Grades 6–8 · Systems
How can nearby traffic signals work together?
A vehicle can leave one green signal and reach the next just as it turns red. Nearby signals can reduce such stops by relating their timing to the journey between them. An offset is the time shift between their repeating cycles. Starting both green intervals together is therefore different from arranging a green interval to meet an arriving vehicle.
In the diagramOne route · 60-second cycleABTrip 15 sDeparture 0 s · B offset 15 s03060 sGreen bars include start, exclude endArrival 15 s · inside B’s green window
This illustration uses a 60-second repeating cycle. Signal A's green interval begins at second 0 and ends just before second 30. Signal B has an equally long interval, but its start is shifted by the pictured offset. Arrival at B is departure time from A plus travel time.
With departure at second 0 and travel lasting 15 seconds, arrival is at second 15. B's 15-second offset means its green interval is beginning at that moment. Departure and arrival can therefore fall inside their respective green intervals even though the intervals do not start simultaneously.
The useful match is between an arrival and an available green interval, not between two lamps changing at the same instant. A different departure or a longer journey changes that match. Crossing traffic, pedestrians, turns and queues also need time, so coordinating one direction does not make every journey uninterrupted. Signal timing organizes competing movements through the same streets.
About this explanation
The diagram isolates one direction between two signals with a repeating 60-second cycle. Its green windows include their start and exclude their end; it is not a complete signal plan.
Starting knowledge: Add time intervals and compare an arrival with an open window.
Sources
- Federal Highway Administration — Traffic Signal Timing Manual, Chapter 6: Coordination (archived conceptual reference)