The most basic way to connect a road to a motorway or motorway-style road is a diamond interchange.
Factfile
- Limited-access roads 1
- Surface roads 1
- Vertical levels 2
- Bridges required 1
- Access between roads Full in all directions
- Number in UK ~250
- First built in UK 1834
Names
Diamond
The universal, internationally accepted name for the junction, and its official name in the UK’s design guidance.
Half-Diamond
A Diamond interchange with only two of the usual four sliproads.
A diamond junction is cheap, easy to build, easy to use, and requires 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.
All other types of motorway-to-road interchange are derived from this simple design, and if you modify it very much you quickly find you have a junction of a different type. If you add roundabouts, you've got a dumbbell. If you bend the sliproads so they're all on the same side of the bridge, you've got a parclo. If you replace the bridge with a single big roundabout, you've got a roundabout interchange.
The diamond's simplicity is a big selling point, but also sometimes a drawback: it can only accommodate four arms, so if you need to add a fifth approach, you're immediately into the territory of adding extra junctions off to the side.
It's possible to build diamonds with lots of lanes and lots of turning capacity, allowing them to handle huge volumes of traffic; the M60 and M8 have several that operate this way and overseas it's common to see some very substantial diamonds indeed. With the right design and signal timings they handle serious traffic loads. But in the UK design guidance has always myopically insisted that they're only suitable for low-traffic situations, so such things are rarer here than they ought to be.
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. Since the diamond's value is played down by highway design manuals, other things tended to be built instead - most commonly, roundabout interchanges.
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.
Advantages
- Low construction costs, with just one bridge and very little land-take.
- Easy to upgrade later by adding traffic lights and more turning lanes.
- Intuitive to understand and navigate.
- Simple to modify the design to fit into difficult locations.
Disadvantages
- Width of the bridge can make it difficult to increase capacity later, by restricting the space available for turning lanes and stacking space.
- Difficult to provide connections for more than one surface road.
- Difficult to substantially upgrade to, for example, a roundabout interchange.
Variations
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.
Finally, the Diverging Diamond is an even higher-capacity version, to which existing diamonds and roundabout interchanges can easily be converted. It crosses over the carriageways of the minor road for a short distance, which (counterintuitively) reduces conflict points and enables incredibly efficient signal timings. They are common in the US and increasingly seen in Europe and Australia, but still so far unknown in the UK.
