Roundabout - Modern Roundabout

Modern Roundabout

The US (right-hand traffic); a similar sign is used in the Republic of Ireland (with directions reversed). UK (left-hand traffic) Australia (left-hand traffic)

A "modern roundabout" is a type of circular junction that was developed by the United Kingdom Transport Research Laboratory in the mid-twentieth century, in which road traffic must travel in one direction around a central island and priority is given to the circulating flow. Signs usually direct traffic entering the circle to slow down and to yield the right of way. The centre of a modern roundabout provides a visual barrier across the intersection to the drivers entering it. This functions to assist the drivers into focusing only on the traffic coming toward them in the path of the circle (right in clockwise flow of traffic, and left in anti-clockwise flow). This significantly reduces the conflicts of concern encountered at conventional intersections. When drivers can see across a roundabout, if there are other vehicles in any other part of the device, drivers tend to stop outside the roundabout rather than merging, waiting for the vehicles (even on the opposite side) to come around to pass them. This interferes with the flow of traffic through the intersection, where many vehicles should be able to circulate in the roundabout at the same time. The high barrier may be a landscaped mound, a raised wall, or thickly planted very tall shrubs. Flag poles at the top of a landscaped mound in a roundabout are popular items. Some communities use the area for monuments, the display of large public art, and in a few, for a fountain, but caution must be exercised for these in case the feature might attract pedestrians into the intersection, which is very dangerous. Any lighting should focus carefully on the centre, but not shine out from the feature in the centre of the circle into the eyes of oncoming drivers. Including light-coloured and variegated plants and a good proportion of white flowered plants among those chosen for landscaping makes the mound visible at a greater distance for drivers, especially for those roundabouts without lighting.

These junctions are called "modern roundabouts" in order to emphasise the distinction from older circular junction types which had different design characteristics and rules of operation. Older designs, called "traffic circles" or "rotaries", are typically larger, operate at higher speeds, and often give priority to entering traffic. In some cases, the term "traffic circle" has been used to describe roundabouts in North America by those who do not recognize the distinction. Originally a British term, "roundabout" now is often used in North America by officials and engineers, but until recently it remained rare in general U.S. usage, although commonplace elsewhere.

In countries where vehicles keep to the right, the traffic flow around the central island of a roundabout is anti- (or counter-) clockwise; where they keep to the left, it is clockwise.

Statistically, roundabouts are safer for drivers and pedestrians than both traffic circles and traditional intersections. Because low speeds are required for traffic entering roundabouts they usually are not used on controlled-access highways, but may be used on lower grades of highway such as limited-access roads. When such roads are redesigned to take advantage of roundabout principles, steps are taken to reduce the speed of traffic, such as curving the approaches. Sometimes the flow through the roundabout exceeds the level anticipated.

Modern roundabouts are commonplace throughout the world, in particularly Australia, Belgium, Canada, China, Costa Rica, Cyprus, Denmark, France, Germany, Hungary, Iceland, Ireland, Israel, Luxembourg, Malaysia, Morocco, the Netherlands, New Zealand, Poland, Portugal, Qatar, Spain, the United Arab Emirates, and the United Kingdom. Half of the world's roundabouts are in France (more than 30,000 as of 2008).