Diamond interchange
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A diamond interchange is a common type of road junction.
Diamond interchanges are used where a freeway crosses a minor road. The freeway itself will be grade-separated from the minor road, a bridge being provided for one or the other. Approaching the interchange from either direction, an off-ramp diverges only slightly from the freeway and runs directly across the minor road, becoming an on-ramp which returns to the freeway in similar fashion.
The two places where the ramps meet the minor road are treated as conventional intersections. In the United States, where this form of interchange is very common, particularly in rural areas, traffic on the off-ramp typically faces a stop sign at the minor road, while traffic turning onto the freeway is unrestricted.
The diamond interchange makes more efficient use of space than most types of freeway interchange, and avoids the interweaving traffic flows that occur in interchanges such as the cloverleaf. Thus, it is most effective in areas where traffic is light and a more expensive interchange type is not needed. But where there is significant traffic, the two intersections within the interchange may cause congestion and accidents, requiring additional features such as traffic lights and extra lanes dedicated to turning traffic, or (as in the United Kingdom) a pair of roundabouts to create a type of junction called a dumbbell interchange. For this reason, many busier junctions that were originally serviced by diamond interchanges have since been upgraded to parclo or SPUI interchanges.
Where HOV lanes are present for carpooling, the ramps of a diamond interchange may be "folded" to the inside lanes instead of the outside. In urban areas this saves some space as well as requiring only one intersection instead of the two one-way intersections. This in turn reduces waiting time for motorists at traffic lights on the smaller road, which may be a large local thoroughfare with heavy traffic.