Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade separated and compact grade separated junctions) dn-geo-03060
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- Figure 4.1: Flow Chart Outlining Junction Selection Procedure (Paragraphs 4.2.1 to 4.2.5)
- 4.2.1 Stage 1 – Initial Choice of Appropriate Junction Form
- 4.2.1.1 Priority Junctions
- 4.2.1.2 Roundabouts
- 4.2.1.3 Grade Separated Junctions
- 4.2.1.4 Compact Grade Separated Junctions
- 4.2.2 Stage 2 – Consider Layout/Size of Junction Type
- 4.2.3 Stage 3 – Assess and Refine Preliminary Design
TII Publications DN-GEO-03060 Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade separated and compact grade separated junctions) April 2017 Page 24 Figure 4.1: Flow Chart Outlining Junction Selection Procedure (Paragraphs 4.2.1 to 4.2.5) TII Publications DN-GEO-03060 Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade separated and compact grade separated junctions) April 2017 Page 25 4.2.1 Stage 1 – Initial Choice of Appropriate Junction Form The most appropriate type of junction to be used depends on a number of factors but primarily safety and operational performance, and will be subject to the evaluation of design year traffic movements at the junction, the nature and proportions of large vehicles and a road safety audit. The following section presents the most appropriate junction types based on projected traffic flows on both the major road and minor road. These values can be used for an initial assessment of the most appropriate junction type, however the final junction type will be subject to traffic analysis by the designer to assess the capacity based on the projected turning movements at the junction. 4.2.1.1 Priority Junctions Simple priority junctions are the most appropriate junction type for all local accesses on single carriageway roads. On dual carriageways simple junctions must be restricted to left in/left out only with the exception of single lane sections of Type 3 Dual Carriageways where right turns off the major road are permitted. Further guidance in relation to simple junctions on Type 3 Dual Carriageways is given in Chapter 5. For junctions with a lightly trafficked minor road the provision of a simple priority junction is the most appropriate junction type where the projected traffic flows (2-way Annual Average Daily Traffic - AADT) are less than those presented in Table 4.1 for both the major road and the minor road. Where traffic flows fall within the ranges outlined in Table 4.1, the provision of a ghost island junction is the most appropriate junction type. The final junction type will be subject to traffic analysis by the designer to assess the capacity based on the projected turning movements at the junction. Table 4.1: Flow Ranges – Ghost Island junctions Major road AADT Minor road AADT < 5,000 > 600 < 5,000 5,000 - 10,000 > 450 < 3,000 > 10,000 > 300 < 1,500 Note: AADT values provided should only be used as an initial assessment of the most appropriate junction type, the final junction arrangement shall be informed by a detailed analysis of peak hour flows (see Appendix D). On Type 2 and Type 3 Single Carriageway schemes, nearside passing bays shall be provided at all simple priority junctions that do not warrant a ghost island right turn lane. At traffic flows greater than those noted in Table 4.1, the provision of an alternative junction design such as a roundabout, compact grade or full grade separated junction should be considered. 4.2.1.2 Roundabouts Roundabouts work most efficiently when vehicular flows are reasonably balanced between the arms, but they may also be the optimum choice in other cases subject to traffic analysis by the designer based on the projected turning movements at the junction. Roundabouts should be designed to match forecast peak hourly flows. The capacity of roundabouts is determined by a number of factors such as their geometric design and whether they are single or multi-lane roundabouts. Entry width and sharpness of flare as described in Chapter 6 are the most important geometric parameters that determine capacity. TII Publications DN-GEO-03060 Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade separated and compact grade separated junctions) April 2017 Page 26 4.2.1.3 Grade Separated Junctions Grade separation to the standards contained in Chapter 7 of this document can be economically justified at design flows above 30,000 AADT on the major road, depending on turning traffic. It is possible to justify grade separation to lower design flows of 20,000 AADT on the major road, but again this is dependent on an analysis of turning traffic at the junction. 4.2.1.4 Compact Grade Separated Junctions Compact grade separation, as discussed in Chapter 8, may be considered for lower traffic flow situations on the major road and effectively extends downwards the range of flows and conditions over which grade separation could be justified economically to around 12,500 AADT on the major road. They appear to be suitable for use for mainline flows between approximately 12,500 AADT and 30,000 AADT. They are normally associated with very low flows (generally below 10% of mainline flow) on the minor road with the majority of traffic on both the major road and minor road being through traffic. Compact grade separated junctions can provide a suitable solution over a roundabout where through traffic on both the major and minor road is relatively high and turning traffic is relatively low. 4.2.2 Stage 2 – Consider Layout/Size of Junction Type Ordinarily, the 2-way AADT design year flows are used to consider junction layout to be provided. However, if there is evidence in the area of the junction of high seasonal variations, or if short, intense peaks in the traffic flows are likely, then the designer shall consider using the appropriate seasonal or peak hour flows in the initial capacity assessment to select a junction layout. A traffic study may be required to confirm that the selected junction type is appropriate at the discretion of TII. Traffic studies shall be conducted as per the guidance in the TII Project Appraisal Guidelines. Computerised methods shall be used by the designer to assess capacity and demonstrate that the particular junction layout chosen is appropriate for the traffic flows and turning movements. It is not realistic to calculate queue lengths and delays manually, reference should be made to PE-PAG-02015 Project Appraisal Guidelines for National Roads Unit 5.1. The range of reference flows developed should be used to produce trial designs for assessment. A flow to capacity ratio (RFC) of not greater than 75% is generally required when considering carriageways with design speeds of greater than 60 km/h. The selection of the junction type should be based on a consideration of the particular site characteristics and should be consistent with adjacent junctions upstream and downstream. Consideration needs to be given to the type of NMU facility incorporated into the junction. The type of crossing will depend primarily on the AADT on the road to be crossed. On a road carrying less than 12,000 AADT, an at grade crossing is acceptable while on roads with greater than 12,000 AADT, the provision of a grade separated crossing should be considered, taking into account the projected number of NMU’s and the availability of land. The design o f any grade separated NMU facility should be in accordance with DN-STR-03005 (Design Criteria for Footbridges) and DN-GEO-03040 (Subways for Pedestrians and Pedal Cyclists Layout and Dimensions). 4.2.3 Stage 3 – Assess and Refine Preliminary Design Stage 3 involves addressing all of the relevant safety issues to ensure as safe a design as possible including consideration of the following: Road users' specific requirements. A preliminary signing, street furniture, vehicle restraint system, and landscape design within the junction. 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