What condition allows for at least one lane of traffic to remain usable?

The condition that allows for at least one lane of traffic to remain usable refers to situations where ongoing traffic can continue to flow despite potential disruptions. When at least one travel lane is open, it means that vehicles are still able to navigate through the area, maintaining a level of mobility and reducing the risk of significant traffic delays or congestion.

In practical terms, this situation typically occurs during road work or maintenance when certain lanes may be closed to facilitate construction or repairs, but at least one lane remains accessible for vehicles. This ensures that traffic can still proceed, albeit perhaps at a reduced speed or in a single lane, which is crucial for preventing gridlock and facilitating better emergency response times.

Other scenarios, such as all lanes being blocked, a partial road closure with multiple lanes closed, or normal traffic flow, do not necessarily guarantee that a lane remains open. Therefore, the presence of at least one open travel lane is the definitive factor that aligns with the correct answer.