roundabout

A roundabout is a type of circular intersection designed to improve traffic flow and safety by channeling vehicles around a central island. At a roundabout, drivers on approaches must yield to traffic already circulating, then exit to their desired road. In countries with right-hand traffic, vehicles circulate counterclockwise around the island; in left-hand traffic countries, the opposite is true. Pedestrians typically cross at designated crosswalks on the approaches, often with splitter islands that provide refuge.

Design features include the central island, splitter islands that split traffic and reduce speed, deflection of entering traffic to slow speeds, a circulating lane, clear signage and lighting, and often elevated or painted guidance. Roundabouts can be classified as mini-roundabouts (small center island, often a circle of raised material), single-lane roundabouts, or multi-lane roundabouts.

They differ from traffic circles and rotaries in that roundabouts are designed to keep traffic moving at low speeds with yield-at-entry rules and fewer conflict points; traffic circles and rotaries are typically larger and may operate with or without yield control, historically allowing higher speeds and more priority conflicts.

Benefits: Roundabouts reduce most severe injury crashes by lowering vehicle speeds and simplifying conflict points; they can improve throughput and reduce delays compared with signalized intersections, especially at moderate to high traffic levels. They may require more space and can present design challenges for pedestrians and cyclists, but modern roundabouts include pedestrian crossings and refuge islands.

History and adoption: Modern roundabouts originated in the United Kingdom in the mid-20th century, with Frank Blackmore credited for redesigning the approach to yield-before-entry and deflection. The concept spread worldwide and is now a standard intersection design in many countries, with guidelines from national transportation agencies.