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.
3.2.1 & 3.2.3
Figure 4.22 - Separation Line removed.
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 (2009), a copy of which can be purchased 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.
2.2.2 Vertical Alignment
2.2.4 Superelevation at or near intersections
In addition to the sight distance requirements in Austroads GRD Part 4A, the following sight distance checks should be made:
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 (August 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:
Design speed (km/h)
Based on approach sight distance for a car1h1 = 1.1, h2 = 0, d = 0.362
RT = 2.0s
RT = 2.5s
Truck stopping capabilityprovided 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:
Combinations of design speed and reaction times not shown in this table are generally not used. Refer to the Austroads GRD - Part 3: Geometric Design (2009) to determine the ASD for trucks around horizontal curves.
3.2.2 Safe Intersection Sight Distance (SISD)
In the application of Table 3.2, the following guidance is provided:
Based on safe intersection sight distance for a car1h1 = 1.1, h2 = 1.25, d = 0.362
Minimum SISD capability provided by the crest vertical curve size3
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:
Combinations of design speed and reaction times not shown in this table are generally not used.
3.2.3 Minimum Gap Sight Distance (MGSD)
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.
Location of Electrical Assets should be considered for the site. Further advise for Electrical Assets are given in Main Roads Roadside Items and Traffic Management. Documents may include:
Main Roads preferred intersection treatment for most roads is a CHR, AUR or Roundabout.
The AUR turn type works well partially due to the fact that in Western Australia the lane marking differs from other states. The type AUR is a relatively low cost and low maintenance solution to solving high rear-end-major accident rates.
Reference Figures 4.5 and 4.6, Main Roads does not support the use of lane separation line at AUR intersections.
A typical type AUR treatment is illustrated in the Figure 4.22 below. Note that the widening (diverge) taper radii shall be designed as per the radii shown therein.
Figure 4.22: Typical Main Roads Type AUR treatment
Reference Figure 4.7, Main Roads does not support the use of the tapered flares at the left turn out of the minor road. Left turn corners should be based on the swept path of the design vehicle.
The type CHR(S) treatment is not a preferred Main Roads treatment.
In the application of Figure 4.9, Main Roads prefers to use the AUR treatment as an alternative to the Type CHR(S).
4.11.1 Rural Staggered T-Intersection Treatments
Reference Figure 4.16(a,) Main Roads does not support the use of the staggered treatment.
4.12.1 Rural Seagull Treatment
Reference Figure 4.19, Continuity lines are not used through merge areas in Western Australia.
5.3.1 Components of Deceleration Turn Lanes
Reference Figure 5.1, Main Roads preferred practice is for 100m radius back to back coincident reverse curves to be applied to the diverge taper.
5.4.3 Merge Taper TM
Main Roads has adopted a merge rate of 0.6 m/s at all auxiliary lane tapers.
5.4.4 Other considerations
For specific requirements relating to merge tapers refer to Main Roads Supplement to Austroads GRD Part 3: Geometric Design Chapter 9.9.2 Tapers.
6.2 Raised Traffic Islands and Medians
Refer to Main Roads Supplement to Austroads GRD Part 3: Geometric Design - Section 4.7. 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.
6.2.1 Raised Islands
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.
Figure 6.1. Splitter Island Geometry
The values in Table 6.2, "Desirable minimum" column should be used wherever possible. The length (L) of the splitter island shall be determined from Austroads - Guide to Road Design Part 4A (Oct, 2009) Section 6.2.2. Table 6.2.
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
Separate traffic flows and a rigid safety barrier
Separate traffic flows and a flexible safety barrier
Shelter turning vehicles and traffic signals
Table 6.2 Minimum island widths
Notes to Table 6.2:
1 For traffic signals, the minimum offset to any part of the signal is 0.6m from the kerb face
This Table replaces Austroads Tables 6.2 & 6.3
6.2.3 Raised High Entry Angle and Free-flow Left-turn Islands
There are two corner approach island types:-
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 184.108.40.206. 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.
Figure 220.127.116.11. High Entry Angle Island
Figure 18.104.22.168. High Entry Angle Island with Adjacent Cycle Lane
The free flow slip lane island shall be designed using the criteria shown in Figure 22.214.171.124 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.
Figure 126.96.36.199. Free Flow Slip Lane Island
An alternative shown at Figure 188.8.131.52 shows a cycle lane cutting through the island to cross to the left of the acceleration lane at 90 degrees.
Figure 184.108.40.206. Free Flow Slip Lane Island with Cycle Lane
Figure 220.127.116.11 shows a left turn island for use only when the left turn movement is signalised.
Figure 18.104.22.168. 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 22.214.171.124.
An example of this is shown on drawing 200131-0085.
Figure 126.96.36.199. Seagull Island (Minimum Island Area 10m2)
Where intersections include kerbed medians or kerbed islands, street lighting should be provided.
6.4 Desirable Clearance to Traffic Islands and Medians
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 Road Widths between Kerb and between Kerb and Safety Barrier
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.
6.6 Kerb and Channel
Main Roads preferred practice is not to use kerb and channel.
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.
10.3.2 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.
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
10.6.3 Pedestrian Treatments
Reference chapter: Pedestrian crossings
At signalised intersections the width between crosswalk lines should:
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 INTERSECTION TURN TREATMENTS
Application of EDD will require the explicit approval of Manager Road and Traffic Engineering.
APPENDIX B CRASH TYPES AT UNSIGNALISED INTERSECTIONS
Main Roads has no supplementary comments for this section
APPENDIX C TRUCK STABILITY AT INTERSECTIONS
Main Roads has no supplementary comments for this section.
APPENDIX D SET-OUT DETAILS FOR HIGH-ENTRY ANGLE CHL
APPENDIX E SWEPT PATHS FOR ROAD TRAINS AT HIGH ENTRY ANGLE LEFT-TURN TREATMENTS