Concurrency (road)
A concurrency in a road network is an instance of one physical roadway bearing two or more different route numbers.[1] When two roadways share the same right-of-way, it is sometimes called a common section or commons.[2] Other terminology for a concurrency includes overlap,[3] coincidence,[4] duplex (two concurrent routes), triplex (three concurrent routes), multiplex (any number of concurrent routes),[5] dual routing or triple routing.[6][7]
Concurrent numbering can become very common in jurisdictions that allow it. Where multiple routes must pass between a single mountain crossing or over a bridge, or through a major city, it is often economically and practically advantageous for them all to be accommodated on a single physical roadway. In some jurisdictions, however, concurrent numbering is avoided by posting only one route number on highway signs; these routes disappear at the start of the concurrency and reappear when it ends. However, any route that becomes unsigned in the middle of the concurrency will still be signed on most maps and road atlases.
Overview[edit]
Most concurrencies are simply a combination of at least two route numbers on the same physical roadway. This is often practically advantageous as well as economically advantageous; it may be better for two route numbers to be combined into one along rivers or through mountain valleys. Some countries allow for concurrencies to occur, however, others specifically do not allow it to happen. In those nations which do permit concurrencies, it can become very common. In these countries, there are a variety of concurrences which can occur.
An example of this is the concurrency of Interstate 70 (I-70) and I-76 on the Pennsylvania Turnpike in western Pennsylvania. I-70 merges with the Pennsylvania Turnpike so the route number can ultimately continue east into Maryland; instead of having a second physical highway built to carry the route, it is combined with the Pennsylvania Turnpike and the I-76 designation.[8] A triple Interstate concurrency is found in Wisconsin along the five-mile (8.0 km) section of I-41, I-43, and I-894 in Milwaukee, Wisconsin.[9] The concurrency of I-41 and I-43 on this roadway is an example of a wrong-way concurrency.
The longest Interstate highway concurrency is I-80 and I-90 for 278 miles (447 km) across Indiana and Ohio, while the longest Interstate highway concurrency with three interstates is I-39, I-90, and I-94 in Portage, Wisconsin for over 29 miles (47 km).[10]
There are at least two examples of eight-way concurrencies. The first example is in Indianapolis, between exits 46 and 47 of the 53-mile (85 km) I-465 beltway, where the highway is concurrent with U.S. Route 31 (US 31), US 36, US 40, US 52, US 421, State Road 37 (SR 37), and SR 67.[11] Once I-69 is extended south of Indianapolis, this segment will have a nine-way concurrency. The second example is in downtown Athens, Georgia, between exits 4 and 8 of Georgia State Route 10 Loop, where the highway is concurrent with US 29, US 78, US 129, US 441, State Route 8 (SR 8), SR 15, and SR 422.[12]
Effect on exit numbers[edit]
Often when two routes with exit numbers overlap, one of the routes has its exit numbers dominate over the other and can sometimes result in having two exits of the same number, albeit far from each other along the same highway. An example of this is from the concurrency of I-94 and US 127 near Jackson, Michigan. The concurrent section of freeway has an exit with M-106, which is numbered exit 139 using I-94's mileage-based numbers. US 127 also has another exit 139 with the southern end of the US 127 business loop in Mount Pleasant, Michigan. (US 127's mile markers in Michigan reflect the cumulative distance north of the Ohio state line; the numbers resume north of the I-94 overlap and reflect the distance accumulated on that concurrency.)[24]
However, there are also instances where the dominant exit number range is far more than the secondary route's highest exit number, for example the concurrency of I-75 and I-85 in Atlanta, Georgia—where I-75 is dominant—the exit numbers range from 242 to 251, while I-85's highest independent mile marker in Georgia is 179.[25]
