Definition of Roundabout
This problem has led to a conflict between the visually impaired and the civil engineering community in the United States. One solution is to provide manually operated pedestrian crossing signals at each entrance. This increases construction and operating costs and requires a way to disrupt traffic long enough for the pedestrian to cross it (such as a HAWK beacon) that cancels out the purpose of the roundabout. Signage also increases delays for most pedestrians during periods of light traffic, as pedestrians have to wait for a signal to change before crossing (legally).  The reduction in the width of roads at roundabouts and at exit and entry points is intended to encourage drivers to slow down.  Engineers use the term modern roundabout to refer to intersections installed after 1960 that incorporate various design rules to increase safety.   However, modern and non-modern roundabouts may have street names or be colloquially identified by local names such as roundabouts or roundabouts. The capacity of a roundabout varies depending on the angle of entry, the width of the lane and the number of entry and traffic lanes. As with other types of seals, operational performance is highly dependent on the flow rates of the different approaches. A single-lane roundabout can accommodate about 20,000 to 26,000 vehicles per day, while a two-lane design can accommodate 40,000 to 50,000.  I saw two bodies on the central island of a roundabout last week – they were still there two hours later. Roundabouts can increase delays in places where traffic would not otherwise have to stop. For example, at the intersection of a high-volume road and a low-traffic road, traffic on the busiest road would only stop if there was cross traffic, otherwise it would not have to slow down for the roundabout.
If the volumes on the roads are relatively the same, a roundabout can reduce delays, as half the time would require a complete stop. Dedicated left turn signals (in countries where traffic is driving on the right side) further reduce throughput. Also known as the Hamburger Roundabout, these intersections are signposted and have a straight lane section for one of the main routes. The name Hamburger is derived from the fact that the top view resembles the cross-section of a Hamburger. The UK has examples on the A580 East Lancashire Road in St. Helens, on Haydock Island in Merseyside (where the M6 also passes) and on the border between Astley and Boothstown.  Other examples include the A6003 at Kettering, the A538 near Manchester Airport, the Showcase junction on the A329 at Winnersh, Berkshire and the A63/A1079 Mytongate junction at Hull. Examples include Bracknell, Hull, Bramcote in Nottinghamshire and Reading, and at the N2/M50 junction in Dublin, Ireland.
In Perth, Western Australia, one is located at the intersection of Alexander Drive, Morley Drive and The Strand.  Passages are very common in Spain, where they are called raquetas (Spanish for “racket [tennis]”) or glorieta/rotonda partida (“divided roundabout”).  Physically separated bike paths best protect cyclists.    What is less optimal is that bike lanes are closed well before roundabout entrances, that cyclists enter the flow of vehicular traffic but keep cyclists in full view of drivers, which increases the speed of motor vehicles with a certain price. Cyclists may also be allowed to use pedestrian crossings. At the Drieschen Kreisel in Bergisch Gladbach, a railway serving a nearby paper mill runs through a roundabout next to a shopping centre and a pedestrian area. Traffic and pedestrians are controlled by 14 barriers, 22 traffic lights and 8 loudspeakers. The gates close three times a day for 7 minutes for trains to pass. Mini-roundabouts may contain a painted circle or a low dome, but must be fully passable by vehicles.
Motorists can pass them if there is no other traffic, but it is dangerous to do it otherwise. Once the practice is established, it can be difficult to discourage them. Mini-roundabouts use the same right-of-way rules as standard roundabouts, but produce different driving behavior. Mini-roundabouts are sometimes grouped in pairs (a double mini-roundabout) or in “chains”, making it easier to navigate otherwise unpleasant intersections. In some countries, road signs distinguish mini-roundabouts from larger ones. The floor markings invite cyclists to enter the sidewalk as they approach the roundabout in Mesa, Arizona. Cyclists are still allowed to use the roundabout like any other vehicle. A new layout of the signposted roundabout has recently been proposed.  It is based on avoiding stops by eliminating points of conflict in roundabouts. This proposed new paradigm (SYROPS) forms trains of vehicles (e.g. 2 x 3 cars) that arrive at the roundabout at a speed identical to the average speed of traffic in the roundabout and in the time interval (visualized as a rotating priority area) assigned to its entrance, avoiding all conflicts of passage and therefore the stops and accelerations required in standard and signaled roundabouts. Signage comes with lights for human drivers and optional wireless for connected and autonomous vehicles.
Since low speeds are required for traffic entering roundabouts, they are physically designed to slow down traffic entering the intersection to improve safety so that roads generally approach the intersection radially. While older style roundabouts can be designed to try to increase speed and have roads that enter the circle tangentially. Cyclists can choose to ride on the sidewalk on the far right or in the main lanes of this roundabout in Mesa, Arizona. According to the simulations, a two-lane roundabout with three exits should offer a traffic flow 12 to 20% higher than that of a conventional three-lane roundabout of the same size. The reason for this is the reduced weaving, which makes entry and exit more predictable. As there are only ten points of conflict (compared to 8 for a conventional single-track roundabout or between 32 and 64 for traffic light control), this construction is also often safer. Research and experiments show that traffic accidents on turbo roundabouts are reduced by 72% compared to multi-lane roundabouts, which have 12 points of conflict.  Research conducted at the University of Windesheim also shows that turbo roundabouts reduce accidents, including injuries, by about 75% compared to regular intersections and by 61% compared to single-lane roundabouts.  The same study clearly indicated that it is safer for cyclists not to have priority over motor vehicles at roundabouts than to have them.  At least 70 were built in the Netherlands, while many turbos (or similar track division designs) can be found in Southeast Asia.  Multi-lane roundabouts in the United States of America generally need to be struck with spiral markings, as most states follow the Federal Manual of Uniform Traffic Control Equipment.
The word roundabout comes from the United Kingdom of the early 20th century.  “Magic roundabouts” direct traffic in both directions around the central island. They are officially known as “ring crossings”. The first magic roundabout was built in 1972 in Swindon, Wiltshire, UK, and designed by Frank Blackmore, the inventor of the mini roundabout.