Design Traffic Control System
Design a Traffic Control Signal System that manages signal states at intersections.
A Traffic Control Signal System is an automated system used to manage the movement of vehicles and pedestrians at road intersections by controlling traffic lights (red, yellow, green) in a coordinated and safe manner.
Its main objectives are to:
- Regulate traffic flow by assigning right-of-way through signal phases
- Reduce congestion and waiting times at busy intersections
- Enhance safety by preventing collisions and ensuring orderly movement
In this chapter, we will explore the low-level design of a traffic control signal system in detail.
Lets start by clarifying the requirements:
1. Clarifying Requirements
Before starting the design, it's important to ask thoughtful questions to uncover hidden assumptions and better define the scope of the system.
Here is an example of how a conversation between the candidate and the interviewer might unfold:
Candidate: Should the system manage a single intersection or multiple intersections?
Interviewer: Let’s design it to handle multiple intersections.
Candidate: Do we need to handle emergency scenarios like green light overrides for ambulances or fire trucks?
Interviewer: Not for this version. Assume normal conditions.
Candidate: Should the signal cycle support left/right turns with dedicated turn signals?
Interviewer: Yes, each direction can optionally have a dedicated turn signal in the cycle, though it's not mandatory for all intersections.
Candidate: Should we support a control panel or admin interface to start/stop or override signals?
Interviewer: Yes, provide basic manual override functionality via a simple admin interface.
After gathering the details, we can summarize the key system requirements.
1.1 Functional Requirements
- Control vehicle signals for all four directions at a single intersection
- Support fixed timing intervals for each signal phase (green, yellow, red)
- Support pedestrian signals that are synchronized with vehicle signals
- Allow configuration of turn signals (e.g., dedicated left turn green)
- Automatically cycle through signal phases in the correct order
- Prevent conflicting green signals across opposing directions
- Provide an admin interface to manually override or reset signals
- Log signal status transitions for diagnostics or analytics
1.2 Non-Functional Requirements
- Modularity: The system should have clearly separated components such as
SignalController,Light,Timer, andAdminPanel - Extensibility: The design should support future additions like adaptive signals, emergency vehicle handling, or IoT sensor integration
- Reliability: Signal transitions must happen consistently and without failure to prevent traffic collisions
- Real-Time Responsiveness: Timing should be precise to the second, ensuring synchronization across lights
- Testability: Signal logic and transitions should be independently testable
- Maintainability: The code should follow object-oriented design and be easy to update or extend
- Safety: The system must prevent conflicting signals from being active simultaneously