Intersection Treatments on
Divided Roads
Operational Instruction 2.21
Transport Services Division
TRAFFIC MANAGEMENT
Operational Instructions
Intersection Treatments on Divided Roads - 2.21
AMENDMENT RECORD
Version
Page(s)
Ed1R0
Date
Amendment Description
15/12/98
Ver 2
All
21/05/08
Init
DH
Format Changes
DW
This document has been prepared by Derek Heneker, Statewide Operational Coordination Group. It has been approved and
authorised for use by Transport SA and its authorised agents by:
Manager, Statewide Operational Coordination
15 December 1998
Extracts may be reproduced providing the subject is kept in context and the source is acknowledged. Every effort has been
made to supply complete and accurate information. This document is subject to continual revision and may change.
For information regarding the interpretation of this document please contact:
Traffic Consultancy Services Unit, Statewide Operational Coordination Group
Telephone: (08) 8343 2289 Facsimile: (08) 8343 2630
For additional copies or to confirm the current status of this document please contact:
Telephone: (08) 8343 2448 Facsimile: (08) 8343 2630
email: [email protected]
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Intersection Treatments on Divided Roads - 2.21
CONTENTS
1. Scope................................................................................................................................. 4
2. Background & Discussion............................................................................................... 4
3. Divided Roads with Narrow Medians.............................................................................. 5
3.1. Junctions .................................................................................................................... 5
3.2. Intersections .............................................................................................................. 7
4. Divided Roads with Wide Medians ................................................................................. 9
4.1. Junctions .................................................................................................................... 9
4.2. Intersections ............................................................................................................. 10
5. Intersection Direction Signs.......................................................................................... 11
6. New Construction........................................................................................................... 12
7. References ...................................................................................................................... 13
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2.21
Intersection Treatments on Divided Roads - 2.21
1.
Scope
This document discusses the treatment of junctions and intersections with divided
roads with the aim of providing better guidance to drivers performing right turn and
cross movements on divided carriageways from side roads.
This document only applies to roads which have a speed limit of 90 km/h or greater.
It is assumed in the following sections that adequate entering sight distance (ESD) is
available at side road junctions and intersections.
Important: As with all regulatory signs and markings, the signs identified in this
Instruction shall be shown on the appropriate traffic control drawing and shall be
approved in the prescribed manner for traffic control devices.
2.
Background & Discussion
Situations have occurred where drivers from side roads have attempted to turn right
onto or cross a divided carriageway without recognising the existence of the second
carriageway. Drivers have been known to turn right into the left carriageway against
oncoming traffic as well as mistakenly continuing across the second carriageway
without caution after having safely crossed the first. It is thought that these incidents
occur because the second carriageway is not visible to the driver approaching from the
side road. The provision of clearer guidance may assist in reducing the occurrence of
these incidents.
How drivers safely negotiate when turning right onto, or crossing, a divided
carriageway from a side road will greatly depend on the available holding space
provided in the median opening and the shape of the median kerbing or edge. Where
the median is wide, drivers may be likely to perform these manoeuvres in two stages,
crossing the first carriageway and then re-assessing the right turn or cross movement
onto the second carriageway from the median opening. However, with a narrow
median, particularly those which are well tapered, drivers are more likely to begin the
turning manoeuvre onto, or decide to cross the second carriageway prior to reaching
the median. The traffic controls and guidance provided at junction and intersections
need to reflect the above behaviour and to some extent encourage it.
It is recommended for a median to be considered wide it should be able to
accommodate the largest legal vehicle that is likely to be using the side as well as
catering for two-way traffic through the median opening. In general a wide median
should have section of straight roadway of at least 20 m in length marked for two-way
traffic.
This should be measured from the tangent points of the straight section of the median
opening and the curved taper of the opening, taking into account any exclusive turn
lanes, as shown in Figure 2.1.
The definition of a wide median should not be rigidly applied as the manner with road
users use the median opening should be the governing factor - even though this is
controlled to some degree by the treatment in place.
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2.21
Intersection Treatments on Divided Roads - 2.21
w
w
(a) Typical arrangement for a “narrow median”
w less than 20 m
2.21
(b) Typical arrangement for a “wide median”
w of 20 m or greater
Figure 2.1: Narrow and Wide Median Definition
Taking care to ensure that directional signs at the junction or intersection, along with
the regulatory signs recommended in this Instruction, are correctly positioned should
provide sufficient information to drivers about the divided nature of the main road.
Although advance warning signs could be used to advise of the divided nature of the
main road, signs at the junction and intersection are considered to be more important
as it is here where drivers from a side road are most likely to make the decision of
which path to take when entering or crossing the main road. Therefore, this
Instruction places a stronger emphasis on the cues to drivers provided by the signs at
the junction or intersection rather than the advance warning signs. The
recommendations for the use of warning signs in this Instruction are consistent with
the use of warning signs as per AS1742.2.
The following Sections set-out recommended sign layouts for junctions and
intersections, categorised as having either a narrow or wide median.
3.
Divided Roads With Narrow Medians
Drivers are likely to commence their turn or decide to cross the second carriageway
prior to reaching the median. Therefore, drivers should be able to position their vehicle
in a manner which they feel comfortable and safe in order to complete their
manoeuvre. In this case marking a separation line or other form of guidance in the
median opening would generally be inappropriate.
3.1
Junctions
Regulatory controls on the side road are not needed, unless there is difficulty in
achieving the required sight distance, as drivers approaching a road from a
terminating road are required by law to give way. Advance warning of the
junction may be provided using a W2-57 erected a distance A, as defined in
Table 3.1, in advance of the junction. In this instance the W2-57 is used as a
replacement for the standard W2-3 T-junction warning sign and shall be used in
the same manner (refer to AS1742.2).
Where advance direction signs and intersection direction signs are provided in
the manner discussed in Section 5, the W2-57 sign shall not be used.
If provided, the size of the warning sign shall be:
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Intersection Treatments on Divided Roads - 2.21
• W2-57B for a side road 85th percentile speed less than 90 km/h.
• W2-57C for a side road 85th percentile speed of 90 km/h or greater.
A R2-3B “KEEP LEFT” sign shall be placed in the centre median, arranged to be
visible to side road traffic.
Where the second carriageway or the nose of the median cannot readily be seen
by drivers from the side road, due to a crest or difference in carriageway levels,
then a turn line may be marked on the pavement to give additional guidance to
turning drivers.
A sight board D4-4 shall be installed facing the side road, placed at a sufficient
height to enable it to be visible from the side road. Intersection direction signs
may be erected with the sight board.
The general layout of signs for a junction with a narrow median is shown in
Figure 3.1.
85th percentile speed
(km/h)
Distance A (m)
<75
80-120
75-90
120-180
>90
180-250
Table 3.1: Location of Warning Signs
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2.21
Intersection Treatments on Divided Roads - 2.21
D4-4
2.21
R2-3B
A
(refer to
table 3.1)
Turn line (if required)
W2-57*
*This sign is not to be used where an
advance direction sign and intersection
direction signs are provided.
Indicates suggested location of
intersection direction sign and
direction of listed destinations.
Figure 3.1: General Sign Arrangement for Narrow Median Junction
3.2
Intersections
At intersections regulatory controls are required on the side roads. In most cases
this will be a R1-2B, Give Way sign, as shown in Figure 3.3. A W3-2, Give Way
Ahead warning sign shall only be used when the sight distance to the give way
sign is less than the stopping sight distance for approaching vehicles. The size
of the warning sign shall be:
• W3-2B for a side road 85th percentile speed less than 90 km/h.
• W3-2C for a side road 85th percentile speed of 90 km/h or greater.
Where advance directions signs (G1 series) are provided on the side road and a
W3-2 sign is required due to insufficient sight distance then the advance direction
sign shall be at least 0.6V85 metres prior to the W3-2, (where V85 is the 85th
percentile speed) as shown in Figure 3.2.
A R1-1, Stop sign shall only be considered where the requirements for the
installation of stop signs contained in AS1742.2 are met. Where a Stop sign is
used, a W3-1, Stop Sign Ahead warning sign may be used in the same manner
as a Give Way Ahead warning sign.
R2-3B “KEEP LEFT” signs shall be placed in the centre median, arranged to be
visible to side road traffic.
Where the second carriageway or the nose of the median cannot readily be seen
by drivers from the side road, due to a crest or difference in carriageway levels,
then a turn line may be provided to give additional guidance.
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Intersection Treatments on Divided Roads - 2.21
R1-2
2.21
A
refer to
table 3.1
G1-?
W3-2
0.6V85
Figure 3.2: Combining Advance Direction and Warning of Traffic Control Signs
W3-2 (if required)
A*
R1-2B
R2-3B
A*
*refer to
table 3.1
R1-2B
W3-2 (if required)
Turn line (if required)
Indicates suggested location of
intersection direction sign and
direction of listed destinations.
Figure 3.3: Sign Arrangement for Narrow Median Intersections
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Intersection Treatments on Divided Roads - 2.21
4.
Divided Roads With Wide Medians
Wide medians provide drivers the opportunity to perform right turn or cross movements
from the side road in two stages. As this type of manoeuvre is considered safer than
trying to complete the turn or cross as a single manoeuvre, traffic controls should be
installed to encourage this movement. With the additional space available in the
median opening, providing clear delineation through the median opening area will also
minimise the potential for vehicle conflict in this area.
4.1
2.21
Junctions
Regulatory controls on the side road are not typically required at junctions.
However, as the first carriageway effectively operates as a “cross-road” for
drivers approaching from the side road and median opening, Give-Way signs
may be installed for the first carriageway only. A Stop sign shall only be used if
the requirements of AS1742.2 are met.
A W3-2, Give Way Ahead warning sign shall only be used when the sight
distance to the Give Way sign is less than the stopping sight distance for
approaching vehicles. The size of the warning sign shall be as shown in
Section 3.2. Where a Stop sign is used, a W3-1, Stop Sign Ahead warning sign
may be used in the same manner as the Give Way Ahead warning sign.
Where a W3-2 is required in conjunction with an advance direction sign the signs
shall be arranged as shown in Figure 3.2.
Duplicated R2-4B, NO ENTRY signs, complimented with a GE9-15B, WRONG WAY
plate shall be placed on the first carriageway, approximately 15 - 20 m to the right
of the junction and arranged to face side road traffic as shown in Figure 4.1. The
signs shall be located so as not to effect sight distance at the junction.
A turn line may be provided where the second carriageway or the nose of the
median cannot be readily seen by drivers on the side road.
A sight board D4-4 shall be installed facing the side road, placed at a sufficient
height to enable it to be visible from the side road. Intersection direction signs
may be erected with the sight board.
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Intersection Treatments on Divided Roads - 2.21
The general layout of signs for a junction with a wide median is shown in Figure
4.1.
D4-4
2.21
Turn line (if required)
R1-2B
A*
15-20 m
*refer to
table 3.1
R2-4B
GE9-15B
R1-2B
W3-2 (if required)
Indicates suggested location of
intersection direction sign and
direction of listed destinations
Figure 4.1: Sign Arrangement for Wide Median Junction
4.2
Intersections
At intersections regulatory controls are required on the side roads. In most cases
this will be a R1-2, Give Way sign. A W3-2, Give Way Ahead warning sign shall
only be used when the sight distance to the Give Way sign is less than the
stopping sight distance for approaching traffic. Size of the warning signs shall be
as shown in Section 3.2.
Where a W3-2 sign is required in conjunction with an advance direction sign the
signs shall be arranged as shown in Figure 3.2.
The use of a R1-1, Stop signs shall only be considered where the requirements
for the installation of stop signs contained in AS 1742.2 are met. Where a Stop
sign is used, the W3-1, Stop Sign Ahead warning sign may be used in the same
manner as a Give Way Ahead warning sign.
Duplicated R2-4B, No Entry signs, complimented with a GE9-15B, WRONG WAY
plate shall be placed on bath carriageways, approximately 15 - 20 m to the right
of each side road approach and arranged to face drivers entering from the side
road. The signs shall be located so as not to effect sight distance at the
intersection.
A turn line may be provided where the second carriageway or the nose of the
median cannot be readily seen by drivers on the side road.
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Intersection Treatments on Divided Roads - 2.21
The general sign arrangement for an intersection with a wide median is shown in
Figure 4.2.
W3-2 (if required)
A*
2.21
R1-2B
R2-4B
R2-4B
GE9-15B
GE9-15B
Turn line (if required)
A*
15-20 m
*refer to
table 3.1
R1-2B
W3-2 (if required)
Indicates suggested location of
intersection direction sign and
direction of listed destinations.
Figure 4.2: Sign Arrangement for Wide Median Intersections
5.
Intersection Direction Signs
Direction signs placed at junctions and intersections shall be positioned to provide
drivers guidance in turning into the correct carriageway as well as towards their
destination. Signs should be positioned so that the driver’s attention is not directed
towards the median opening (other than checking for traffic) unless they need to cross
or turn right onto the divided road. Suggested locations for directional signs are shown
in Figures 3.1, 3.2, 4.1 and 4.2, although other locations may be considered depending
on the actual layout of the junction or intersection layout.
The placement of directional signs must not adversely effect the sight distance at the
junction or intersection.
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Intersection Treatments on Divided Roads - 2.21
6.
New Construction
The main emphasis of this Operational Instruction is addressing operational difficulties
drivers are experiencing with the layouts of junctions and intersections with divided
roads. For any new works or major upgrade/improvement work, it is important that the
driver error problems occurring at the existing side road treatments is addressed.
Where a side road joins a divided road at a point where the median is wide (not less
than 20 m) consideration should be given to the wide median treatment shown in
Figure 5.9 of the Austroads 1998, Guide to Traffic Engineering Practice, Part 5:
Intersections at Grade.
The wide median treatment is appropriate for median widths of:
a) 20 m or greater without right turn lanes.
b) 18 m or greater with right turn lanes provided as shown in Figure 6.1.
Where the side road traffic is less than 50 vehicles per day the wide median treatment
may be designed without right turn lanes provided on the divided road. The impact of
traffic operation as a result of removing the right turn lanes will need to be carefully
evaluated taking into account the traffic volume and speed on the main divided road.
Right turn lanes shall be provided where long vehicles (greater than the legal length of
20 m) need to be accommodated, as the turn lanes will provide the necessary storage
space to prevent longer vehicles from protruding into the through lanes.
The raised (or kerbed) medians shown on the side roads in Figure 6.1 shall only be
installed where adequate road lighting is also provided. Painted medians are to be
used where lighting is not provided.
Figure 6.1: Recommended Wide Median Treatment
(source: Austroads 1998, Guide to Traffic Engineering Practice, Part 5)
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2.21
Intersection Treatments on Divided Roads - 2.21
7.
References
AS 1742.2 - 1994 “Manual of Uniform Traffic Control Devices, Part 2: Traffic Control
Devices for General Use”.
AS 1742.8 - 1990 “Manual of Uniform Traffic Control Devices, Part 8: Freeways”.
Austroads 1998 “Guide to Traffic Engineering Practice, Part 5: Intersections at
Grade”.
2.21
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