Rotary (roundabout) control: For an at-grade intersection using a roundabout, beyond what total entering traffic volume does a roundabout generally become unsuitable in standard highway practice?

Introduction / Context:
Roundabouts (rotaries) work best where approach flows are moderate and well balanced. When total entering traffic becomes too high, weaving sections and entry deflections no longer provide adequate capacity or safety, and other forms (signals/grade separation) are preferred.

Given Data / Assumptions:

• Total entering traffic is considered (all approaches combined), expressed in vehicles per hour.
• Standard design assumptions for urban conditions and typical passenger-car flow apply.
• No unusual geometric enhancements (e.g., multilane weaving lanes) are assumed.

Concept / Approach:
The capacity of a conventional single-lane roundabout is constrained by weaving and entry conflicts. Many design guides use a practical threshold near 3000 vehicles/hour (combined) for conventional rotaries; above this, queue spillback and crash risk increase rapidly, suggesting signals or grade separation.

Step-by-Step Solution:
Identify the control type: conventional roundabout with weaving-based capacity.Compare total entering volume to practical thresholds noted by standard practice.Conclude that beyond about 3000 vehicles/hour, performance and safety degrade.

Verification / Alternative check:
Weaving capacity formulas show sharp drops in level of service once flows exceed moderate levels. Field experience similarly supports converting to signals or multilane/grade-separated solutions at higher volumes.

Why Other Options Are Wrong:

• 4000–6000 vehicles/hour: typically too high for basic roundabouts; heavy delays occur.
• Not applicable: a practical upper limit is commonly recognized.

Common Pitfalls:

• Ignoring unbalanced approach flows that reduce effective capacity even below thresholds.
• Overestimating weaving section performance without adequate width and entry geometry.

Final Answer:
3000 vehicles/hour