upgrade later by adding traffic lights at the top of the sliproads, for example.
- Intuitive to understand and navigate.
- Simple to modify the design to fit into difficult locations.
- Low capacity because of the junctions between the sliproads and the minor road.
- Difficult to provide connections for more than one surface road.
- Difficult to substantially upgrade to, for example, a roundabout interchange.
The simplest variations take away two sliproads to make a limited access junction, leaving what's called a half-diamond.
If space on one side of the minor road is limited, it's possible to put all the sliproads to one side, looping two of them around, making a folded diamond. These are sometimes also known as partial cloverleafs (or "parclos") as they're halfway to being a cloverleaf. This layout takes up more space but is a useful option where the surrounding area limits space for building.
One important variation is the Single Point Urban Interchange, or SPUI, which arranges the junction between the sliproads and the minor road in such a way that the paths of opposite right-turning traffic flows never cross each other, and in doing so vastly improves the throughput of the junction. They are increasingly prevalent in the USA, but only one exists in the UK: it's on the A12 Westlink in Belfast.