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A Diamond Interchange is a compact and relatively low-cost Grade Separated Junction where the minor road and major road are connected by simple slips, with simple give way junctions at the end of the slip roads onto the minor road. This forms a rough diamond shape between the four slips.
Favoured for connections to B roads and quiet A roads in the early days of motorway building, many have needed to be improved over the years to cope with modern traffic levels. The easiest solution is to convert them to a variant of the Dumbbell interchange, but this is not always sufficient, and so a few have been completely rebuilt. However, this does not mean that the Diamond is no longer built. Indeed, the newest motorways, such as the M74 extension through Glasgow feature diamonds, although in this case it is perhaps due to space constraints in the urban environment.
Typically, Diamond junctions see the minor road cross the major route, either at right angles or a skew. Occasionally, they are used on spurs, but due to conflicting traffic on the far side of the major route, such junctions are more commonly built as a Diamond-Dumbbell hybrid with a roundabout (or teardrop) on the far side.
As with all Junction types, there is no simple one size fits all design. Diamond junctions can be built as 'Half Diamonds', where there are just two south-facing slips (for example), or 'Three-quarter Diamonds', where a single sliproad is omitted due to a perceived lack of need for the movement, or perhaps land constraints. Some Diamonds are also split, with the north and south facing slips meeting different cross-roads some distance apart. Such junctions may occur at either end of a village bypass.
Folded diamond interchange
A folded Diamond is one where the slip roads on one side of the major route both meet the same side of the minor road, one being folded with a 270 degree turn to connect the the two roads. This can be done either due to land constraints limiting the available room in a corner of the junction, or because there is another road joining in the normal position for the sliproad.