Highway multiplexes, sometimes referred to as concurrencies or overlaps, are a widespread feature in transportation networks across the globe. Found in nearly every U.S. state, Canadian province, and many other nations, these sections of road allow multiple routes to share the same stretch of pavement.
In essence, a multiplex signifies that two or more highways share the same physical roadway, with each highway retaining its distinct route number.
Common examples include portions of major interstate systems like the Pennsylvania Turnpike (I-70/I-76) and the Ohio Turnpike (I-80/I-90), but they can also occur on U.S. Routes, state highways, provincial routes, and even European Autoways.
Glossary of Key Terms
- Multiplex: A segment of highway shared by two or more routes, whether signed or unsigned. The thru lanes are shared, meaning the main flow of traffic is carried by all concurrent routes. Intersections, ramps, or roundabouts typically do not count as multiplexes.
- Concurrency/Overlap: Alternate terms for multiplex used by departments of transportation. These terms are interchangeable with “multiplex.”
- Multiplexed Routes: Highways that share the same stretch of pavement.
Departments of transportation may use these terms interchangeably, but “multiplex” allows for more specific classifications based on the number of routes sharing the highway.
Highway multiplexes, where multiple routes share the same roadway, are common in transportation networks. For instance, the Pennsylvania Turnpike features a duplex with I-70 and I-76.
In some cases, highways are repurposed to serve local needs. A notable example is the I-70 Eastern Terminus Park and Ride Lot near Baltimore, where the unused section of the highway was converted into a park-and-ride facility.
Numbers of Routes in Multiplexes
- Uniplex: Technically not a multiplex, as it involves only one route.
- Duplex: Two concurrent routes (e.g., I-70/I-76).
- Triplex: Three concurrent routes.
- Quadruplex: Four concurrent routes.
- Quintuplex: Five concurrent routes.
- Sextuplex: Six concurrent routes.
- Septuplex: Seven concurrent routes.
- Octuplex: Eight concurrent routes.
- Nonuplex: Nine concurrent routes.
- Decuplex: Ten concurrent routes.
For cases involving more than ten concurrent routes, the terms grow increasingly complex, such as:
- Undecuplex (11 routes)
- Duodecuplex (12 routes)
- Tridecuplex (13 routes)
While rare, such high-number multiplexes can occur in urban areas with densely overlapping highway systems.
Directional Types of Multiplexes
The direction in which routes are signed plays a significant role in defining multiplexes. The types of directional relationships include:
Same-Way Multiplex
| Multiplex Type | Configuration | Example |
|---|---|---|
| Same-Way Multiplex | Duplex Example: Both routes are signed in the same direction | N-S/N-S or E-W/E-W |
| Triplex Example: All three routes share the same direction | N-S/N-S/N-S | |
| Quadruplex Example: Four routes aligned in the same direction | N-S/N-S/N-S/N-S | |
| Perpendicular Multiplex | Duplex Example: One route is signed north-south, while the other is signed east-west | N-S/E-W |
| Triplex Example: Two routes share the same direction, and the third is perpendicular | N-S/N-S/E-W | |
| Quadruplex Example: Two routes in one direction and two in a perpendicular direction | N-S/N-S/E-W/E-W | |
| Wrong-Way Multiplex | Duplex Example: The signed directions of the two routes are opposite | N-S/S-N or E-W/W-E |
| Triplex Example: Two routes share the same direction, while the third is opposite | N-S/N-S/S-N | |
| Quadruplex Example: Two pairs of routes in opposite directions | N-S/N-S/S-N/S-N |
Three-Way or Four-Way Multiplexes
These involve three or four distinct signed directions.
- Three-Way Triplex: Each route has a distinct direction (e.g., N-S/E-W/S-N).
- Four-Way Quadruplex: Each of the four routes has its own signed direction (e.g., N-S/E-W/S-N/W-E).
Such classifications can extend to multiplexes with more than four routes, though these cases are exceedingly rare.
Types of Signing in Multiplexes
How routes are signed in a multiplex varies based on local transportation practices.
- Signed Multiplex: All routes are visibly signed along the multiplex, making it clear to motorists that multiple highways share the pavement.
- Hidden Multiplex: Also known as a secret or unsigned multiplex, this occurs when only one route is signed, creating the appearance of a single highway.
- Partially Signed Multiplex: Some, but not all, of the routes in a multiplex are signed. This approach implies continuity for certain routes while leaving others unsigned.
- False Multiplex: This occurs when one route is continuous, while another appears to overlap it but begins and ends at separate junctions, creating the illusion of a multiplex.
The approach to signing often depends on whether the routes belong to the same system (e.g., interstates) or are part of separate systems (e.g., state and U.S. routes).
Significance of Highway Multiplexes
Highway multiplexes serve an essential role in transportation infrastructure by:
- Streamlining navigation for motorists who may rely on multiple highways to reach their destinations.
- Reducing the need for additional pavement by consolidating overlapping routes.
- Simplifying maintenance and administration for departments of transportation.
Despite their utility, multiplexes can also lead to confusion, especially in cases of hidden or partially signed routes. Clear signage and consistent route numbering are crucial to mitigating this issue.
The Bottom Line
Highway multiplexes are an integral part of modern road systems, offering efficient solutions for overlapping highways and simplifying route navigation.
Whether they involve two routes or more than ten, multiplexes demonstrate the complex yet practical design of transportation networks.
By understanding their types, signing practices, and directional classifications, both motorists and transportation professionals can better appreciate the purpose and functionality of these shared roadways.