Diamond interchange between Interstate 285 and Georgia State Route 6 in Atlanta. This example has since been converted into a diverging diamond interchange.Visual showing how diamond interchanges are used, with an expressway running across and a local road running in the center. Left image is for left-side traffic (UK), right image is for right-side traffic (US). Large arrows show where turns are made; smaller arrows show traffic flow.
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.
Variations
Dumbbell
For the at-grade intersection design analogous to dumbbell and dogbone interchanges, see Bowtie (road).
The ramp intersections may also be configured as a pair of roundabouts[1] to create a type of diamond interchange often called a dumbbell interchange[citation needed] (due to its aerial resemblance to a dumbbell), and sometimes called a double roundabout interchange.[2][1] 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.[3]
A variation of the dumbbell interchange, 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,[1] 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, 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.[3] An example of a dogbone interchange in the United States is located on Interstate 70 in Avon, Colorado, United States;[4] more compact examples, which show less of the characteristic "dog bone" shape, are located along Keystone Parkway in Carmel, Indiana, United States. Several interchanges similar to those along Keystone Parkway are being built along the new US 31 freeway under construction in northern Indiana.[5]
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.[1][7] 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.[citation needed]Caltrans classifies this type as Type L-1.[8]
A single-point urban interchange (SPUI) is built with a large over- or clear underpass providing space for a single traffic signal controlled intersection with the ramps and the cross street. Caltrans classifies this type as Type L-13.[8]
In a diverging diamond interchange (DDI) or (DCD), the two directions of traffic on the non-freeway road cross to the opposite side on both sides of the bridge at the freeway to save the third traffic signal phase.
Three-level diamond
In a three-level diamond interchange, the cross street is built in a third level with free flowing traffic as a second arterial road. The intersection is split up into four intersections, handling just two conflicting directions each.
Its two-level variant is the split diamond interchange.
A single-leg continuous-flow intersection (CFI) was built in 2014 in San Marcos, Texas, at the intersection of Aquarena Springs Drive (Loop 82), Interstate 35's southbound frontage road and I-35's southbound-to-northbound Texas U-turn.[11][12] A two-leg CFI, also in San Marcos, was built in 2015 at the intersection of Hopkins Street (State Highway 80), I-35's frontage roads and I-35's Texas U-turns.[13] In both intersections, the displaced left turn lanes merge with the Texas U-turn lanes.
Split diamond
A split diamond interchange has its ramps "split" between two crossroads, typically with an exit ramp/entrance ramp pair serving each of the crossroads. The crossroads themselves may be one-way or two-way, and are most often connected by frontage roads, usually one-way.[14][15]
Other
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.
In Henrietta, New York, Jefferson Road (NY-252) crosses West Henrietta Road (NY-15) on a modified diamond interchange, where traffic merges onto NY-252 at signalized intersections without any merge lanes, as well as Texas U-turns and RIROs on the ramps.
^Chatterjee, Indrajit & Sharma, Siddharth (2007). Comparative Analysis of Conventional Diamond Interchange and Contra Flow Left Turn (CFL) Interchange (Report). Center for Transportation Research and Education at Iowa State University. S2CID108139034.
^Garcia, Rene, P.E. (October 1, 2014). "Freeways". Roadway Design Manual. Texas Department of Transportation. Chapter 3: New Location and Reconstruction (4R) Design Criteria, Section 6. Archived from the original on May 14, 2013. Retrieved November 27, 2017.{{cite web}}: CS1 maint: multiple names: authors list (link)
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