Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade separated and compact grade separated junctions) dn-geo-03060
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- Figure 2.6: Single Lane Roundabout
- 2.3.2 Multi-lane Roundabouts
- 2.4.1 Dumb-Bell Roundabout
- Figure 2. 8: “Dumb - bell” Grade Separated Junctions 2.4.2 Half-Cloverleaf
- Figure 2.9: Half Cloverleaf Grade Separated Junctions
TII Publications DN-GEO-03060 Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade separated and compact grade separated junctions) April 2017 Page 12 Figure 2.6: Single Lane Roundabout NOTES: Entries can be more perpendicular on single lane roundabouts to promote lower speeds Central overrun area may be required Road markings to be designed in accordance with the TSM TII Publications DN-GEO-03060 Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade separated and compact grade separated junctions) April 2017 Page 13 2.3.2 Multi-lane Roundabouts Multi-lane Roundabouts (Figure 2.7) require designers to achieve multi-lane entries and exits on each arm and allow two cars to travel on the circulatory carriageway. Figure 2.7: Multi-lane Roundabout NOTES: Lane markings on circulatory carriageway may or may not be required depending on roundabout size Road markings to be designed in accordance with the TSM TII Publications DN-GEO-03060 Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade separated and compact grade separated junctions) April 2017 Page 14 2.4 Grade Separated Junctions A grade separated junction is a separation of the levels at which major and minor roads cross each other in order to reduce traffic conflicts. A grade separated junction involves the use of an at-grade junction at the commencement or termination of slip roads. e) There are two main types of grade separated junction: Dumb-bell roundabout and Half-Cloverleaf. These junction types are mandatory on Motorways and Type 1 Dual Carriageways and are described in the following sections. Grade Separated Junction types are described in further detail in Chapter 7. 2.4.1 Dumb-Bell Roundabout The dumb-bell roundabout junction comprises two roundabouts situated either side of the major road. The minor road crosses over or under the major road at one location using a bridge – see Figure 2.8. A Departure from Standards shall be required for a half diamond dumb-bell junction where access onto and off of the major road is provided in one direction only. Figure 2. 8: “Dumb - bell” Grade Separated Junctions 2.4.2 Half-Cloverleaf A half-cloverleaf is used where site conditions are such that a dumb-bell roundabout junction cannot be provided and the use of all four quadrants is not possible for the provision of a full interchange in accordance with DN-GEO-03041 (see Figure 2.9). The at-grade junction element should normally utilise two roundabouts. The half-cloverleaf requires the use of only 2 quadrants, which if possible should be chosen so as to minimise any right turn movements where priority junctions are provided. Consideration should be given to future improvement when considering this layout. TII Publications DN-GEO-03060 Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade separated and compact grade separated junctions) April 2017 Page 15 Figure 2.9: Half Cloverleaf Grade Separated Junctions TII Publications DN-GEO-03060 Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade separated and compact grade separated junctions) April 2017 Page 16 2.5 Compact Grade Separated Junctions An alternative to at-grade junctions are Compact Grade Separated Junctions. This provides a junction to a standard intended to enforce low traffic speeds and minimise land take. See Figures 2.10 and 2.11. Compact grade separated junctions are suitable for use on Type 2 dual carriageways where mainline flows are high but turning traffic may be relatively low. They are also suitable for use on Type 3 dual carriageways and on single carriageway roads where there is a high level of through traffic to reduce the conflict points for turning traffic. The objectives of compact grade separated junctions are to:- a) reduce the environmental impact by providing a compact junction layout with less landtake; b) extend the use of grade separation on the national road network; The benefits that a compact grade separated junction is intended to provide when compared to an at- grade junction are:- a) improvement of safety by reducing the number and severity of collisions; b) separation of high through traffic from minor road off the major road. c) a safer means of crossing high speed routes for all road users, including NMUs; d) regulation and continuity of vehicle speed for minor route traffic through the junction at a level appropriate to the layout standards; e) removal of right turn manoeuvres from high speed roads; The layout of the compact grade separated junction will in many cases be dictated by the constraints imposed by the existing junction to be modified, or site topography in the case of new routes. Yüklə 8,57 Mb. Dostları ilə paylaş:
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