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Guide to Highway Multiplexes

Highway multiplexes (also called concurrencies or overlaps) are found in nearly every state in the United States, as well as in many Canadian provinces and many nations worldwide. Multiplexes are simply highways that carry two or more official route numbers, indicating that two or more routes share the same pavement. Portions of the Pennsylvania Turnpike (I-70/I-76) and the Ohio Turnpike (I-80/I-90) are common examples involving interstates, though thousands of other examples can be found in US Routes, state routes, provincial routes, European Autoways, etc. This guide elaborates on the different types of multiplexes, presents their terminology, lists some of the superlative multiplex examples, and links to multiplex photo galleries on the web, with an emphasis on the multiplexes in the United States.

Glossary

• multiplex (also, concurrency or overlap): a segment of highway shared by two or more signed or unsigned routes. Typically, shared interchange ramps, roundabouts, and single intersections are not considered a multiplex; the thru lanes are shared in multiplexes. Departments of transportation often use the terms concurrency or overlap, but all three terms are interchangeable. Multiplex allows specific names for multiplexes of two, three, etc. routes. The routes that share pavement in a multiplex are called multiplexed, concurrent, or overlapping.

Numbers of Routes

• uniplex: one route (not really a multiplex)
• duplex: two routes
• triplex: three routes
• quadruplex: four routes
• quintuplex: five routes
• sextuplex: six routes
• septuplex: seven routes
• octuplex: eight routes
• nonuplex: nine routes
• decuplex: ten routes
• undecuplex: eleven routes
• duodecuplex: twelve routes
• tridecuplex: thirteen routes
• quatuordecuplex: fourteen routes
• quindecuplex: fifteen routes
• multiplex: any number of routes (typically at least two)

Directions

• same-way duplex: both routes have the same signed directions, e.g., N-S/N-S or E-W/E-W
• perpendicular duplex: one route is signed N-S while the other is signed E-W, e.g., N-S/E-W or N-S/W-E
• wrong-way duplex: the signed directions of one route are opposite those of the other route, e.g, N-S/S-N or E-W/W-E
• same-way triplex: all three routes have the same signed directions, e.g., N-S/N-S/N-S or E-W/E-W/E-W
• perpendicular triplex: two routes have the same signed directions, while the third route is signed in a perpendicular direction, e.g., N-S/N-S/E-W or E-W/E-W/S-N
• wrong-way triplex: two routes have the same signed directions, while the third route is signed in the opposite direction, e.g., N-S/N-S/S-N or E-W/E-W/WE
• three-way triplex: all three routes have different signed directions, e.g., N-S/E-W/S-N or E-W/S-N/WE
• same-way quadruplex: all four routes have the same signed directions, e.g., N-S/N-S/N-S/N-S or E-W/E-W/E-W/E-W
• perpendicular quadruplex: the four routes have a total of two distinct signed directions, and those two directions are perpendicular, e.g., N-S/N-S/E-W/E-W or E-W/E-W/E-W/S-N
• wrong-way quadruplex: the four routes have a total of two distinct signed directions, and those two directions are opposite, e.g., N-S/N-S/S-N/S-N or E-W/E-W/E-W/WE
• three-way quadruplex: the four routes have a total of three distinct signed directions, e.g., N-S/E-W/E-W/S-N or E-W/N-S/WE/WE
• four-way quadruplex: each of the four routes has a distinct signed direction, e.g., N-S/E-W/S-N/WE
• The above terms can be extended to quintuplexes, sextuplexes, etc.
• same-way multiplex: all of the routes have the same signed directions
• wrong-way multiplex: at least one pair of routes have opposite signed directions

Signing

Some of the following terms rely on the fact that some states, provinces, etc. have different internal and external route systems. In such areas, a section of road may be signed with a certain number of designations (according to the external system) while the department of transportation's referencing or bookkeeping system (the internal system) may have a different number of designations. For example, consider the case of a section of highway that is signed with two designations (like a duplex) so that motorists can follow either route, while the department of transportation refers to it with a single designation for simplicity.

• signed multiplex: all routes are signed along the multiplex
• hidden multiplex (also, secret or unsigned multiplex): only one route is signed along the multiplex, implying continuity of one route and discontinuity of the other route(s), yet all routes are truly continuous
• partially signed multiplex: two or more, but not all, routes are signed along the multiplex, implying continuity of some routes and discontinuity of other routes, yet all routes are truly continuous
• false multiplex: one route is continuous while another route begins/ends at two different junctions along the first route, so that the second, discontinuous route does not share pavement with the first, continuous route.

Other terms

• unnecessary multiplex: one or more routes end at the end of the multiplex