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📘 How does a traffic light coordinate an intersection?

Learn how phases, detectors, conflict checks, and clearance intervals share an intersection safely.

lessons

~10 min

to learn

🔬 Science

subject

Adults

level

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What you’ll learn

Movements
Phase
Controller
Detection
Green timing
Yellow
Conflicts
Pedestrians
Coordination
The complete cycle

Questions this course answers

What is a traffic-signal phase?

FHWA defines a phase as the right of way, yellow change, and red-clearance intervals for a movement or group.

What does an actuated controller use detectors for?

Detector calls let the controller vary phase service within programmed limits.

Why is an all-red clearance interval used?

The interval separates conflicting movements while the intersection clears.

What prevents incompatible green indications?

Safety logic checks conflicting outputs and forces a restrictive state when needed.

What does an offset do in coordinated signals?

Offsets align phase timing across intersections to support planned progression.

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