Urban Junction Design: Corner Radius Guide

Urban Junction Design
Corner Radius Selection for Safety & Efficiency

flowchart TD A["""Design Vehicle Selection Step"""] --> B{"""Vehicle Type Classification"""} B --> C1["""Speed Control Vehicle Passenger Car Controls turning speed at corners"""] B --> C2["""Design Vehicle City Bus, WB-12 Truck Regular users Determines lane width and turning path"""] B --> C3["""Control Vehicle Fire Truck, WB-33 DM Occasional users Allowed to encroach over curbs/lanes"""] B --> C4["""Non-Motorised Users Cyclists, Pedestrians Require safe, continuous protected routes"""] C1 --> D["""Corner Radius Design Process"""] C2 --> D C3 --> D C4 --> D D --> E{"""Corner Radius Size Decision"""} E --> F1["""Smaller Radius → Slower turning speeds → Shorter pedestrian crossing → Higher pedestrian safety → Better cyclist alignment"""] E --> F2["""Larger Radius → Faster turning speeds → Longer pedestrian crossing → Lower pedestrian safety → Forces wide cyclist swings"""] F1 --> H["""Recommended Corner Radii by Context"""] F2 --> H H --> I1["""Low-Density Residential 5 m radius 7.5 m max for large vehicles Prioritizes safety"""] H --> I2["""City / Town Commercial Areas 7.5 m radius Balanced design for buses and safety"""] I1 --> J["""Ensure 6 m effective travel width Avoid refuge islands in tight corners"""] I2 --> J J --> K["""Final Outcome Safe, Efficient Balanced Junction Geometry"""] style F1 fill:#d4edda,stroke:#28a745,strokeWidth:2px style F2 fill:#f8d7da,stroke:#dc3545,strokeWidth:2px style K fill:#d4edda,stroke:#28a745,strokeWidth:3px,color:#155724

1. Vehicle Hierarchy in Junction Design

All vehicle types influence corner design, but with distinct roles:

Vehicle Type	Role in Design
Speed Control Vehicle Passenger Car	Sets maximum safe turning speed at corners
Design Vehicle City Bus, WB-12 Truck	Determines minimum lane width and turning path geometry
Control Vehicle Fire Truck, WB-33 DM	Allowed to encroach over curbs or lanes — do not oversize for them
Non-Motorised Users Cyclists, Pedestrians	Must have continuous, protected routes — priority in radius choice

2. Corner Radius Trade-Off

Impact	Smaller Radius	Larger Radius
Turning Speed	↓ Slower	↑ Faster
Pedestrian Crossing Distance	↓ Shorter	↑ Longer
Pedestrian Safety	↑ Higher	↓ Lower
Cyclist Comfort	↑ Better path alignment	↓ Forced wide swings

Design Principle: Use the passenger car to control speed, not the fire truck. Let large vehicles encroach — do not widen corners for them.

3. Recommended Corner Radii

Context	Recommended Radius	Design Notes
Low-Density Residential	5 m 7.5 m max for large vehicles	Prioritizes pedestrian safety Discourages high-speed turns
City / Town / Commercial	7.5 m	Balanced design for buses Maintains safety with moderate speeds

Lane & Island Rule: Always ensure 6 m effective travel width for design vehicle.
Avoid refuge islands in tight corners — they complicate turning paths.

Final Design Outcome:
Safe, Efficient, Balanced Junction Geometry

Key Takeaways for Urban Planners & Engineers

• Prioritize pedestrian safety with smaller radii where possible
• Use passenger cars to control speed, not emergency or freight vehicles
• Allow larger vehicles to encroach instead of oversizing corners
• Balance needs of regular users (buses) vs. occasional users (fire trucks)
• Integrate cyclist and pedestrian routes from the conceptual stage

Urban Design Guideline • Based on Multimodal Safety Principles • Last Updated: November 2025