Which of the following factors primarily contributes to queueing delay?

The primary factor that contributes to queueing delay is the presence of vehicles already waiting. Queueing delay occurs when vehicles are placed in a line, typically at intersections or traffic signals, due to demand exceeding the available capacity. When there are vehicles already waiting, it leads to an increased queue length and consequently prolongs the time any new vehicles must wait to clear that intersection or reach their destination.

The accumulation of vehicles creates a situation where the flow of traffic is impeded, and new arrivals must wait until those already in the queue can move forward. This delay is inherently tied to the existing traffic conditions, particularly the volume of traffic and the rate at which it can move through the intersection or road segment.

In contrast, while signal timing adjustments and control delays can influence overall traffic flow, they are not the primary cause of queueing delay. These factors may help optimize traffic movement but do not create the queue itself. Similarly, the negotiation of turns at intersections can lead to delays, but again, the actual queueing delay is fundamentally tied to the presence of vehicles already waiting in that queue.