Design an intersection traffic-light system
Overview
This question evaluates system design competency across real-time control and safety-critical logic, API and data modeling, sensor/actuator integration, and operational scalability for traffic-signal controllers.
Amazon
Sep 6, 2025, 12:00 AM
Software Engineer
Technical Screen
System Design
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System Design: Traffic-Light Control for a Multi-Road Intersection
Context
Design the software and systems for controlling a signalized, multi-approach intersection. Assume you are building the on-controller logic, the local APIs, and the central management interfaces used to deploy and monitor a network of intersections.
Requirements
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State explicit goals and assumptions:
- Examples to address: number of approaches and lanes, protected/permissive turn arrows, pedestrian and bike crossings, emergency vehicles, public transit priority, railroad preemption (if any), and sensor availability/types.
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Define core components:
- Controllers, sensors, actuators, communications.
- Data model and key APIs (configuration, control, telemetry).
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Specify the control logic:
- State machine for signal phases and pedestrian intervals.
- Timing calculations: fixed-time vs. actuated/adaptive.
- Coordination across approaches and between intersections (peak/off-peak plans, offsets).
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Safety and reliability:
- Conflict avoidance, intergreen (yellow/all-red) intervals, concurrency rules.
- Preemption/priority (emergency, transit), failure detection, and safe fallback modes.
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Operations:
- Monitoring/observability, deployment strategy, and scaling to a citywide network.
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Follow-ups:
- With more time, what optimizations would you implement?
- How would you design comprehensive unit tests and simulations, including deterministic time control and edge cases?
Solution
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