A green light is part of a timed handoff
At a signalized intersection, the controller cannot simply give green to whichever approach looks busiest at this instant. It must first end the current movement safely. That usually means a yellow interval, followed by a brief all-red interval in which every direction is held at red while vehicles already in the intersection clear. Only then can a conflicting direction receive green.
Those intervals are built into the signal plan. Even if the road has emptied, the controller may still be completing a phase it has already started. Cutting it short whenever traffic appears would make the handoff harder for drivers to predict and could leave someone entering at the end of a green with too little time to clear.
The signal may be serving traffic you can’t see
Some lights follow a fixed schedule; others use detectors to extend or request green based on approaching vehicles. A detector does not necessarily make the signal switch instantly: the current phase still has to end safely, and the next phase may have a minimum green or other timing requirements. Pedestrian buttons can also place a crossing request in the sequence. The walk interval and the time afterward for people to finish crossing must be accounted for before traffic is released across their path.
Signals may also be coordinated with nearby intersections so a stream of vehicles can move through several lights without stopping at each one. That can mean holding a green or delaying a change even when this particular junction looks quiet. An empty approach is only one input; the controller is also managing safe transitions, crossing time, and the rhythm of the surrounding network.
