Design an intersection traffic-light system

Q: Design an intersection traffic-light system

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.

Q: How do I approach System Design interview questions?

System Design questions require understanding of core concepts and practice. PracHub provides solutions with explanations to help you master system design interviews.

Q: What difficulty level is this interview question?

This is a hard difficulty System Design question, commonly asked during Technical Screen rounds at Amazon.

Q: What role is this question designed for?

This question is commonly asked for Software Engineer candidates at Amazon during technical interviews.

Question

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

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.
Define core components:
- Controllers, sensors, actuators, communications.
- Data model and key APIs (configuration, control, telemetry).
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).
Safety and reliability:
- Conflict avoidance, intergreen (yellow/all-red) intervals, concurrency rules.
- Preemption/priority (emergency, transit), failure detection, and safe fallback modes.
Operations:
- Monitoring/observability, deployment strategy, and scaling to a citywide network.
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?

Design an intersection traffic-light system

Quick Overview

System Design: Traffic-Light Control for a Multi-Road Intersection

Context

Requirements

Solution

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