TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 101
The entry taper length for the segregated left turn lane shall be provided in accordance with Table 6.4.
The segregated left turn lane width shall be a minimum of 3.5m at the start of the entry taper, as shown
on Figure 6.31.
Any widening required to accommodate the swept paths of HGVs shall be developed along the length
of the entry taper. The length of the entry taper shall be calculated using the following method:
The length of the entry taper shall be dependent on the widening required to accommodate either the
segregated left turn lane island width, subject to a minimum width of 1.0m for a non-physical and 2.1m
(1.5m island width plus 0.3m marking offset on each side) for a channelising island, or the widening
required to accommodate the swept path of a HCV (Table 6.3). The larger of the two values shall be
used to calculate the entry taper length using the factors contained in Table 6.4.
Figure 6.34 shows an example of how to calculate the entry taper length for a 1.5m wide physical
island less than 50m in length, for a 70km/h single carriageway approach.
The taper for the hatching shall be developed asymmetrically on the segregated left turn lane side of
the entry taper as shown on Figure 6.34 and shall terminate in a position offset 0.3m from the edge of
a channelising island as shown on Figure 6.33. The 0.3m offset may be reduced to 0.15m where the
speed limit is 70 km/h or less.
Figure 6.33: Termination of Taper Hatching at Physical 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 102
Figure 6.34: Calculation of Entry/Exit Taper Lengths
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 103
Table 6.4: Desirable Minimum Entry/Exit Taper Length Factors
Design Speed (km/h)
Entry/Exit Taper Length Factor
50
20
60
20
70
20
85
25
≥100
30
6.12.5
Exit Layout of Segregated Left Turn Lanes
The exit layout from a segregated left turn lane shall be a dedicated lane consisting of an exit taper
as shown on Figure 6.31.
The dedicated exit lane (see Figure 6.31), provides a free running exit for traffic from the segregated
left turn lane.
The exit taper for the segregated left turn lane shall be provided in accordance with Table 6.4. The
segregated left turn lane width shall be a minimum of 3.5m at the end of the exit taper, as shown on
Figure 6.31. Any widening required to accommodate the swept paths of HGVs through the segregated
left turn lane shall be removed along the length of the exit taper. As with the entry taper, the length of
the exit taper shall be calculated by using the larger value when comparing the width of the segregated
left turn lane island with the width reduction required between the start and end of the exit taper, as
shown on Figure 6.34.
The taper for the hatching shall be developed asymmetrically on the segregated left turn lane side of
the exit taper as shown on Figure 6.34 and shall terminate in a position offset from the edge of a
channelising island in accordance with the details provided in the previous section of this Chapter.
The Entry Flaring section of this Chapter recommends that at the beginning of a roundabout exit, its
width should allow for an extra traffic lane over and above that of the link downstream. This extra width
should be reduced on the nearside, normally at a taper of 1:15 to 1:20. Where a segregated left turn
lane is present, the exit width reduction must be completed upstream of the end of the segregated left
turn lane exit taper. This may require extending the segregated left turn lane exit taper to
accommodate the roundabout exit width reduction.
The end taper will be dependent on site constraints and the use of the minimum taper values contained
in Table 6.5 is recommended.
Table 6.5: Minimum End Tapers
Design Speed
Minimum Taper
60 km/h
> 60 km/h
1:10
1:15
Where signs and street furniture are placed on the channelising island in the vicinity of the exit, they
should be located so as not to obstruct intervisibility between the segregated left turn lane exit and
adjacent roundabout exit lane.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 104
6.12.6
Signage of Segregated Left Turn Lanes
The presence of a segregated left turn lane shall be signed on the approach using an advance
direction sign.
The use of road markings to supplement Advance Direction Signs may also be considered.
6.12.7
Kerbing of Segregated Left Turn Lanes
The kerb height above the carriageway for raised and kerbed channelising islands should be 125mm.
6.12.8
Traffic Signs and Road Markings of Segregated Left Turn Lanes
The layout of the approaches to physical segregated left turn lane islands shall be marked using
Chevron markings detailed for the particular road type in Chapter 7 of the TSM but with 100mm line
widths for entry taper road markings.
Reflecting road studs shall comply with Chapter 7 of the TSM but with 100mm solid line width in
conjunction with Chevron markings to Chapter 7 but with 100mm lines, 800 min parallel width and
shall be used only when laid on the nearside. Guidance on the use of reflecting road studs is contained
in Chapter 7 of the TSM.
Designers should ensure that where the segregated left turn lane has been widened to accommodate
the swept paths of HGVs, the widened lane does not encourage high vehicular speeds or two vehicles
to attempt to use the lane side by side. The operational lane width shall be narrowed down on the
nearside to a minimum of 3.5m width through the use of suitable hatching to Chapter 7 of the TSM
but with 100mm solid line width. Hatching shall be a minimum width of 1.0m.
6.12.9
Straight ahead movements
Designers considering the use of a segregated lane for a straight ahead traffic movement should be
aware that there are a number of issues that could result in unsafe layouts. They are:
a)
High entry speed;
b)
Higher speed of vehicles exiting the straight ahead lane compared with slower
traffic leaving the roundabout, which can result in merging problems;
c)
The use of reverse curves both on the approach and through the roundabout and
abrupt changes in crossfall;
d)
Difficulties in signing the layout;
e)
Difficulties presented to Non-Motorised Users.
Designers should exercise particular caution in the design of segregated lanes for straight ahead
traffic movements in order to avoid these problems, and if necessary consider alternative layouts.
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