Roundabout
[1][2] In the United States, engineers use the term modern roundabout to refer to junctions installed after 1960 that incorporate design rules to increase safety.
[4] Variations on the basic concept include integration with tram or train lines, two-way flow, higher speeds and many others.
[15]: 3:02 Widespread use of the modern roundabout began when the UK's Transport Research Laboratory engineers re-engineered and standardised circular intersections during the 1960s.
Frank Blackmore led the development of the "priority rule" and subsequently invented the mini-roundabout[16][17] to overcome capacity and safety limitations.
A Transportation Research Board guide reports that the modern roundabout represents a significant improvement, in terms of both operations and safety, when compared with older rotaries and traffic circles.
[3] In the United States modern roundabouts emerged in the 1990s after years of planning and educational campaigning by Frank Blackmore and Leif Ourston, who sought to bring the by then well-established increased safety and traffic flow in other countries to America.
In the mid-2010s, about 3% of the then circa 4,000 U.S. modern roundabouts were located in Carmel, Indiana, whose mayor James Brainard had been actively promoting their construction; because of increased safety, injuries caused by car accidents in the city dropped by 80% after 1996.
[23] A "modern roundabout" is a type of looping junction in which road traffic travels in one direction around a central island and priority is given to the circulating flow.
In the United States, traffic engineers typically use the term rotary for large-scale circular junctions between expressways or controlled-access highways.
Denmark has begun widespread adoption of particular high islands, or if not possible, obstacles such as hedges or a ring of trees in larger examples.
[54] The central island may be surrounded by a truck apron that is high enough to discourage drivers from crossing over it, but low enough to allow wide or long vehicles to navigate the roundabout.
A 1992 study[70] found that the risk to cyclists is high in all such intersections, but much higher when the junction has a marked bicycle lane or sidepath around its perimeter.
When the volumes on the roadways are relatively equal, a roundabout can reduce delays, because half of the time a full stop would be required.
The TRL approach is derived from empirical models based on geometric parameters and observed driver behaviour with regard to lane choice.
A recent NCHRP survey of US state transport agencies found that Sidra Intersection is the most widely used software tool in the US for roundabout analysis.
The New Zealand researchers propose that low vehicle speeds, circulatory lane markings and mountable centre aprons for trucks can reduce the problem.
The next most common crash type involves motorists leaving the roundabout colliding with cyclists who are continuing farther around the perimeter.
Poorly designed walkways increase risks for the vision-impaired, because it is more difficult than at a signalised intersection to audibly detect whether there is a sufficient gap in traffic to cross safely.
This increases construction and operation costs, and requires some way to disrupt traffic long enough for the pedestrian to cross (such as a HAWK beacon) that defeats the purpose of the roundabout.
As the overall or external size of a roundabout (in the UK referred to as the Inscribed Circle Diameter – ICD) is reduced, so the maximum practicable (and prescribed) diameter for the central island is also reduced, whilst the width of the circulatory carriageway increases (due to the greater width of vehicle swept path at smaller turning radii).
The effect of this is to discourage drivers from taking a more direct path through the roundabout, their line of least resistance is more tightly curved (and therefore slower) but more bearable.
In the Netherlands, Belgium, Bosnia and Herzegovina, the United Kingdom, Finland, Spain, Poland, Hungary, Slovenia, Slovakia, the Czech Republic, North Macedonia, Croatia, Germany, and the U.S. states of Alabama,[96] California, and Florida, a relatively new type of two-lane roundabout designs is emerging, called "turbo roundabouts".
An early application of the principle was a six-arm and therefore relatively large (and fast) non-circular roundabout at Stairfoot, Barnsley, South Yorkshire, which was given spiral marking about 1984.
The use of raised lane separators prevents road users from weaving (thereby reducing conflicts) but can make manoeuvring more difficult for large vehicles.
Because there are only ten points of conflict (compared with 8 for a conventional single lane roundabout, or between 32 and 64 with traffic signal control), this design is often safer as well.
The traffic volume of several of these junctions increased to a level higher than the capacity such roundabouts can accommodate, and in turn, have been converted into partially or fully free-flowing interchanges.
In Northern Ireland, the junction between the M1 and M12 (Craigavon connector motorway) is via a standard roundabout with a raised centre, three onslips and three offslips, and two lanes.
The landscaped central island bisected by the tracks was originally curbed/kerbed, but 18-wheelers had trouble negotiating the roundabout, so the curbs were replaced with painted concrete strips.
At the Driescher Kreisel[121] in Bergisch Gladbach, Germany, a railway serving a nearby paper factory crosses a roundabout located next to a shopping centre and pedestrian zone.
The United Kingdom has examples on the A580 East Lancashire Road in St Helens, on Haydock Island in Merseyside[124] (which also features the M6 passing overhead), and on the Astley/Boothstown border.
