Field Guide To Interchanges

Glossary

Culture
interchanges in movies, computer games, etc.

Diamond
and other 4-ramp designs

Six-ramp
partial cloverleaf

Cloverleaf

Trumpet
and other 3-way interchanges

Stack
and other heavy-duty 4-way interchanges

Volleyball
an odd 3-level 4-way treatment

SPUI
Single-Point Urban Interchange

Oddities
Some strange or fictional interchanges

Other Roads

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Kurumi's Field Guide to Interchanges

Have you ever read an article about a planned "trumpet" interchange, or a "single-point diamond," and wondered what exactly those were? The Field Guide, with diagrams and descriptions of several common interchanges, might help.
    Keep in mind that civil engineers can make theses, journal articles, and entire careers out of geometric interchange design. The Field Guide will entertain and inform, but it falls short of a scholarly exposition on the topic. Your local university engineering library will likely have some good books on the topic.
    Nonetheless, I do want to keep the information here correct; if you have something to add or correct, or just want to talk about interchanges, please send me an email.
    Thanks to those of you who have done so, including Steve McCartney, Anthony Argyriou, Vince Del Vecchio, Mark Bozanich, Joe Keane, Dan Moraseski, Mike Moroney, Michael G. Koerner, Eric S Vymyslicky, Chris Sampang, Jim Poston, Raymond C. Martin and Jason Proctor.

Introduction
interchange with traffic arrows An interchange is a grade-separated intersection (one road passes over another) with ramps to connect them, as pictured. This is a Good Thing for busy roads; in the picture, thru traffic on the freeway (red) can pass through without stopping. Even if traffic signals are installed at the ends of the ramps, traffic on the surface street (blue) also flows smoother without interference from the freeway.
    All descriptions and diagrams in this guide assume you are driving on the right. However, at our level of detail, the same interchange diagrams also apply to left-side driving, with all traffic directions reversed.
    For brevity, the other interchange diagrams you see will not have arrows indicating traffic flow. To trace the traffic flow, start with right-side or left-side driving on a freeway, and note that ramps merging on or off the freeway do so at very gradual angles.

Some terms defined
An exit ramp leaves the main roadway for another road; an entrance ramp enters the roadway. These terms make the most sense when one freeway intersects a surface street; entrance and exit are from the point of view of the freeway.
    A complete interchange has enough ramps to provide access to and from any direction. Full freeway to street access with a conventional interchange requires four ramps, to get on and off in each direction. A four-way interchange of two freeways requires eight ramps.
    For more interchange lingo, see the Glossary.

Identifying an interchange
An interchange is primarily a solution to a capacity problem. As other factors such as safety, cost, environment, development and politics can vary at each site, there are hundreds if not thousands of unique, one-of-a-kind interchanges worldwide.
    However, many interchanges are slight variations of a few basic types, that adapt well to various situations and have been proven over the decades. (I'll have icons for all these in a later version of the Guide). For now, check which conditions best match the interchange you have in mind:

  • Freeway and surface street, 4 ramps: Diamond, or a functional equivalent
  • Freeway and surface street, 4 ramps, meeting at single point above or below freeway: Single-point urban interchange (SPUI)
  • Freeway and surface street, 5-7 ramps: Partial cloverleaf (see Cloverleaf)
  • Freeway and surface street, 6 ramps: Six-ramp or 3/4 partial cloverleaf
  • Freeway and surface street, 8 ramps, including 4 loops: Full Cloverleaf
  • 3-way interchange, one loop, no stopping or crossing: Trumpet
  • 3-way interchange, no loops: Three-way (various)
  • 4-way interchange, 8 ramps including 4 loops: Full Cloverleaf
  • 4-way interchange, 8 ramps meeting in a square at separate grade: "Volleyball"

Beefed-up at-grade intersections
The Center Turning Overpass (CTO) is a new design combining the at-grade arterial intersection with bits of the SPUI and the Volleyball to separate left-turning traffic from thru and right-turning traffic. All four left-turning movements climb ramps to meet at a four-way signalized intersection above the main intersection below (which no longer handles left turns). Instead of one four-phase signal, you have 2 two-phase signals working in parallel. The CTO website has 3D renderings of the design and other supporting information. ::br The Echelon Interchange lifts the northbound and eastbound roads above the opposite directions. Each pair of one-way roadways meets at a two-phase signalled intersection. Again, the intent is to have fewer opposing movements, and higher traffic flow through the intersection.

History
The first cloverleaf opened in New Jersey (at routes 4 and 25, which are now US 1/9 and NJ 35) in 1929; it is scheduled to be replaced with a diamond interchange around 2004. The interchange was under consideration for the National Register of Historic Places, but was declined, primarily because of the many safety and capacity-related alterations to it over the years: "widening and curb replacements for both roadways, revised geometry for one ramp, placement of a center barrier on Rt.US 1&9, removal of bridge pilasters, and the addition of modern lights and guide rails." Thanks to Raymond C. Martin for this information.

The first four-level stack interchange was built in Los Angeles, around 1952. US 101 and CA 110 intersect there, and leading north is the area's first freeway, the Pasadena.

Other Resources
http://www.kurumi.com/roads/interchanges/
Updated Aug. 19, 2003