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

1. 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.
2. Define core components:
   • Controllers, sensors, actuators, communications.
   • Data model and key APIs (configuration, control, telemetry).
3. 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).
4. Safety and reliability:
   • Conflict avoidance, intergreen (yellow/all-red) intervals, concurrency rules.
   • Preemption/priority (emergency, transit), failure detection, and safe fallback modes.
5. Operations:
   • Monitoring/observability, deployment strategy, and scaling to a citywide network.
6. 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?