Concurrency (road)

Concurrency (road)

A concurrency, overlap, or coincidence in a road network is an instance of one physical road bearing two or more different highway, motorway, or other route numbers. When it is two freeways that share the same right-of-way, it is sometimes called a common section or commons.

Road enthusiasts often use the term multiplex—as well as the more specific duplex and triplex—to refer to such instances although those type are the more common instances.

Concurrency is a relatively common phenomenon: where two routes must pass through a single geological feature, or crowded city streets, it is often both economically and practically advantageous for them both to be accommodated on one road.

Often when two routes with exit numbers overlap (concurrency), 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. 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 where I-75 is dominant—the exit numbers range from 242 to 251 while I-85's highest mile marker in Georgia is 179. I-40 in Tennessee (a neighboring state of Georgia), which is also concurrent with I-75, has the instance of its exit numbers in the concurrency range being higher than I-75's highest exit number in the state.