An extreme example: I-40, I-85 Business, US 29, US 70, US 220, and US 421 formerly ran concurrently in Greensboro, North Carolina. In 2008, I-40 was rerouted off this road, but was put back on it later that same year with US 220 being rerouted instead. US 421 was also rerouted in 2009. I-85 Business was truncated in 2018, and then eventually decommissioned between 2019-2023.
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.
^Kanillopoolos, John J. (March 17, 1983). "Dual and Triple Routing on State Trunklines". Letter to Trunkline Numbering Committee. Lansing: Michigan Department of Transportation. Retrieved June 3, 2019 – via Wikisource.