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Roundabout Design Basics

These statements are very general but follow the process of design.

Horizontal Design: As modern roundabouts replace cross intersections, right angle crashes become less severe and less frequent, and left turning crashes do not occur. Rear-end crashes become less frequent because roundabouts have less queuing. To provide good vehicle path deflection, an important safety design feature that slows traffic on entry, the approach roads are aimed toward left of the central island. They veer back to the right just before the yield lines. Roundabouts are typically designed for speeds from 20 to 25 mph.

Once the horizontal alignment and location of the center island are determined the geometry is checked by turning templates to see if the design vehicle (usually a truck or a bus)can travel through the roundabout without leaving the pavement. Changes are made to the entries, circulatory lane, and exits as needed.

Vertical design follows horizontal design to ensure that approach roads enter and exit the roundabout at optimal grades and smoothness of curves. Care is taken to design the roundabout plateau with a cross slope of 5% or less.

The capacity of the roundabout is analyzed based on the design parameters. Modern roundabouts are designed for minimum speed and size. The number of entry lanes depends on the future capacity needs. Generally, single-lane roundabouts experience a lower number of accidents than multi-lane roundabouts.

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