In a road network, a concurrency is an instance of one physical roadway bearing two or more different route numbers. The practice is often economically and practically advantageous when multiple routes must pass between a single mountain crossing or over a bridge, or through a major city, and can be accommodated by a single right-of-way. Each route number is typically posted on highways signs where concurrencies are allowed, while some jurisdictions simplify signage by posting one priority route number on highway signs. In the latter circumstance, other route numbers disappear when the concurrency begins and reappear when it ends. In most cases, each route in a concurrency is recognized by maps and atlases.

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.

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 between New Stanton and Breezewood, 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. The longest Interstate Highway concurrency is I-80 and I-90, which run concurrent on the Indiana Toll Road and Ohio Turnpike for 278 miles (447 km) between Lake Station, Indiana and Elyria Township, Ohio.

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 I-69, U.S. Highway 31 (US 31), US 36, US 40, US 52, US 421, and State Road 67. The second example is in downtown Athens, Georgia, between exits 4 and 8 of SR 10 Loop, where the highway is concurrent with US 29, US 78, US 129, US 441, SR 8, SR 15, and the unsigned SR 422.

Since highways in the United States and Canada are usually signed with assigned cardinal directions based on their primary orientation, it is possible for a stretch of roadway shared between two highways to be signed with conflicting, even opposite, cardinal directions in a wrong-way concurrency.^{[citation needed]} For example, near Wytheville, Virginia, there is a concurrency between Interstate 77 (which runs primarily north–south, as it is signed) and Interstate 81 (which runs primarily northeast–southwest, but is also signed north–south). A vehicle might simultaneously be on I-77 northbound and I-81 southbound, while actually traveling due westbound. An unusual example of a three-directional concurrency occurs southeast of Rhinelander, Wisconsin, where US 8 westbound (the actual compass direction) converges with southbound Wisconsin Highway 17 and northbound Wisconsin Highway 47, and vice-versa.

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.)

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.