Guide to Road Design Part 4A: Unsignalised and Signalised Intersections in Australia

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Designing safe and efficient road intersections is crucial to managing traffic flow and minimizing accidents. The Austroads Guide to Road Design Part 4A provides comprehensive guidance on the design, selection, and implementation of unsignalised and signalised intersections across Australia. This guide helps urban planners, engineers, and policymakers create safer roads while ensuring smooth traffic movement.

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In this blog, we will explore the key aspects of unsignalised and signalised intersections, their design criteria, and safety considerations based on the Austroads Guide to Road Design Part 4A​.

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Understanding Unsignalised and Signalised Intersections

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1. Unsignalised Intersections

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Unsignalised intersections are those without traffic signals and rely on road signs, markings, and priority rules for traffic control. These intersections are commonly found in low-traffic areas but require careful design to ensure adequate visibility, clear priority rules, and safe pedestrian crossings.

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Types of Unsignalised Intersections:

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• Unchannelised Intersections: No dedicated turning lanes; suitable for low-traffic areas.
• Flared Intersections: Wider approaches to accommodate turning vehicles.
• Channelised Intersections: Use of traffic islands and lanes to guide vehicles safely.

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2. Signalised Intersections

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Signalised intersections use traffic lights to regulate the movement of vehicles and pedestrians. These are common in urban and high-traffic areas where controlling traffic flow is essential to prevent congestion and accidents.

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Key Features of Signalised Intersections:

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• Traffic signals and phases for different vehicle movements.
• Dedicated turn lanes to reduce waiting time and improve efficiency.
• Pedestrian crossings and safety measures to enhance accessibility.
• Advanced traffic management systems for coordination with nearby intersections.

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Key Design Considerations for Intersections

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1. Sight Distance Requirements

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Visibility is crucial in intersection design. The guide outlines minimum sight distance requirements to ensure drivers can see approaching vehicles and pedestrians in time to react safely.

Types of sight distances include:

• Approach Sight Distance (ASD): Ensures a clear view of the intersection ahead.
• Safe Intersection Sight Distance (SISD): Allows enough time for vehicles to enter an intersection safely.
• Minimum Gap Sight Distance: Ensures drivers can judge gaps in traffic to cross safely​.

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2. Traffic Islands and Medians

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Traffic islands and medians help manage traffic flow by guiding vehicles and providing safe waiting areas for pedestrians. The guide recommends:

• Raised islands and medians for better visibility and safety.
• Painted traffic islands in low-speed areas where raised structures are unnecessary.
• Appropriate dimensions to accommodate different vehicle types, including trucks​.

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3. Auxiliary Lanes for Smooth Traffic Flow

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Auxiliary lanes improve traffic efficiency by providing:

• Deceleration lanes for vehicles slowing down before turning.
• Acceleration lanes for vehicles merging into fast-moving traffic.
• Storage lanes for waiting vehicles to avoid blocking through traffic​.

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4. Safe Intersection Design for Cyclists and Pedestrians

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The guide emphasizes designing intersections that are safe for all road users, including cyclists and pedestrians.

• Dedicated bicycle lanes at intersections prevent conflicts with motor vehicles.
• Pedestrian crossings should have clear visibility, sufficient waiting space, and appropriate traffic signal timing.
• Cyclist bypass lanes at signalised intersections allow safer navigation through heavy traffic​.

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Intersection Safety and the Safe System Approach

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Austroads promotes the Safe System Approach, which aims to reduce fatal and serious injury crashes by:

• Designing intersections to minimize high-speed conflicts.
• Incorporating clear road markings, barriers, and safety buffers.
• Using traffic signals, signs, and advanced road technology to improve visibility and predictability​.

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Conclusion

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The Austroads Guide to Road Design Part 4A is a crucial resource for designing safe and efficient intersections in Australia. By implementing its guidelines, road designers can enhance traffic flow, reduce accidents, and improve road safety for all users.

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