TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 95
6.11
Heavy Goods Vehicles
The problem of HGVs overturning or shedding their loads at roundabouts has no obvious solution in
relation to layout geometry. Whilst this type of collision may infrequently cause personal injury, there
are considerably more damage only incidents. Load shedding often results in congestion and delay,
and is expensive to clear, especially if occurring at major junctions. Experience suggests that
roundabouts where these problems persist usually exhibit one or more of the following features:
a)
Inadequate entry deflection leading to high entry speeds;
b)
Long straight sections leading into deceptively tight bends;
c)
Excessive visibility to the right;
d)
Low circulating flow past the entry;
e)
Tightening of the line on circulation;
f)
Sharp turns into exits;
g)
Excessive crossfall changes;
h)
Excessive adverse crossfall on circulatory sections;
i)
Double or reverse curvature.
A problem for some vehicles may be present even if speeds are not high. Research has shown that
an articulated HGV with a centre of gravity height of 2.5m above the ground can overturn on a 20m
radius bend at speeds as low as 25 km/h. Layouts designed in accordance with the recommendations
in this standard should avoid the problems listed above. However, designers should recognise that
each site will be different and designs should be specifically checked to ensure that such problems
are avoided. During construction, particular attention should be paid to ensure that pavement surface
tolerances are complied with and that abrupt changes in crossfall are avoided.
6.12
Segregated Left Turn Lanes at Roundabouts
6.12.1
General
This section outlines the criteria for the provision of segregated left turn lanes at roundabouts and the
geometric features to be considered in their design.
Segregated left turn lanes should only be considered where there is a significant left turn movement
at a particular entry.
Segregated left turn lanes shall not be used at single lane exits unless they form the introduction of a
lane gain arrangement.
Segregation shall be provided by physical separation as shown in Figure 6.30. Vehicles are
channelled into the left hand lane by road markings, supplemented by advance direction signs. They
proceed to the first exit without having to yield to other vehicles at the entry onto the roundabout.
All traffic signs and road markings shall be designed and applied in accordance
with the Road Traffic
Acts, the Road Traffic (Signs) Regulations, the Road Traffic (Traffic and Parking) Regulations and 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 96
Figure 6.30: Generic Physical Left Turn Lane (No provisions made for cyclists)
6.12.2
Traffic Flows and Capacity of Segregated Left Turn Lanes
When considering the use of segregated left turn lanes, vehicle composition and the total inflow at the
roundabout entry, the proportion of left turning vehicles and the number of entry lanes should all be
examined. The following procedure can be used as an initial assessment to determine whether the
provision of a segregated left turn lane merits further consideration.
The inclusion of a segregated left turn lane should be considered if:
L ≥
F
E
Where:
L is the flow of left turning vehicles;
F is the total entry arm inflow in vehicles per hour;
E is the number of proposed entry lanes onto the roundabout including the segregated left turn lane.
The following examples illustrate the use of this initial assessment process. In Example 1, a
segregated left turn lane is being considered at an existing roundabout and in Example 2, as an
addition to a new roundabout layout.
Example 1
An existing roundabout currently has a 3-lane entry, a left turn flow of 500 vehicles per hour and a
total entry inflow of 1200 vehicles per hour. The provision of a left turn lane will result in the loss of
one entry lane onto the circulatory carriageway. L is 500, which is greater than F/E (1200/3 = 400),
indicating that further examination is worthwhile.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 97
Example 2
A new roundabout is proposed, the left turn flow is 250 vehicles per hour, the total inflow is 1000
vehicles per hour and the left turn lane can be provided in addition to two entry lanes. L is 250, which
is less than F/E (1000/3 = 333), indicating that a segregated left turn lane may not improve the capacity
of the roundabout entry. Designers should therefore consider alternative measures such as additional
entry width (i.e. a three lane entry) or a longer flare length.
For marginal cases where the value of F/E is close to L, the provision of a segregated left turn lane
may merit further consideration where other factors such as safety need to be considered.
The composition of the turning proportions at the entry, the number of exit lanes and the capacity of
the approach road should also be examined when considering the provision of a segregated left turn
lane.
The capacity of a segregated left turn lane is dependent on the entry and exit treatments and lane
width. To ensure maximum capacity a dedicated lane on the approach and exit shall be provided with
a minimum lane width of 3.5m.
The impact of introducing a segregated left turn lane should be assessed taking into account the entry
and exit treatments. A traffic micro-simulation program shall be used by the designer to assess the
impact of introducing a segregation.
The relevant peak periods for the junction being analysed should be used, and should include forecast
commuter, development usage and other peak periods. The effect of the lane on traffic flows at
different periods of the day should also be considered. The results of any assessment should be
carefully examined to ensure that the provision of a segregated left turn lane is the most appropriate
form of improvement, compared with alternatives such as modifications to flare lengths or entry widths.
The use of segregated left turn lanes on roundabouts forming part of grade separated junctions shall
be considered where queuing at roundabout entries becomes a problem.
6.12.3
Geometric Design Standards of Segregated Left Turn Lanes
Segregated left turn lanes should not be designed to induce high speeds. Any desirable speed
reduction should be achieved on the approach to the lane rather than within it. Where the segregated
left turn lane follows a tight radius
relative to the approach speed, the use of “slow” road marki
ngs to
TSM Chapter 7 is recommended in conjunction with associated Bend Ahead warning signs (W 051),
plated with a Max Speed (RUS 041, RUS 042, RUS 043 or RUS 044) per Chapter 5 of the TSM. Care
should be taken to ensure that these signs are located to avo
id “sign clutter” or confusion to drivers
not using the segregated left turn lane.
Countdown markers to TSM Chapter 4 should only be provided on the approach to the roundabout
when there is no risk of confusion between the distance to the commencement of the segregated left
turn lane and the distance to the roundabout yield line.
The curve radius used for the segregated left turn lane will be dependent on both the design speed of
the approach road and site constraints. The driver’s perception of the approac
h and segregated left
turn lane radii will be a determining factor in their approach speed. The designer should therefore
consider the need for speed reduction measures on the approach depending on the minimum curve
radii used. Inside curve radii of less than 10m are not recommended. The exit radius used should be
greater than, or equal to, the entry radius.