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https://nap.nationalacademies.org/catalog/27432/critical-issues-in-transportation-for-2024-and-beyond

Two-lane roundabouts. Design principles

2-kaistaisten kiertoliittymien suunnitteluperiaatteet

Finland has about 15 years of experience with new types of roundabouts and about 10 years of experience with two-lane roundabouts. Eleven two-lane roundabouts have been built since the 1990s, and several are currently being planned for both the street and road networks. Ex-periences with roundabouts have been positive, so two-lane roundabouts can be recom-mended where traffic volume necessitates them, i.e. in built-up areas. This study examined what types of sites two-lane roundabouts are suited for, compared different types of two-lane roundabouts and assessed their special features. Design guidelines for intersections currently describe two different types of two-lane roundabouts: a two-lane roundabout without lane lines and a two-lane roundabout partly closed by road markings. Nevertheless, Finland also has other types of two-lane roundabouts, such as a fully two-lane roundabout containing two lanes marked by lane lines and a so-called turbo roundabout developed in Holland. The in-coming direction of a roundabout requires two lanes if the traffic volume entering and circling the roundabout from that direction exceeds 1400 vehicles/hr. Two lanes are required in the exiting direction if the traffic volume in that direction exceeds 1500 vehicles/hr. If the exiting traffic volume is 1200 - 1500 vehicles/hr., the need for two lanes must be assessed case by case. The capacity of a two-lane roundabout is estimated to be around 40 % greater than that of a one-lane roundabout. A two-lane roundabout is a safe type of roundabout, but not as safe as a one-lane roundabout. The traffic environment of a two-lane roundabout is more difficult and there are more places for conflicts than in a one-lane roundabout. The most typical acci-dents are collisions between exiting and circling vehicles and between circling and entering cars. Rear-end collisions, minor sideswipes and collisions with traffic islands or traffic signs also occur in roundabouts. When designing two-lane roundabouts it is necessary to ensure that vehicles in adjacent lanes and the geometry of the roundabout do not form dangerous obstacles to lines of sight. Intersecting driving paths, i.e. places for conflicts must be designed to be as safe as possible. Lane changes in the roundabout should be minimised. Entering and exiting lane arrangements in a two-lane roundabout must take traffic regulations and driving behaviour into consideration. Public transport, pedestrian and bicycle traffic and special trans-ports place special requirements on the design of two-lane roundabouts. If a bus stop is located along the exit immediately after the roundabout, public transport must be able to exit from the roundabout using the outside lane. This requirement may even influence selection of the type of roundabout. It is recommendable to implement pedestrian and bicycle traffic grade-separated. Roundabouts should be equipped with lighting. The design of lighting and signs should take into account the space required by special transports. Two-lane round-abouts without lane lines are suitable for built-up areas with considerable local traffic and only one-lane exits. A partly two-lane roundabout is the most common type of two-lane roundabout in Finland. It is suitable for situations where there is a busy primary direction or much turning traffic. The basic principle of a turbo roundabout is that the desired lane is chosen before en-tering the roundabout, and the roundabout is circled toward the exit direction without changing lanes. A turbo roundabout is especially suitable when traffic on the main road is busy or there is much turning traffic, but also where there are three busy directions. A fully two-lane round-about with two lanes in the exiting direction is not suitable for Finnish driving culture, and it is not recommended for use in Finland. This report may be found at http://alk.tiehallinto.fi/julkaisut/pdf/3201018-v-2kaistkiertoliitt.pdf

Language

  • Finnish

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Filing Info

  • Accession Number: 01094812
  • Record Type: Publication
  • Source Agency: TRL
  • ISBN: 978-951-803-778-4
  • Files: ITRD
  • Created Date: Apr 25 2008 7:58AM