MRWA Supplement to Austroads Guide to Road Design - Part 4A

Document No:  D11#308726
 
Revision:  2A
 
Date amended:  18-Sep-2018

Image: orange line.RCN-D13^23151823.GIF 

 

The information below is intended to reflect the preferred practice of Main Roads Western Australia ("Main Roads"). Main Roads reserves the right to update this information at any time without notice. If you have any questions or comments please contact Nick De La Motte by e-mail or on (08) 9323 4448.

To the extent permitted by law, Main Roads, its employees, agents, authors and contributors are not liable for any loss resulting from any action taken or reliance made by you on the information herein displayed.

Revision Register

 

Ed/Version Number Clause Number Description of Revision Date

1

All

Guideline Developed.09-Dec-2011 

1A

3.2.1 & 3.2.3

Tables 3.1 & 3.2 corrected.21-Dec-2011 

1B

10.6.4

Drawings 200531-0006 and 200531-0007 amended.02-Feb-2012

1C

10.6.4

Drawings 200531-0006 and 200531-0007 amended.11-Feb-2012

1D

4.1

New Clause 4.1 "General" added.09-Aug-2012

1E

4.6

Figure 4.22 - Separation Line removed.

29-Sep-2012
​1F​6.2.3Drawings 200131-0084, 200131-0085 and 200131-0086 amended.22-Nov-2012
​1G​6.2.1​Figure 6.1 - Text "Edge of Shoulder" amended to "Edge of Lane". ​17-Apr-2013

1H

Header

​Contact person changed to Kyle Smith.​16-May-2013
​1I​4.1 & 4.6Links to Roadtrains at Rural Intersections Drawings added.
Figure 4.22 updated.
​19-Jun-2014

1J

4.6

​Figure 4.22 amended.​23-Jun-2014

1K

10.6.4

​Reference Figure 10.10 Example (a) and (b) - Position of stop line for motor vehicle lane changed from 4m to 5m. Example (d) - Length of the bicycle head start storage facility changed from 4m to 5m.​03-Oct-2014

1L

10.6.4

​Re-link drawings 200531-0006 and 200531-0007.​06-Oct-2014
​1M​4.6​1st paragraph amended.​18-Aug-2015
​1N​9.2 & 9.3​Drawing 200731-0071 amended.​16-Jun-2016
​1O​6.2​Table 6.2 Minimum island widths amended.​01-Aug-2016
​1P​10.6.4​Drawings 200531-0006 and 200531-0007 amended.​08-Aug-2016
​1Q​Header​Contact person changed to Nick De La Motte​25-Jan-2017
​1R​4.1​Drawings 201431-0001 and 201431-0002 amended.​18-Apr-2018
2All​​Updated to supplement Austroads Guide to Road Design (GRD) Part 4A (2017) release.​20-Jul-2018
​2A​4.1​Drawings 201431-0001 and 201431-0002 amended.​18-Sep-2018

Table of Content


MRWA Supplement to Austroads Guide to Road Design

Part 4A - Unsignalised and Signalised Intersections

This Supplement has been developed to be read as a supplement to the Austroads Guide to Road Design (GRD) Part 4A: Unsignalised and Signalised Intersections (2017), a copy of which can be obtained via the Austroads website.

In Western Australia, state-based information, in this website and elsewhere, takes precedence over Austroads Guides and Standards Australia Standards.National Guides and Standards take precedence over International Guides and Standards, unless specifically stated otherwise.

This Supplement has the same structure as the equivalent Austroads Guide and only additional requirements, clarifications, or practices different from Austroads appear.Where appropriate, this Supplement may also contain additional sections and figures not covered by Austroads, but the numbering sequence found in the Austroads Guide remains. Figures and tables in this Supplement replace those with the same figure or table number in the equivalent Austroads Guide.

 

GENERAL STANDARDS AND APPLICATION

This guideline applies to all new works on roads managed by Main Roads. It is noted that many existing intersections were constructed to the design standards of the time and do not necessarily meet all current design requirements. Whilst it is not economically feasible to upgrade existing intersections each time revisions are made to design standards, Project Managers should consider making improvements whenever major works are completed in the vicinity of existing intersections.

It is important for Road Planners and Designers to be aware of the effects that different types of intersection control may have on delays to traffic (under various traffic demand situations) and the resultant emissions. A range of factors should be taken into consideration when deciding the most appropriate traffic management treatment at any given intersection.  These are outlined in the Guide to Road Design Part 4B: Roundabouts, the Guide to Traffic Management Parts 3 and 10, and the Main Roads Supplement to Austroads Guide to Road Design Part 4 - Intersections and Crossings - General.


1. INTRODUCTION

Main Roads has no supplementary comments for this section.

 

2. LAYOUT DESIGN PROCESS

2.2.2 Vertical Alignment

The general requirement is that the maximum grade for at least the last 30m from the intersection edge of pavement is 3%. It is good practice that this requirement is applied at all intersections - particularly where there are a high number of trucks.

Reference Figure 2.5, Approach Sight Distance (ASD) is the minimum level of sight distance to be provided at intersections. ASD is measured from a driver's eye height (1.1m) to object height (0.0m), which ensures that a driver is able to see any line marking and kerbing at the intersection.

 

2.2.4 Superelevation at or near intersections

Intersection design must consider instability of heavy vehicles. Refer to Austroads GRD Part 4A, Appendix B for assessment of critical truck turning speeds. Where truck operating speeds are likely to exceed those provided in Table B2, measures should be considered to reduce truck speeds or mitigate the risk.

The maximum effective adverse crossfall for turning movements at intersections is 5%.  At intersections with higher speed turning movements (i.e. traffic signal controlled intersections) the safe effective adverse crossfall may need to be less than the maximum.

 

3. SIGHT DISTANCE

3.1 General

In addition to the sight distance requirements in Austroads GRD Part 4A, the following sight distance checks should be made:

    • intersections with restricted lateral sight distance (for stopping sight distance);
    • intersections on or near crest vertical curves (for approach sight distance);
    • on approaches to speed change and lane drop areas (for approach sight distance);
    • on the approaches to underpasses (for stopping sight distance); and
    • on the approaches to railway level crossings (for approach sight distance)

Intersections should be positioned in safe locations using the Sight Distance criteria in Section 3.2. The sight distance values shown in Table 3.1 of the Austroads documents should be increased generally for design on unsealed roads. Refer to Australian Road Research Board - Unsealed Roads Manual, Guidelines to Good Practice (2009).

 

3.2.1 Approach Sight Distance (ASD)

Where it is unreasonable or extremely difficult to achieve ASD, then as an absolute minimum, Stopping Sight Distance (SSD) should be provided. 

In the application of Table 3.1, the following guidance is provided:

    • A reaction time of 2.5s shall be used as the Main Roads desirable minimum and a reaction time of 2.0s shall be used as the Main Roads absolute minimum. 
    • Absolute minimum reaction time should not be used in combination with other minimum design standards. 
    • A reaction time of 1.5 seconds shall not be used in Western Australia.

 

Design speed (km/h)

Based on approach sight distance for a car1
h1 = 1.1,  h2 = 0,  d = 0.362

 

RT = 2.0s

RT = 2.5s

ASD
(m)

K

ASD
(m)

K

 40

40

7.2

45

9.3

 50

55

13.8

62

17.5

 60

73

24.0

81

29.8

 70

92

38.9

102

47.5

 80

114

59.5

126

71.6

 90

139

87.3

151

103.8

 100

165

123.6

179

145.3

 110

193

170.1

209

198.0

 Truck stopping capability
provided by the
minimum crest curve size3

d = 0.22,  h1= 2.4m,  h2= 0m

Table 3.1 Approach sight distance (ASD) and corresponding minimum crest
vertical curve size for sealed roads (S<L)

 

Notes to Table 3.1:

  1. If the roadway is on a grade, calculate the approach sight distance (ASD) values using the correction factors in Table 3.4 (or use Equation 1 in Section 3.2.1) by applying the average grade over the braking length.
  2. A coefficient of deceleration (d) of 0.36 shall be used in Western Australia.

 

3.2.2 Safe Intersection Sight Distance (SISD)

Main Roads requires that the viewing point on the minor road approach is measured 5.0 m (minimum of 3.0 m) from the hold line, or in the absence of a hold line from the projected major road face of kerb or edge of seal.

For vehicles on the minor road consideration should be given to observation angles (i.e. Maximum of 110° to the left and 120° to the right)

In the application of Table 3.2, the following guidance is provided:

    • A reaction time of 2.5s shall be used as the Main Roads desirable minimum and a reaction time of 2.0s shall be used as the Main Roads absolute minimum. 
    • A reaction time of 1.5 seconds shall not be used in Western Australia. 

 

Design speed (km/h)

Based on safe intersection sight distance for a car1
h1 = 1.1,  h2 = 1.25,  d = 0.362

RT = 2.0s

RT = 2.5s

SISD
(m)

K

SISD
(m)

K

 40

73

5.7

79

6.6

 50

97

10.0

104

11.5

 60

123

16.0

131

18.3

 70

151

24.2

161

27.4

 80

181

34.9

192

39.3

90

214

48.6

226

54.4

100

248

65.6

262

73.2

110

285

86.6

300

96.1

 Minimum SISD
capability provided
by the crest
vertical curve size3

Truck

d = 0.29,  h1= 2.4m,  h2= 1.25m,  observation time = 3.0s

Table 3.2 Safe Intersection sight distance (SISD) and corresponding minimum crest
vertical curve size for sealed roads (S<L)

Notes to Table 3.2:

  1. If the roadway is on a grade, calculate the safe intersection sight distance (SISD) values using the correction factors in Table 3.3 (or use Equation 2 in Section 3.2.2) by applying the average grade over the braking length.
  2. A coefficient of deceleration (d) of 0.36 shall be used in Western Australia.
  3. These check cases assume the same combination of design speed and reaction time as those listed in the table. 

 

3.2.3 Minimum Gap Sight Distance (MGSD)

 
Main Roads has not adopted Minimum Gap Sight Distance (MGSD). 
 
 
3.4 Sight Distance at Property Entrances

Safe Intersection Sight Distance should be provided at driveways in accordance with Table 3.2. However, where this is not possible due to constraints, sight distance equal to the Stopping Sight Distance for the design speed of the road shall be provided as an absolute minimum.

For information relating to property access refer to Main Roads Supplement to Austroads GRD Part 4: Intersections and Crossings General.



4. TYPES OF INTERSECTIONS AND THEIR SELECTION

4.1 General

Location of Electrical Assets should be considered for the site. Further advice for Electrical Assets are given in Main Roads Roadside Items and Traffic Management. Documents may include:

In relation to Roadtrains at Rural Intersections, Main Roads has developed some typical layouts which can be found in Guideline Drawings 201431-0001 and 201431-0002.
 

5. AUXILIARY LANES

5.2.1 Components of Deceleration Turn Lanes

Main Roads preferred practice for diverge tapers is to use 100 m radius back to back reverse curves for roads with design speeds of 80 km/h and higher, and to use 50 m radii for design speeds lower than 80 km/h. These radii are based on a turn lane width of 3.5 m.

For wider turn lanes Equation 5 should be used to calculate the taper length based on a design speed of 110 km/h for roads with design speeds of 80 km/h and higher, and 80 km/h for roads with design speeds lower than 80 km/h. The values calculated for T should be from TP to TP of the reverse curves, and not IP to IP.

 

5.2.2 Determination of Deceleration Turning Lane Length

For design Main Roads preferred practice is to use a deceleration rate of 2.5 m/s².


5.3.3 Merge Taper TM

Main Roads has adopted a merge rate of 0.6 m/s at all auxiliary lane tapers. 

 

For specific requirements relating to merge tapers refer to Main Roads Supplement to Austroads GRD Part 3: Geometric Design Chapter 9.9.2 Tapers.


6. TRAFFIC ISLAND AND MEDIANS

6.1 Raised Traffic Islands and Medians

Raised islands on the centre line of the side road (minor road) approaching intersections are referred to as splitter islands by Main Roads. Raised islands on the centre line of the main carriageway (major road) are referred to as medians by Main Roads.


6.1.2 Raised Medians

Splitter Islands (excluding roundabout and corner approach islands) shall be constructed as shown in Figure 6.1. The island width ('W') and refuge gap ('X') can be determined from Table 6.3, after assessing usage by pedestrians and cyclists or a combination of both.
 

Image: splitter island geometry.RCN-D13^23144617.GIF 

Figure 6.1. Splitter Island Geometry

The length (L) of the splitter island shall be determined from Austroads - GRD Part 4A (2017), Section 6.2.2. Table 6.1. The values in Table 6.2, "Desirable Minimum" column should be used wherever possible.

 

Island/Median used for protection/refuge
for:-

Island width 'W'

Gap in island
'X' Min (m)

Desirable Minimum (m)

Absolute Minimum (m)

Signs or street lighting

2.0

1.52

2.5

Signals1- single aspect width​2.0​1.52​2.5
Signals1- dual aspect width​2.5​2.0​2.5

Pedestrians

2.5

2.0

2.5

Cyclists

3.0

2.0

2.5

Shelter turning vehicles and traffic signals

6.0

5.5

 

Table 6.2 Minimum island widths

 

Notes:

1. For traffic signals, the minimum offset to any part of the signal is 0.6m from the kerb face.

2. This width does not allow for a gap in the island for pedestrians

This Table replaces Austroads Table 6.2.


6.1.3 Raised High Entry Angle and Free-flow Left-turn Islands

There are two corner approach island types:-

    • High entry (70o) angle island, and
    • Free flow slip lane (directional) island

Examples of the use of these islands are shown on drawings 200131-0084, 200131-0085 and 200131-0086. The High entry angle island shall be designed using the criteria shown in Figure 6.1.3.1. Note that if cut through pedestrian access is to be used rather than pedestrian ramps, it is recommended that the minimum island size be increased from 6.0m by 12.0m to 8.0m by 15.0m.
 

high entry angle island.GIF 

Figure 6.1.3.1. High Entry Angle Island

Where an exclusive cycle lane runs adjacent to an island, the island may be placed parallel to the cycle lane with a 0.3m offset as shown in Figure 6.1.3.2 provided the cycle lane width exceeds the 0.2m per 10km/h offset criteria.

high entry angle island with adjacent cycle lane.GIF 

Figure 6.1.3.2. High Entry Angle Island with Adjacent Cycle Lane



The free flow slip lane island shall be designed using the criteria shown in Figure 6.1.3.3 and must exit into its own lane on the cross road. The island has an extended parallel departure nose which ensures that vehicles are aligned correctly in the acceleration lane prior to merging. 

free flow slip lane island.GIF 

Figure 6.1.3.3. Free Flow Slip Lane Island

 


An alternative shown at Figure 6.1.3.4 shows a cycle lane cutting through the island to cross to the left of the acceleration lane at 90 degrees. 

free flow slip lane island with cycle lane.GIF 

Figure 6.1.3.4. Free Flow Slip Lane Island with Cycle Lane

 

Figure 6.1.3.5 shows a left turn island for use only when the left turn movement is signalised. 

 

left turn island for signalised intersection.GIF 


Figure 6.1.3.5. Left Turn Island for Signalised intersection

 

Seagull islands are used in the median opening at 'T' intersections - where the median width is 10 metres or greater - to fill the large expanse of pavement and give direction to turning traffic. The seagull island shall be designed using the criteria shown in Figure 6.1.3.6.

An example of this is shown on drawing 200131-0085.  

seagull island.GIF 

Figure 6.1.3.6. Seagull Island (Minimum Island Area 10m2)

 

Where intersections include kerbed medians or kerbed islands, street lighting should be provided.


6.4 Road Widths between Kerb and between Kerb and Safety Barrier

6.4.1 General

Main Roads requires a 0.5m clearance beyond the swept path (which includes vehicle body overhang) of the design vehicle to the face of kerb.

To enable stopped vehicles to be passed, the desirable minimum through carriageway width between kerbs is 6.0m. The absolute minimum through carriageway width between kerbs is 5.5m. The desirable minimum width for single lane one-way traffic carriageways (including shoulder) should be 6.0m with an absolute minimum width of 5.5m.


6.5 Kerb and Channel

Main Roads preferred practice is not to use kerb and channel.


7. RIGHT-TURN TREATMENTS - LAYOUT DESIGN DETAILS

For physical tapers (T) refer to section 5.2.1.

 

7.5.2 Urban Channelised T-junction – Short Lane Type CHR(S)

Refer to Main Roads Supplement AGRD Part 4 Appendix A.8

 

8. LEFT-TURN TREATMENTS

For physical tapers (T) refer to section 5.2.1.

 

9. U-TURN TREATMENTS

9.1 General

Refer to the Policy on U-turns at Intersection Traffic Lights guideline, for Main Roads requirements.


9.2 Rural Roads

Refer to Main Roads Drawing 200731-0071 for details of signs and pavement markings.


9.3 Urban Roads

Refer to Main Roads Drawing 200731-0071 for details of signs and pavement markings.



10. SIGNALISED INTERSECTIONS

10.3.2 Proximity to Other Intersections

Main Roads preferred practice is that intersection proximity is typically determined on the basis of at least five seconds of travel time between an intersection and the start of auxiliary lanes for the next downstream intersection.

When planning locations for new intersections this separation distance should not be solely referred to. Other issues such as road hierarchy, function and operation also need to be considered.

 

 

10.4 Sight Distance

Main Roads does not use Minimum Gap Sight Distance (MGSD).


10.5.2 Service Road Treatments to Other Intersections

Reference Figure 10.3 the following treatments are not preferred practices used by Main Roads:

    • In the top left quadrant, the traffic island separating the left turning lane from through lanes is not supported.
    • In the top left quadrant, the egress should be separated by a minimum length of 10m of parallel kerb.
    • In the bottom right quadrant, the left turn lane consisting of taper only is not supported.
    • In the bottom right quadrant, the egress from the service road blending into the taper for the left turning lane should be separated by a minimum length of 10m of parallel kerb.


10.6.3 Pedestrian Treatments

Reference chapter: Pedestrian crossings

At signalised intersections the width between crosswalk lines should:

    • match the pedestrian ramp width (minimum = 2.5m); and
    • where necessary, be wide enough (e.g. = >5.0m) to accommodate high pedestrian demands."


10.6.4 Cyclist Facilities

Bicycle lanes on signalised intersection approaches

Main Roads does not support the use of exclusive right turn lanes for bicycles. Right turn head start facilities may be provided at particular locations however access to these facilities are via bicycle lanes located to the left of the leftmost through lane. Bicycles are not allocated exclusive space through the intersection. Refer to Main Roads Standard Drawings 200531-0006 and 200531-0007 for further details. Note that right turn head start storage facilities for bicycles are not used where there is more than one through lane for motor vehicles.

Head start and expanded storage areas

Reference Figure 10.10 Example (a):

Main Roads position the stop line for motor vehicles 5m back from the adjacent bicycle stop line (in accordance with Main Roads Standard Drawing 200531-0006), not 2m as shown in the example.

Reference Figure 10.10 Example (b):

Main Roads position the stop line for all motor vehicle lanes 5m back from the bicycle stop line (in accordance with Main Roads Standard Drawing 200531-0006), not with the through / right motor vehicle lanes further forward than the left turn lane as shown in the example.

Reference Figure 10.10 Example (c):

Main Roads would typically provide a left turn head start storage facility in conjunction with the bicycle through lane.

Main Roads does not support the use of exclusive right turn lanes for bicycles. The right turn head start storage facility would be accessed via the exclusive bicycle lane located to the left of the leftmost through lane. Refer to Main Roads Standard Drawings 200531-0006 and 200531-0007 for further details. Right turn head start storage facilities for bicycles are not used where there are more than one through lane for motor vehicles.

Reference Figure 10.10 Example (d):

Main Roads does not support the use of Hook Turn facilities for bicycles. Refer also to Figure 10.11. The length of the bicycle head start storage facility shall be 5m in accordance with Main Roads Standard Drawings 200531-0006 and 200531-0007. Right turn head start storage facilities for bicycles are not used where there is more than one through lane for motor vehicles.

Reference Left-turn bypass treatment

Main Roads does not support the use of left turn bypass facilities (as shown in Figure 10.12), due to land constraint issues and conflict with services. Proposals to use such facilities shall be treated on a case by case basis taking into account the aforementioned issues and interaction with other road users such as pedestrians.

Reference Bypass of T-intersection

Main Roads does not support the use of bypass treatments at intersections. Proposals to use such facilities shall be treated on a case by case basis and may result in modification to the treatment shown in Figure 10.13.


 
APPENDIX A   EXTENDED DESIGN DOMAIN (EDD) FOR INTERSECTIONS

Application of EDD will require the explicit approval of Manager Road and Traffic Engineering. 


APPENDIX B   TRUCK STABILITY AT INTERSECTIONS

Main Roads has no supplementary comments for this section.


APPENDIX C   SWEPT PATHS FOR ROAD TRAINS AT HIGH ENTRY ANGLE LEFT-TURN TREATMENTS

Main Roads has no supplementary comments for this section.