TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 98
Where segregated left turn lanes are considered on grade separated junctions, designers shall
provide a 3.75V m distance between the mainline and the segregated lane diverge noses, where V is
the design speed in km/h. Distances that are less than 3.75V m will require a Departure from
Standards. Refer to Chapter 7 for further information.
Superelevation along the segregated left turn lane shall be applied in accordance with Table 1.3 of
DN-GEO-03031 subject to a maximum value of 5%.
The Desirable Minimum Stopping Sight Distance (SSD) on the segregated left turn lane shall be the
lesser of (a) the SSD obtained from DN-GEO-03031 Table 1.3 for the design speed of the approach
or (b) the SSD given in Table 6.2 in this standard appropriate to the maximum nearside curve radius.
The D6/3esirable Minimum SSD shall be applied to the section of segregated left turning lane between
the end of the entry taper and the start of exit taper.
The maximum curve radius used to determine the SSD from Table 6.2, shall be the greater of either
the entry or exit radius of the segregated left turn lane, these being defined as the radius that occurs
immediately after the entry taper and immediately before the exit taper in the direction of travel. See
Figure 6.31 for definition of entry and exit tapers.
Figure 6.31: Dedicated approach and exit for segregated left turn lane introducing a lane gain
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 99
The carriageway widths specified in column 2 of Table 6.3 shall be used to accommodate the swept
path of a HGV and hatch markings provided on the inside of the curve to reduce the marked lane
width to a minimum of 3.5m.
As physical segregation is to be provided, this shall permit a left turn at the roundabout in
the normal
way from the non-segregated part of the approach as shown on Figure 6.30. Where a channelising
island in excess of 50m in length is proposed, the lane widths specified in Column 3 of Table 6.3 shall
be used.
Fo
r roundabouts ≤ 50m inscribed circle diameter, the segregated left turn lane width used shall be
based on the minimum curve radius on the entry or exit. For roundabouts with an inscribed circle
diameter > 50m, the designer shall have discretion to reduce the segregated left turn lane width on
the section of segregated left turn lane between the entry and exit, depending on the radius used on
that section. The widths specified in Table 6.3 shall be used.
The use of two-lane segregated left turn lanes is not permitted, as these can result in high vehicle
speeds and potential conflict at the exit or merge point. Where left turn flows are very high, alternative
junction forms or methods of junction control shall be considered.
1m hardstrips shall not be provided on segregated left turn lanes. They shall be terminated at the start
of the entry taper and recommenced at the end of the exit taper.
Table 6.2: Desirable Minimum Stopping Sight Distances
Maximum Curve Radius (m)
Desirable Minimum Stopping
Sight Distance (m)
less than or equal to 20
35
21 to 40
70
41 to 80
90
81 to 100
120
Greater than 100
215
Table 6.3: Minimum Corner and Curve Radii and Carriageway Widths
Minimum Inside Corner
Radius or Curve Radius (m)
(1)
Segregated Left Turn Lane
Carriageway Width (for
channelising island lengths
≤ 50m) (m)
(2)
Segregated Left Turn Lane
Carriageway Width (for
channelising island lengths
> 50m) (m)
(3)
10
8.4
10.9
15
7.1
9.6
20
6.2
8.7
25
5.7
8.2
30
5.3
7.8
40
4.7
7.2
50
4.4
6.9
75
4.0
6.5
100
3.8
6.3
> 100
3.5
6.0
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 100
Channelising islands shall be a minimum width of 4 m, to facilitate the safe crossing of NMU and the
requirements for bollards and signs. Channelising islands shall extend a minimum of 1.5m and 6m
into the entry and exit roads respectively where no pedestrians are expected, as shown on Figure
6.32.
Figure 6.32: Generic Physical Left Turn Lane with no provisions made for cyclists
Segregated left turn lanes shall not be used at the end of steep downhill gradient approaches, this
being defined as a longitudinal gradient in excess of 4% within the immediate approach to the junction
as defined in DN-GEO-03031, applicable to the design speed of the approach measured back from
the start of the entry taper. The longitudinal gradient along the segregated left turn lane shall not
exceed 4%.
Traffic signs and street furniture may be placed on channelising islands. Their number should be
limited however, as proliferation can create confusion, distract, reduce visibility, add to sign clutter and
have significant maintenance implications. Reference should be made to DN-REQ-03034 for vehicle
restraint system details for protection from roadside hazards.
The use of physical segregated left turn lanes at unlit junctions is not permitted.
Where a channelising island is to be provided, a retroreflective or transilluminated bollard shall be
installed at the start of the island. A minimum clearance of 0.6m between the edge of the sign or
bollard and edge of the island shall be provided.
6.12.4
Approach Layout of Segregated Left Turn Lanes
The approach arrangements shall consist of a dedicated lane. Dedicated lanes on approach, see
Figure 6.31, provide the highest capacity entry to a segregated left turn lane. They require careful
design of signing and road markings such as the use of lane destination signs and Bifurcation Arrow
markings in accordance with TSM chapter 4 on the approach, to avoid driver confusion that may result
in lane changing manoeuvres occurring adjacent to the segregated left turn lane entry.
The entry arrangements consist of an entry taper as shown on Figure 6.31.