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.
Introduction
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.
Definitions
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.
History
The first cloverleaf opened in Woodbridge Township, New Jersey (quite the surprise) in 1929, at the junction of Routes 4 and 25. This is now the intersection of US 1/9 and NJ 35, and as of about 2004, the interchange is no longer a cloverleaf. (Fittingly, an adjacent cemetery is named Cloverleaf Cemetery.)
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." In the end, for safety, three of the loop ramps were removed.
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.
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. 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"
Links
- 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 Resources
- Interchanges (Wikipedia; more thorough than this page now)
- Open Directory: Interchanges. (DMOZ directory)
- Interchange Types and Commentary (Donovan Martin)
- Interchange Dictionary (Chris Marshall)