Interchange of the Week
Monday, 22 September 2003
Theme and Variations: Diamond Interchanges
(click any image to enlarge)
Standard Diamond (rural)

The most basic interchange form is the diamond, a four-ramp configuration used where the freeway meets a relatively low-traffic surface road. The example shown above is the NY 531 freeway at NY 259 (Union Street) near Spencerport. The ramp widths and allocation of turning lanes in this example favor traffic between points north (Spencerport) and east (Rochester).

Standard Diamond (urban)

Diamond interchanges are as common in urban areas as rural. In urban areas, however, space is usually limited, and the urban diamond interchange is usually more compact. In this case, Interstate 490 (Exit 19) at Culver Road in Rochester, the ramps ride directly alongside the freeway on vertical concrete retaining walls. Although this interchange handles far more traffic than the rural example, it has narrower ramps and fewer turning lanes. As a result, Culver Road between the interchange ramps is often choked with traffic. A modern design that solves this problem (and might be appropriate at this location) is the Single Point Urban Interchange or SPUI.

Sometimes in urban areas a diamond interchange will intermingle with nearby streets. At Exit 18 (right), I-490 meets NY 31 (Monroe Avenue) at an interchange very similar to Exit 19. However, Field Street (bottom, left of center) joins the eastbound exit ramp before meeting Monroe Avenue. Therefore, the end of the offramp is a two-way divided road, to allow a full connection between Field Street and Monroe Avenue. As well, Pembroke Street (bottom left) meets Field Street immediately alongside the offramp.


At left is an example of a non-freeway diamond interchange in the Town of Henrietta. Here, NY 252 (left to right) crosses over NY 15 in a busy retail area. The ramps entering NY 252, two lanes wide, are controlled by traffic signals, with both lanes feeding directly into the divided highway. Note that the ramps also serve adjoining properties, and to provide left turn movements from these areas, the interchange features "Texas U-turns" on either side of NY 15.

When a freeway crosses two closely-spaced surface roads, an extended diamond can be used. At right is Exit 11 on Interstate 390, where the freeway crosses NY 251 (left to right) and NY 15 in the Town of Rush. In this case, since the two surface roads intersect each other, the interchange provides some redundant connections between them.

A half diamond (left) is used where traffic patterns require a connection in one direction only, or as a supplement to a nearby interchange. This interchange, NY 104 at Phillips Road in Webster, primarily serves traffic between the Rochester area to the west, and a major plant of the Xerox Corporation on Phillips Road.

Half diamonds are often used in pairs with parallel surface streets. Such a pair exists on I-590 at Exit 3 (Elmwood Avenue) and Exit 4 (Highland Avenue). This has the effect of providing full access to a residential area without encouraging much on-and-off traffic activity.

A very rare type of half diamond provides ramps on and off in the same direction (on only one side of the freeway), instead of the usual offramp in one direction and onramp in the other.

Other examples showing further variations

Week 6 (diamond with traffic circle and U-turns)
Week 7 (overlapped diamond)
Week 8 (half diamond)
Week 13 (modified diamond)
Week 16 - Exit 6A (modified extended diamond)
Week 21 - Exit 7 (half diamond)
Week 24 (urban half diamond)
Week 28 (overlapped diamond)
Week 29 (modified half diamond)
Week 30 (half diamond)
Week 36 - Exit 14B (diamond)
Week 47 (extended diamond)
Week 50 (interior diamond)

Diamond Interchanges at Kurumi's Field Guide to Interchanges by Scott Oglesby.

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