Shoulder Sealing on High Speed
Rural Roads
Operational Instruction 19.12
Transport Services Division
ROAD MANAGEMENT
Operational Instructions
Shoulder Sealing on High Speed Rural Roads – 19.12
AMENDMENT RECORD
Version
Page(s)
Date
Draft
All
25/08/10
Ver 1.0
All
02/08/11
Amendment Description
Init
Draft (Prep by Amit Dua)
AD
(prep by N. Sim)
NS
This document has been jointly prepared by Safer Roads and Vehicles Section and Traffic and
Access Standards Section. It has been approved and authorised for use by Transport Services and its
authorised agents by:
Manager, Traffic & Access Standards Section
02 / 08 / 2011
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:
Statewide Traffic and Operations Unit, Department for Transport, Energy and Infrastructure
Telephone: (08) 8343 2166 Facsimile: (08) 8343 2630
For additional copies or to confirm the current status of this document please contact:
Traffic & Access Standards Section, Department for Transport, Energy and Infrastructure
Telephone: (08) 8343 2849 Facsimile: (08) 8343 2630
Email: [email protected]
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Shoulder Sealing on High Speed Rural Roads for Road Safety – 19.12
CONTENTS
1.
2.
3.
4.
5.
6.
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SCOPE..................................................................................................................... 4
Background ............................................................................................................ 4
Purpose of Sealed Shoulders ............................................................................... 4
Shoulder Seal Width Determination for Rural Roads ......................................... 5
4.1
Shoulder Seal Width on a Straight Section .................................................. 5
4.2
Sealed Shoulder Width on Curves ............................................................... 6
4.3
Provision for Cyclists .................................................................................... 8
4.4
Pavement Marking........................................................................................ 8
Determination of Traffic Lane and Seal Shoulder Width.................................... 8
Bibliography & References ................................................................................. 13
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Shoulder Sealing on High Speed Rural Roads for Road Safety – 19.12
1.
SCOPE
The key objectives of this guideline are to provide a policy and clear direction on the
minimum carriageway seal width and the minimum shoulder seal width for Shoulder
Sealing of the network based on traffic composition, Annual Average Daily Traffic
(AADT) volumes, operating speed, existing seal widths and road alignment (curves).
This policy is being driven by the annual Shoulder Sealing Program for the purpose of
road safety and provides guidance to determine the width of the sealed shoulder on
straights and curves of high speed rural roads.
2.
Background
A study undertaken in 1997 by K. W. Ogden suggested that a 43% saving in casualty
accidents from shoulder sealing could be achieved. This was based on having a 7.4 m
seal width and then sealing another 0.6 to 0.8 m each side to provide an 8.6 to 9 m
total seal width. Edge line was added to keep traffic away from the weaker shoulder.
The prime objective of the Shoulder Sealing Strategy, developed in September 2000,
was to achieve a significant decrease in road crashes, targeting single vehicle run off
road crashes and head on type crashes. Based on the Black Spot crash reduction
matrix (1), a reduction of up to 40% in crashes can be expected along the improved
length of roads.
Between 2004 and 2008 in rural South Australia, run off road crashes (hit fixed object,
roll-over & left road-out of control) and head on crashes accounted for 81% of the total
crashes (2).
The report ‘Traffic Management and Infrastructure-Lessons from In-depth Crash
Investigation’, CASR (draft) March 2010 reveals that unsealed shoulders were a factor
in the majority of non-intersection crashes. According to the study, approximately 79%
of the total crashes that occurred in the study area (3) were on roads without sealed
shoulders. The report concludes that the most appropriate infrastructure
countermeasure to address head-on and overtaking type crashes would be the
provision of sealed shoulders.
3.
Purpose of Sealed Shoulders
The key function of a sealed shoulder is to provide a higher friction recovery width
outside the edge line for errant vehicles deviating from the traffic lane. Further safety
benefits are delivered to cyclists using unkerbed arterial roads particularly those in high
speed open road environments.
Benefits associated with shoulder sealing include the following:



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Reducing up to 40% of head-on crashes and run-off road crashes(4) (hit fix
object, left road out of control & roll over)
Allowing cyclists more rideable width to travel outside the traffic lane
Increasing the ‘effective’ width of the sealed carriageway
May allow left turning traffic to use the sealed shoulder which may assist in
reducing delays for the through traffic
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



Reduce maintenance through the elimination of edge breaks near the traffic lane
and the reduction of shoulder drop offs
Assist with pavement performance as water is directed away from the pavement
structure that is under traffic
Protect the road base from water ingress
Safer overtaking on carriageways with narrow lane width
It is important to recognise how the allocation of sealed road space between lane width
and shoulder width might affect crash rates. Generally, a wide shoulder with a narrow
lane allows a greater recovery time for errant vehicles and is likely to result in lower
speeds compared to a narrower shoulder with a wide lane – although both would have
the same overall seal width. An Austroads (2000) study contained the following key
finding:
“A tendency to higher crash rates with no sealed shoulder, but no consistent tendency
for crash rates to diminish with wider seals”.
Thus, in terms of gaining the greatest safety benefits, the evidence favours the sealing
of shoulders adjacent to existing traffic lanes. Consequently this strategy does not
propose lane widening, although minor lane widening undertaken in conjunction with
shoulder sealing may be advantageous in isolated cases, i.e. where the minimum is not
met (3.0m lanes).
4.
Shoulder Seal Width Determination for Rural Roads
The desirable width of a sealed shoulder may depend on a number of factors such as
traffic composition, AADT volumes, operating speed, existing seal widths and horizontal
road alignment.
4.1 Shoulder Seal Width on a Straight Section
Shoulder seal widths based on designed AADT are shown in Table 4.1. It is
desirable that forecasted vehicle volumes are considered.
Table 4.1 Single carriageway rural road widths (m)
Design AADT (5)
Elements
Total shoulder (m)
(6) (7) (8)
Minimum shoulder
seal (m)
Desirable shoulder
seal (m)
< 1500
(Less than)
> 1500
(Greater than)
1.5-2.0
2.5
1.0
1.2
1.2
1.5
Note: Refer to table 5.2 for width determination
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Figure 4.1 shows the typical cross-section for a sealed shoulder on a straight
section of road.
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Figure 4.1 Typical cross section on a straight section of road
4.2 Sealed Shoulder Width on Curves
Crash investigation shows that ‘run off road’ crashes occurring on high speed,
undivided roads tend to have ‘run off the road to the left on a right hand curve’
(OL/RHC) crashes as the most common.
The curves with a radius of between 200m and 600m should be identified and
treated first as the majority of run off road casualty crashes occur within this radii
range as shown in Figure 4.2.
The priority for shoulder sealing on curves on roads with a posted speed limit
greater than 80km/hr can be broken down into the following range of curve
radii (9).
Priority 1 - Radius 200m to Radius 600m
Priority 2 - Radius less than 200m
Priority 3 - Radius greater than 600m
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Shoulder Sealing on High Speed Rural Roads for Road Safety – 19.12
Other factors like curve length, roadside hazards (trees and culverts), batter slopes,
traffic volumes and composition are equally important when prioritising potential sites.
19.12
Figure 4.2 Retro-fitting road safety to existing Rural Roads, 2007, Stephen Levett,
RTA
On the outside of such a curve with a radius less than 1500m a wider sealed
shoulder of 2.0-2.5m should be provided. Appropriate taper length should also be
provided through the curve. Where these shoulder widths are unattainable due to
terrain constraints, then the sealed shoulder requirements may be reduced to the
maximum width that is economically feasible.
For the inside of a curve, the width should be consistent with the width of the
sealed shoulder on the straight adjacent to the curve section as shown in Figure
4.3.
Figure 4.3 Horizontal alignment with single isolated curve
Where TP is ‘Tangent Point’.
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Shoulder Sealing on High Speed Rural Roads for Road Safety – 19.12
The following cross-section is recommended on curves for the above mentioned
categories as shown in Figure 4.4.
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Figure 4.4 Typical right hand curve alignment
4.3 Provision for Cyclists
Sealed shoulders provide a significant safety benefit by enabling cyclists to ride
outside the traffic lane.
A shoulder surfacing treatment must be selected which ensures an acceptable
riding surface for cyclists on roads which attract appreciable bicycle usage. This
applies to urbanised areas, tourist areas (including Barossa Valley, Adelaide Hills,
Fleurieu Peninsula & Clare Valley) and within rideable proximity (say 20 km) of
townships with a moderate or large population.
An acceptable riding surface may be achieved by using a 10/5 double spray seal,
asphalt, slurry or similar treatment. However, if a segment of spray sealed
shoulder is subject to regular heavy commercial vehicle loading, a larger
aggregate size may need to be used to provide an adequate surfacing life. Consult
Pavements and Structures Section for further advice regarding appropriate
shoulder surface treatments.
4.4 Pavement Marking
The edge line pavement markings must be implemented in accordance to DTEI’s
Pavement Marking Manual.
The use of audio tactile line markings should be considered (Refer to Operational
Instruction 2.13, Audio Tactile Line Marking for details).
5.
Determination of Traffic Lane and Seal Shoulder Width
Austroads, Guide to Road Design Part 3 – section 4.8.9 should be considered where
shoulder sealing is proposed on arterial roads with cycling significance.
A flow chart (Table 5.2) has been developed to assist the determination of the
appropriate lane and seal shoulder width requirement. This flow chart was developed in
conjunction with the Road Freight Operations and Regulations Group (now Vehicles
Services Section) and later modified by Safer Roads and Vehicles.
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Shoulder Sealing on High Speed Rural Roads for Road Safety – 19.12
The criteria used when developing this flow chart includes, the role and function of the
route, the daily traffic volumes, freight requirements, current speed environment and
current roadside environment.
The following information has been used in the development of the Flow Chart:

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When traffic volumes are high there may be a greater risk of head-on type
crashes. Austroads guidelines indicate a preferred lane width of 3.5m for
National Highways and a minimum sealed shoulder width of minimum 1.2m for
traffic volumes greater than 1500 AADT and minimum 1.0m for volumes less
than 1500 AADT. However, it is considered that, under certain conditions lane
width may be reduced.

Austroads suggests that roads with high heavy vehicle use i.e. 10% of total
AADT or more will cause drivers towards the edge of the road to avoid conflict
with the opposing traffic. This will increase the need for the pavement marking
to be remarked. There could be an increase rate of failures along the edge of
the pavement. Therefore the shoulder width requirement for this type of route is
preferably a minimum of 1.0m (desirable 1.2m) if traffic volumes are less than
1000 vehicles per day and a preferred minimum of 1.2m if volumes are greater
than 1000 vehicles per day.

If the 85th percentile speed of any road under consideration is 80 km/hr or less
due to the general geometric restrictions, then a lane width of 3.1m as apposed
to 3.3m lanes may be appropriate. However, a total seal width of 8.6m should
be targeted.

Where the 85th percentile is generally greater than 80km/hr and the road is
subject to a number of significant alignment issues (i.e. sub standard curves
treated with advisory speed signs) then a lane width of 3.1m may also be
appropriate and a shoulder seal width of minimum 1.2m if traffic volumes are
greater than 1500 AADT, otherwise a minimum of 1.0m should apply provided
that a total seal width of 8.6m is achieved.

It should be noted that depending on the severity of the curves and the mix of
vehicles, substantial shoulder material depth/strength may be required on the
inside of the curves.

Where the roadside environment width is restricted and the risk of hitting fixed
objects increases as the lane width increases or the cost to remove the
restrictions are far greater than the benefits achieved from increasing the lane
width, a lane width of 3.1m is also likely to be appropriate, given that the total
seal width of 8.6m is targeted.

The flow chart complies with the Route Access Assessment Guidelines for
Restricted Access Vehicles which has identified the minimum lane width for
26m B-Doubles and 36.5m Road Trains (Attached is the lane width for a BDouble route).
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Shoulder Sealing on High Speed Rural Roads for Road Safety – 19.12
LANE WIDTH FOR B-DOUBLES
4200
4100
4000
3900
LANE WIDTH (mm)
3800
3700
3600
LOWER LIMIT LINE FOR GAZETTE APPROVAL
3500
3400
3300
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3200
3100
3000
2900
LOWER LIMIT LINE FOR PERMIT APPROVAL
2800
2700
2600
2500
0
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
2400
2600
2800
3000
AADT
Figure 5.1 Lane Width for B-Doubles
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Shoulder Sealing on High Speed Rural Roads for Road Safety – 19.12
Table 5.2 Flow Chart
Start checking process through 1 to 5
Traffic
Volumes
Lane Width
(m)
Minimum Shoulder
Sealing Width (m)
National Highway
AADT>1500
3.5
1.2 minimum
(If not applicable, go to
2)
AADT<1500
3.5
1 minimum
AADT>1000
3.5
1.2 minimum
AADT<1000
3.3
1 minimum,
1.2 desirable
B Double (or
larger) Route,
AADT>1000
1000<AADT
<1500
3.3
1 minimum,
total seal width 8.6
(If not applicable, go to
3)
AADT>1500
3.3
1.5 minimum
3.3
1 minimum,
total seal width 8.6
AADT>1500
3.3
1.2 minimum
AADT<1500
3.3
1 minimum,
total seal width 8.6
AADT>1500
3.3
1.2 minimum,
1.5 desirable
AADT<1500
3.3
1 minimum,
total seal width 8.6
AADT>1500
3.1
minimum
1.2 minimum
AADT<1500
3.1
minimum
1 minimum,
total seal width 8.6
AADT>1500
3.3
1.2 minimum
AADT<1500
3.3
1 minimum,
total seal width 8.6
Road Category
1
2
Primary or
Seconday Freight
or Gazetted
Route with
AADT>500
(Note: A wide lane
width is not required
for 500 AADT with
large freight
vehicles.)
(If not applicable, go to
3)
3
Amenities
Road Train Route
Road With
AADT<1000
(If not applicable, go to
4)
85th Percentile
Speed < 80KPH
(Note: Is the 85% ' ile
speed for the section
of road under
consideration (i.e.
between nodes) < 80
Kph. Nodes may be
chosen by 85% 'ile.)
With
Substandard
Curves
4
85th Percentile
Speed > 80KPH
(If not applicable, go to
5)
(Note: Is the
horizontal alignment
between selected
nodes substandard
whereby advisory
signs are required.)
(If not applicable, go to
5)
Restricted by
roadside
environment
5
(Note: Is the road
between selected
nodes restricted
whereby a width of
3.3m can not be
achieved due to clear
zone i.e. vegetation)
Not restricted by
roadside
environment
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Shoulder Sealing on High Speed Rural Roads for Road Safety – 19.12
Notes:
1) DTEI State Black Spot Program Guidelines, Sep 2009
2) Source – Rural Crashes in South Australia (Refer
http://www.dtei.sa.gov.au/roadsafety/road crash facts)
3) Study area is within 100 Km or 1 hr drive of Adelaide
4) Nation Building Program, Black Spot Projects, Notes on Administration, Sep 2009
5) Design AADT is Projected AADT for future years.(Road Planning and Design Manual,
QLD)
6) Traffic lane widths include centre-lines are exclusive of edge-lines.
7) Full width shoulder seals may be appropriate adjacent to safety barriers and on high
side road superelevation.
8) A minimum 7.0m seal should be provided on designated heave vehicle routes (or
where the AADT contains more than 15% heavy vehicles)
9) Retro-fitting road safety to existing Rural Roads, 2007, Stephen Levett, RTA
For further information on Traffic Management issues refer to:
Traffic and Access Standards
Traffic and Access Standards
Department for Transport, Energy and Infrastructure
77 Grenfell Street, Adelaide 5000
Telephone: 8343 2166
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Shoulder Sealing on High Speed Rural Roads for Road Safety – 19.12
6.
Bibliography & References

AUSTROADS 1999 ‘Estimation of Lane Width Requirements for Heavy Vehicles
on Straight Paths’, Research Report No. ARR 342. September 1999.

AUSTROADS 2009d, Guide to Road Design - Part 6: Roadside Design, Safety
and Barriers.

AUSTROADS 2010, Guide to Road Design - Part 3: Geometric Design.

AUSTROADS 2009, Guide to Road Design - Part 6A: Pedestrian and Cyclists
Path.

AUSTROADS 2007, RSERA Stage 3 - Part 2A: Relationships between crash risk
and the standards of geometric design elements.

AUSTROADS 2000 ‘Relationship between Crash Risk and Geometric of Rural
Highways’, Austroads, Sydney, p iv.

ROAD PLANNING AND DESIGN MANUAL Sep 2004, Queensland, Chapter 7
‘Cross Section’.

CASR Draft Report, March 2010, ‘Traffic Management and InfrastructureLessons from In-depth Crash Investigation’.

Stephen Levett, RTA, ‘RETRO-FITTING ROAD SAFETY TO EXISTING RURAL
ROADS’, 2007.

CHOUEIRI, E.M., LAMM, R. and MAILENDER, T. 1994 ‘Safety Aspects of
Individual Design elements and their interactions on two-lane highways:
international perspective’, Transport Research Record No 1445 pp 34-46.

DTEI, Traffic Management, Operational Instruction 2.13, 2009, ‘Audio Tactile Line
Marking’.

‘National Transport Commission’ Guidelines, July 2007, Performance Based
Standards Scheme Network Classification Guidelines.

K.W OGDEN, 1996 Safer Roads, Guide to Road Safety Engineering’. Monash
University, Melbourne, Australia.
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