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Design a Traffic Signal Control System - Low-Level Design | OneNoughtOne

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Design a Traffic Signal Control System

Intermediate

Design an object-oriented traffic signal control system that manages signal cycling at intersections, coordinates multiple roads so only one has GREEN at any time, and supports emergency vehicle overrides and peak-hour timing adjustments. The system should model signals as state machines, intersections as coordinators, and expose a central controller that manages an entire road network.

Core Use Cases

Signal cycling

Automatically cycle signals through RED → GREEN → YELLOW → RED on a timer

Multi-road intersection management

Coordinate signals so only one road has GREEN at any time

Configurable timing

Each signal has independently configurable red, yellow, and green durations

Emergency vehicle override

Force a specific road to GREEN and all others to RED when an emergency vehicle approaches

Peak-hour mode

Extend green time for high-traffic roads during configurable peak periods

Multi-intersection control

A central controller manages multiple intersections across the road network

Constraints

Only one road in an intersection can have a GREEN signal at any time
A YELLOW phase must always occur between GREEN and RED transitions
Signal timings must be configurable per intersection and per road
Emergency overrides must take priority over normal cycling
The system must gracefully resume normal cycling after an emergency override ends

Assumptions

Standard 3-state signals (RED, YELLOW, GREEN) — no turn arrows
Each road at an intersection has exactly one signal
Timer-based cycling (no sensor or camera input)
Single-threaded simulation (real concurrency is out of scope)

In Scope

Signal state management and transitions
Intersection coordination logic
Emergency override mechanism
Peak-hour timing adjustments
Central controller for multiple intersections

Out of Scope

Vehicle detection sensors / cameras
Pedestrian crossing signals
Turn-arrow signals
GPS-based emergency vehicle detection
Physical hardware interface

Approach Guide(Click to expand each section)

Before diving into code, clarify the use cases and edge cases. Understanding the problem deeply leads to better class design.

List Core Use Cases

Identify the primary actions users will perform. For a parking lot: park vehicle, exit vehicle, check availability. Each becomes a method.

Identify Actors

Who interacts with the system? Customers, admins, automated systems? Each actor type may need different interfaces.

Clarify Constraints

What are the limits? Max vehicles, supported vehicle types, payment methods. Constraints drive your data structures.

Ask About Edge Cases

What happens on overflow? Concurrent access? Payment failures? Thinking about edge cases reveals hidden complexity.

Follow-up Questions(Questions an interviewer might ask)

💡 Draw UML class diagrams, sequence diagrams, or any design visualizations. Submit from the Code tab.

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00:00:00

Design a Traffic Signal Control System

Intermediate

Design an object-oriented traffic signal control system that manages signal cycling at intersections, coordinates multiple roads so only one has GREEN at any time, and supports emergency vehicle overrides and peak-hour timing adjustments. The system should model signals as state machines, intersections as coordinators, and expose a central controller that manages an entire road network.

Core Use Cases

Signal cycling

Automatically cycle signals through RED → GREEN → YELLOW → RED on a timer

Multi-road intersection management

Coordinate signals so only one road has GREEN at any time

Configurable timing

Each signal has independently configurable red, yellow, and green durations

Emergency vehicle override

Force a specific road to GREEN and all others to RED when an emergency vehicle approaches

Peak-hour mode

Extend green time for high-traffic roads during configurable peak periods

Multi-intersection control

A central controller manages multiple intersections across the road network

Constraints

Only one road in an intersection can have a GREEN signal at any time
A YELLOW phase must always occur between GREEN and RED transitions
Signal timings must be configurable per intersection and per road
Emergency overrides must take priority over normal cycling
The system must gracefully resume normal cycling after an emergency override ends

Assumptions

Standard 3-state signals (RED, YELLOW, GREEN) — no turn arrows
Each road at an intersection has exactly one signal
Timer-based cycling (no sensor or camera input)
Single-threaded simulation (real concurrency is out of scope)

In Scope

Signal state management and transitions
Intersection coordination logic
Emergency override mechanism
Peak-hour timing adjustments
Central controller for multiple intersections

Out of Scope

Vehicle detection sensors / cameras
Pedestrian crossing signals
Turn-arrow signals
GPS-based emergency vehicle detection
Physical hardware interface

Approach Guide(Click to expand each section)

Before diving into code, clarify the use cases and edge cases. Understanding the problem deeply leads to better class design.

List Core Use Cases

Identify the primary actions users will perform. For a parking lot: park vehicle, exit vehicle, check availability. Each becomes a method.

Identify Actors

Who interacts with the system? Customers, admins, automated systems? Each actor type may need different interfaces.

Clarify Constraints

What are the limits? Max vehicles, supported vehicle types, payment methods. Constraints drive your data structures.

Ask About Edge Cases

What happens on overflow? Concurrent access? Payment failures? Thinking about edge cases reveals hidden complexity.

Follow-up Questions(Questions an interviewer might ask)

💡 Draw UML class diagrams, sequence diagrams, or any design visualizations. Submit from the Code tab.

Loading canvas...

Design a Traffic Signal Control System

Core Use Cases

Constraints

Assumptions

In Scope

Out of Scope

Approach Guide(Click to expand each section)

Follow-up Questions(Questions an interviewer might ask)

Explore the full editorial.

Sign in to view submissions

Design a Traffic Signal Control System

Core Use Cases

Constraints

Assumptions

In Scope

Out of Scope

Approach Guide(Click to expand each section)

Follow-up Questions(Questions an interviewer might ask)

Explore the full editorial.

Sign in to view submissions

Design a Traffic Signal Control System

Core Use Cases

Constraints

Assumptions

In Scope

Out of Scope

Approach Guide(Click to expand each section)

Gather Requirements~3 min

Identify Classes & Entities~4 min

Define Relationships~3 min

Apply Design Patterns~4 min

Apply SOLID Principles~3 min

Code Organization~2 min

Follow-up Questions(Questions an interviewer might ask)

1How would you add pedestrian crossing signals that sync with traffic lights?

2How would you implement adaptive signal timing based on real-time traffic density?

3How would you coordinate a 'green wave' across consecutive intersections?

4How would you handle a signal malfunction (e.g., stuck on GREEN)?

Key Topics

Asked At

Explore the full editorial.

Sign in to view submissions

Design a Traffic Signal Control System

Core Use Cases

Constraints

Assumptions

In Scope

Out of Scope

Approach Guide(Click to expand each section)

Gather Requirements~3 min

Identify Classes & Entities~4 min

Define Relationships~3 min

Apply Design Patterns~4 min

Apply SOLID Principles~3 min

Code Organization~2 min

Follow-up Questions(Questions an interviewer might ask)

1How would you add pedestrian crossing signals that sync with traffic lights?

2How would you implement adaptive signal timing based on real-time traffic density?

3How would you coordinate a 'green wave' across consecutive intersections?

4How would you handle a signal malfunction (e.g., stuck on GREEN)?

Key Topics

Asked At

Explore the full editorial.

Sign in to view submissions