Diamond interchange

A diamond interchange is a common type of road junction, used where a freeway crosses a minor road. The freeway itself is grade-separated from the minor road, one crossing the other over a bridge. 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 that returns to the freeway in similar fashion. The two places where the ramps meet the 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 uses less space than most types of freeway interchange, and avoids the interweaving traffic flows that occur in interchanges such as the cloverleaf. Thus, diamond interchanges are most effective in areas where traffic is light and a more expensive interchange type is not needed. But where traffic volumes are higher, the two intersections within the interchange often feature additional traffic control measures such as traffic lights and extra lanes dedicated to turning traffic. The at-grade variant of the diamond interchange is the split intersection. The ramp intersections may also be configured as a pair of roundabouts to create a type of diamond interchange often called a dumbbell interchange (due to its aerial resemblance to a dumbbell), and sometimes called a double roundabout interchange. Because roundabouts can generally handle traffic with fewer approach lanes than other intersection types, interchange construction costs can be reduced by eliminating the need for a wider bridge. This configuration allows other roads to form approach legs to the roundabouts and also allows easy U-turns. This type of interchange is common in the United Kingdom and Ireland, and is becoming increasingly common in the United States. Examples of dumbbell interchanges in the United States are located on Interstate 35 in Medford, Minnesota, on Interstate 87 in Malta, New York, on Interstate 17 at Happy Valley Road north of Phoenix, Arizona, and on Interstate 80 at California State Route 89 (exit 185) in Truckee, California. An example in Canada is found on the Pat Bay Highway in North Saanich, British Columbia, near Victoria International Airport. One or both roundabouts in the dumbbell interchange may also contain side lanes to increase the capacity. A good example of such a 'turbo' dumbbell interchange, which was formerly a half cloverleaf, can be seen in Jülich, Germany at 50°54′51″N 6°19′24″E / 50.914055°N 6.323368°E / 50.914055; 6.323368.