Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade separated and compact grade separated junctions) dn-geo-03060
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- Figure 6.18: Staggering of Arms to Increase Deflection Figure 6.19: Typical Single Carriageway Exit at a roundabout with a Long Channelising Island
- Exit Kerb Radius
- 6.7.2 Forward Visibility on Approach (Stopping Sight Distance)
- Figure 6.20: Measurement of Stopping Sight Distance on Curved Approach
- 6.7.3 Forward Visibility at Entry
- 6.7.4 Visibility to the Right
- Table 6.1: Roundabout Minimum Visibility Requirement Inscribed Circle Diameter (m) Visibility Distance (m) (‘a’ in Figures)
TII Publications DN-GEO-03060 Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade separated and compact grade separated junctions) April 2017 Page 82 Figure 6.18: Staggering of Arms to Increase Deflection Figure 6.19: Typical Single Carriageway Exit at a roundabout with a Long Channelising Island TII Publications DN-GEO-03060 Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade separated and compact grade separated junctions) April 2017 Page 83 At a Single Lane Roundabout, the exit width should be similar to the entry width. On exits, the edge line should continue along the projected line of the kerbing once this is terminated (see Figures 6.29 and 6.30). Exit Kerb Radius The exit kerb radius (a) is shown in Figure 6.19. Values for the exit kerb radius should exceed the largest entry radius (except at Single Lane Roundabouts, where they should be equal). At a Single Lane Roundabout, the value of the exit kerb radius should be between 15m and 20m. At other roundabouts, the exit kerb radius should not be less than 20m or greater than 100m. A value of 40m is desirable, but for larger roundabouts on high speed roads, a higher value may suit the overall junction geometry. A compound curve starting with a 40m radius and developing to a larger radius, of up to 100m, will usually offer the best solution. Larger values of exit radii may lead to high exit speed, which will not be appropriate if there are significant numbers of cyclists using the junction or where pedestrian/cyclist crossing facilities are located immediately downstream. The shortest distance possible between an entry arm and the next exit is governed by the minimum entry radius (10m) and the minimum exit radius for the type of roundabout in question (15m at a Single Lane Roundabout, otherwise 20m). If a roundabout is to be modified to include an additional arm, care should be taken to ensure that this does not affect safety at the preceding entry and following exit. It may be necessary to redesign the whole junction if adequate spacing and deflection between entries and adjacent exits cannot be achieved. Exits should be checked to ensure that vehicle paths are smooth and vehicles are not directed towards channelising islands. Channelising islands should end at a tangent (or, at least, parallel) to the centre line and be long enough to prevent an exiting vehicle from crossing the centre line into oncoming traffic. If the peak exit volume approaches the capacity of the downstream link, tapers longer than 1:20 may be needed to merge the traffic as the traffic density in each lane will be high. If circulatory speeds are high sharp turns into exits can increase the likelihood of load shedding by HGVs and decrease the traffic capacity of the junction. TII Publications DN-GEO-03060 Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade separated and compact grade separated junctions) April 2017 Page 84 6.7 Visibility 6.7.1 General Except for visibility to the right at entry and across the central island as set out in this Chapter, the envelope of visibility for the measurement of stopping sight distance on the approaches to the roundabout shall be in accordance with DN-GEO-03031. Where signs are to be erected on a central reserve, verge or channelising island within the envelope of visibility, including to the right, the mounting height must not be less than 2m above the carriageway surface. 6.7.2 Forward Visibility on Approach (Stopping Sight Distance) Visibility on the approach (Desirable Minimum Stopping Sight Distance for the design speed of the road) must be measured to an object at the yield line as indicated in Figure 6.20. Visibility shall be measured and meet the requirement stated in DN-GEO-03031. Chevron signs on the central island must also be visible to approaching drivers in all lanes from a distance equal to the Desirable Minimum Stopping Sight Distance. Chevron signs should not be stacked. If conspicuity of the signs is a problem, grey backing boards or larger signs should be used. If the approach to the roundabout is over a crest, a higher mounting height may be necessary. Refer to the Traffic Signs Manual. TII Publications DN-GEO-03060 Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade separated and compact grade separated junctions) April 2017 Page 85 Figure 6.20: Measurement of Stopping Sight Distance on Curved Approach TII Publications DN-GEO-03060 Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade separated and compact grade separated junctions) April 2017 Page 86 6.7.3 Forward Visibility at Entry Drivers of all vehicles approaching the yield line must be able to see objects on the roadway for the full width of the circulatory carriageway for the Visibility Distance given in Table 6.1 (measured along the centre of the circulatory carriageway as shown in Figure 6.21). The height of objects shall be consistent with those used for measurement of stopping sight distance as outlined in DN-GEO-03031. The visibility must be checked from the centre of the nearside lane at a distance of 15m back from the yield line, as shown in Figure 6.21. 6.7.4 Visibility to the Right Drivers of all vehicles approaching the yield line must be able to see the full width of the circulatory carriageway to their right, from the centre of the offside lane at the yield line, for the Visibility Distance given in Table 6.1 (measured along the centre of the circulatory carriageway), as shown in Figure 6.22. Table 6.1: Roundabout Minimum Visibility Requirement Inscribed Circle Diameter (m) Visibility Distance (m) (‘a’ in Figures) <40 Whole junction 40 – 60 40 60 – 100 50 >100* 70 *ICD >100m shall require a Departure from Standards. Visibility to the right must also be provided from the centre of the offside lane at a distance of 15m back from the yield line, as shown in Figure 6.23. The envelope of visibility to the right must be obtainable from a driver’s eye height of between 1.05m and 2m to an object height of between 1.05m and 2m. Object height is adjusted for this visibility check and is expected to be for oncoming traffic and not objects on the roadway. Excessive visibility to the right can result in high entry speeds, potentially leading to collisions. On dual carriageway approaches where the speed limit is greater than 60km/h, limiting visibility to the right by screening until the vehicle is within 15m of the yield line can be helpful in reducing excessive approach speeds. The screening should be at least 2m high, in order to block the view of all road users. Screening can also be used on flared approaches on high speed single carriageway roads where there is a long channelising island. Yüklə 8,57 Mb. Dostları ilə paylaş:
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