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.
Dumbbell and dogbone interchanges
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.
A variation often called a dogbone interchange (due to its aerial resemblance to a real or toy dog bone), and sometimes also called a double roundabout interchange, occurs when the roundabouts do not form a complete circle but instead have a "raindrop" or "teardrop" shape. These two raindrop roundabouts are fused together, forming a single "squashed" roundabout. This configuration reduces conflicts between vehicles entering the raindrop roundabouts from the ramps, increasing the roundabouts' capacity, and reducing queueing and delays, compared with the dumbbell interchange. Direct U-turns are not possible, although the movement can be made by circulating around both raindrop roundabouts. An example of a dogbone interchange in the United States is located on Interstate 70 in Avon, Colorado; more compact examples, which show less of the characteristic "dog bone" shape, are located along Keystone Parkway in Carmel, Indiana.
Tight diamond interchange
A tight diamond interchange (TDI), also known as a compressed diamond interchange or a tight urban diamond interchange (TUDI), is sometimes used in areas where there is insufficient right-of-way for a standard diamond interchange. The pair of intersections where the ramps meet the minor road are closely spaced. This spacing forces the turn lanes for each direction to run beside each other, causing the minor road to be wider than it would be if it were a standard diamond.
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, which in rural or suburban areas can be turned into a single-point urban interchange. 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.
- Staff (April 2010). "Chapter 9. Other Interchange Configurations (FHWA-HRT-09-060)". Alternative Intersections/Interchanges: Informational Report (AIIR) (FHWA-HRT-09-060). Federal Highway Administration. Retrieved October 8, 2012.
- Chris Marshall (2012). "Dumbbell Interchange". Interchanges. Chris's British Road Directory. Retrieved February 25, 2013.
- "Double Roundabout Interchange: Design and Operations". An Applied Technology and Traffic Analysis Program: Unconventional Arterial Intersection Design. University of Maryland, College Park / Maryland State Highway Administration. 2011. Retrieved February 25, 2013.
- Staff (August 2004). "Chapter 10 — Alternative Intersection Treatments (FHWA-HRT-04-091)". Signalized Intersections: Informational Guide (FHWA-HRT-04-091). Federal Highway Administration. Retrieved October 8, 2012.
- Scott Oglesby. "Diamonds and other 4-ramp interchanges". Field Guide to Interchanges. Kurumi.com. Retrieved October 8, 2012.