Roundabouts: What They Are & Why They Work
by Michael Wallwork
Roundabouts are an increasingly popular approach to improving traffic flow and safety at intersections. Transportation engineer Michael Wallwork explains how roundabouts work, and compares their use to traffic signals.
From PCJ #26, Spring 1997
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For many years traffic signals have been seen by traffic
engineers, officials, and the public as the only solution to
capacity or crash problems at intersections. Unfortunately,
traffic signals often fail to meet expectations, and frequently
worsen delays and increase congestion, without improving safety.
To make matters worse, traffic signals are aesthetically ugly,
consume significant quantities of electricity, and require
costly maintenance.
In recent years, a growing number of state departments of
transportation, cities, and developers have adopted modern
roundabouts -- small, circular landscaped islands -- as an
alternative form of traffic control. The reason is that
roundabouts are the safest form of traffic control available. In
addition, they are cheaper, more efficient, have a higher
capacity, and can be attractively landscaped.
Roundabouts are usually small (ten feet in diameter for a small,
single lane, residential roundabout, up to 180 feet for a three
lane arterial roundabout). The approaches to the roundabout are
tangential to the central island with about a 60-degree angle of
intersection between the entering and circulating roadways. This
forced "deflection" means that vehicle speeds are constrained to
10 to 22 mph.
Motorists enter by selecting a gap in the circulating traffic. Their only decision is whether or not the gap is large enough for them to enter safely. If it is, they enter the roundabout. If there are no circulating vehicles, they can adjust their speed and enter without stopping.
The small diameter, restricted entry, and low circulating speeds provide a slow speed environment where drivers can and do make use of very small gaps in the circulating traffic stream. Interestingly enough, the slower circulating speeds actually increase traffic capacity, as drivers can enter much smaller gaps in
the traffic flow. Moreover, with roundabouts, no time is lost waiting for the traffic signal to change.
...
A roundabout in Hilton Head Island, South Carolina.
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