A concurrency in a road network is an instance of one physical roadway bearing two or more different route numbers. When two roadways share the same right-of-way, it is sometimes called a common section or commons. Other terminology for a concurrency includes overlap, coincidence, duplex (two concurrent routes), triplex (three concurrent routes), multiplex (any number of concurrent routes), dual routing or triple routing.
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
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. A triple Interstate concurrency is found in Wisconsin along the section of I-41, I-43, and I-894 in Milwaukee, Wisconsin. 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 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).
There are at least two examples of eight-way concurrencies. The first example is in Indianapolis, between exits 46 and 47 of the 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. 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.
Regional examples
North America
In the United States, concurrencies are simply marked by placing signs for both routes on the same or adjacent posts. The federal Manual on Uniform Traffic Control Devices prescribes that when mounting these adjacent signs together that the numbers will be arranged vertically or horizontally in order of precedence. The order to be used is Interstate Highways, U.S. Highways, state highways, and finally county roads, and within each class by increasing numerical value.
Several states do not officially have any concurrencies, instead officially ending routes on each side of one. There are several circumstances where unusual concurrencies exist along state borders. One example occurs along the Oklahoma–Arkansas state line. At the northern end of this border Oklahoma State Highway 20 runs concurrently with Arkansas Highway 43 and the two highways run north–south along the boundary.
Concurrencies are also found in Canada. British Columbia Highway 5 continues east for concurrently with Highway 1 and Highway 97, through Kamloops. This stretch of road, which carries Highway 97 south and Highway 5 north on the same roadway (and vice versa), is the only wrong-way concurrency in British Columbia. Concurrencies are also very common in Quebec. Most notably, the Samuel-de-Champlain Bridge features a concurrency with three Autoroutes: A-10, A-15, and A-20. Another example is A-55, which runs concurrently with A-10, A-20, and A-40, all of which are major highways.