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 Kyle Smith by e-mail or on (08) 9323 5442.
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.
MRWA Supplement to Austroads Guide to Road Design Part 4C - Interchanges
This Supplement has been developed to be read as a supplement to the Austroads Guide to Road Design (GRD) Part 4C: Interchanges(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
The purpose of this document is to detail the requirements for the design of grade separated interchanges in Western Australia and to provide guidance in the application of those principles. The design of grade separated interchanges has the following primary design objectives:
Absolute minimum standards are to be avoided except where absolutely critical to achieving the most suitable outcome. Generally, if a minimum is used for any particular design element it becomes necessary to avoid using a minimum for any other element on that particular section of road. This is necessary to allow an appropriate factor of safety to road users.
1.8 Staged Development of Interchanges
1.8.2 Flexibility and Constructability
Where sufficient space is available and a satisfactory alignment of the minor road through the initial at grade intersection with the major road can be achieved, the minor road should be constructed clear of any future bridgeworks. In determining a suitable alignment, consideration shall be given to providing sufficient clearances to proposed worksites e.g. future bridge site allowance for deep excavation and clearance for machinery such as pile driving. Wherever possible, the working zone should be separated from the traffic route.
Additional factors influencing the type of interchange may include:
Further guidance on the different types of interchanges can be found in:
Investigations regarding the stability of heavy vehicles suggest that the safe operating speed of these vehicles on loop ramps is considerably lower than cars, and may result in the need for significantly longer deceleration and acceleration lengths.
Structural clearances shall be designed in accordance with:
Clearances between roadway profiles at grade separations shall allow for crossfall, gradient, bridge profile, structural depth and settlement. The design clearances shall be confirmed upon completion of the structural design of the bridge structure by the Designer.
For concept design an indicative structural depth of 1/17th of the longest span plus an allowance of 60mm surfacing on the structure and 50mm settlement should be used.
Bridge piers and abutments should be offset from the road by the dimensions shown on Figure 12.1 of the Bridge Branch Design Information Document No. 3912/02-1.
4.3 Cross-sections on Bridges
A reduction to shoulder widths may be made where the structure length is more than 50m.
All bridge cross sections should be determined on a case by case basis in accordance with Bridge Branch Design Information Document No. 3912/02/11, Bridge Widths.
5.2.2 Ramp Lane Widths
Main Roads generally provides a 3.0m left shoulder on ramps and no right hand shoulder.
For two-lane ramps, 2 x 3.5m wide traffic lanes are to be provided. Lane widths of two-lane ramps with a radius of less than 75m should be designed using swept path simulation software.
For ramps on High Wide Load routes a 0.3m wide right hand shoulder is to be provided, so that a minimum width of 7.3m is achieved between kerbs.
For extent of kerbing on exit and entry ramps refer to the Main Roads drawings listed in Section 6.4.1.
The same design speed should be used throughout an interchange.
The design speed chosen for elements of an interchange should reflect any potential for future increases in the ultimate posted speed of the freeway/highway.
6.4.1 Ramp Design Speed
At exit ramp noses, the ramp design speed should be equal to the posted speed of the adjacent through carriageway.
The design speed on entry ramps for passenger vehicles at the edges meet point is equal to the posted speed of the adjacent through carriageway.
The design speed of all ramp terminals shall be in accordance with:
Main Roads' drawings:
Single Lane Exit Ramp - Typical Tapered Design
Dual Lane Exit Ramp - Typical Tapered Design
Single Lane Exit Ramp- Typical Parallel Design
Dual Lane Exit Ramp - Typical Parallel Design
Single Lane Loop Exit Ramp
Exit Ramp Shoulder Tapers
Entry Ramp Shoulder Tapers
The absolute minimum reaction time that can be used is 2.0 seconds. Absolute minimum reaction time should not be used in combination with other minimum design standards.
Approach Sight Distance (ASD) on the minor road, in between the ramp intersections, may be based on a reaction time of 2.0 seconds. All other ASD calculations must be based on a reaction time of 2.5 seconds. Refer to Main Roads Supplement to Austroads GRD: Part 4A: Unsignalised and Signalised Intersections; Table 3.1.
For the purpose of this document "ramps terminals" are defined as the ramp intersection with the minor road.
Main Roads has not adopted Minimum Gap Sight Distance (MGSD).
Interchange geometry including major and minor roadways, ramps and turning roadways shall be designed in accordance with:
Acceleration and deceleration lengths required for entry and exit ramps shall be designed in accordance with drawings listed in Section 6.4.1.
Ramp shoulder tapers shall be designed in accordance with Main Roads Drawings 201131-0028 and 201131-0029.
8.3.4 Service Interchanges
Exit loop ramps
Large plan transitions should not be used on the approach to loop ramp exits because they may lead drivers to overestimate the safe speed of the loop ramp. If required only plan transitions based on Main Roads - Horizontal Curve Tables should be used on the approach to a loop ramp.
Refer to Main Roads Drawing 201131-0020 for ramp lane and shoulder widths on single lane exit ramps. Lane widths for loop ramps with radii of less than 75m should be designed using swept path simulation software. It is acceptable for vehicles over 23.5m to track onto the ramp shoulder.
Entry loop ramps
The compounding of larger radius curves or adding a plan transition to the departure from a loop ramp can be used to provide the required acceleration distance for vehicles entering the through carriageway.
Refer to Main Roads Drawing 201431-0017 and 201431-0053 for ramp lane and shoulder widths on single lane entry ramps. Lane widths for loop ramps with radii of less than 75m should be designed using swept path simulation software. It is acceptable for vehicles over 23.5m to track onto the ramp shoulder.
Loop ramp shoulders
Loop ramp shoulders should generally be located on the outside of the ramp curve to ensure that vehicles stopped on the shoulders do not adversely impact horizontal sight distance. At the ramp nose a 3m wide shoulder should be developed on the left hand side of the ramp as per the typical entry and exit ramp details. A smooth transition of the shoulder from one side of the ramp to the other should be provided. (Refer to Main Roads drawing 201531-0020).
Loop ramp shoulder tapers should be designed in accordance with Main Roads drawings 201131-0028 and 201131-0029.
Where horizontal sight distance on the outside of a loop ramp is affected by safety barriers, vegetation, retaining walls or any other structure, consideration may be given to locating the shoulder on the inside of the loop ramp.
9.2 Minor Road
Where required, crossfall on the cross street may be reduced to a minimum of 2% at ramp terminals to reduce the impact of adverse crossfall on the turning movements. Crossfall on the cross street bridge structure may be reduced to match that at the ramp terminal where a suitable profile on the cross street cannot be achieved. The impact on drainage spread widths should be considered before reducing crossfall on the bridge. In general a higher crossfall on the bridge will result in less drainage structures being required.
The minimum vertical design speed on a ramp at the intersection with the minor road is 40km/h.
The maximum upgrade for at least the last 30m from the intersecting edge of pavement for an exit ramp should be limited to 3%.
9.3.2 Ramp Gradients on Service Interchanges
Ramp grades in the range of 3% to 5% or flatter are desirable. It should be noted that ramp grades are controlled by the distance from the ramp nose to the minor road. This in turn is controlled by the distance required for either acceleration or deceleration and storage distance.
10.3 Ramp Terminal at Minor Road
It is not Main Roads preferred practice to install concrete aprons at ramp terminals with minor roads.
Ramp terminals shall generally be configured as follows:
Exit ramp terminals should desirably be located on tangents. The exit must appear as an obvious diversion from the through alignment and should be located to prevent a driver inadvertently entering the exit ramp. The exit ramp taper should not be located just beyond or straddling a curve/tangent point on the major alignment where the ramp alignment may appear to be a continuation of the through carriageway.
The distance between ramp terminals shall be designed in accordance with:
Exit and entry ramp minimum kerbing extents are to be in accordance with Main Roads drawings 201131-0020 and 201431-0053.
11.2 Exit Ramps
Exit ramp terminal horizontal geometry shall be designed in accordance with Main Roads Drawings listed in Section 6.4.1.
For exit ramps the cross slope between the edge of the left carriageway shoulder and the ramp right lane edge in the first 60m of the ramp beyond the nose should be a maximum of 6H to 1V.
Entry ramp terminal horizontal geometry shall be designed in accordance with Main Roads Drawings listed in Section 6.4.1.
Where a single-lane entry ramp leads into an auxiliary lane, a varying taper of approximately 1 in 30 may be used between the ramp nose and the edges-meet point.
The maximum difference in bearings between the ramp and through carriageway at the nose of single-lane entry ramps shall be between 3o 20' and 4o30' for curved alignments depending on the design speed. Refer to Main Roads drawing 201431-0053 for details.
11.3.3 Entry with auxiliary lane
Parallel entry ramp auxiliary merge tapers are to be designed at a rate of 0.6m/sec not 1.0m/sec as shown in AUSTROADS - GRD - Part 4C: Interchanges (2009) ; Section 11.3.3 and on Figure 11.7.
11.3.4 Two lane entry
For two lane entry ramp details refer to Main Roads drawing 201431-0055.
11.4.3 Geometric and layout design
Any merging of lanes on the ramp beyond the ramp signals should be completed before the ramp nose.
Main Roads policy is to provide grade separation for Principal Shared Paths at grade separated interchanges. (Policy for Cycling Infrastructure Document No. 37/09/01) Shared path facilities for cyclists and pedestrians through interchanges shall be designed in accordance with the Geometric Design of Pedestrian and Cyclist Facilities.
14.2 Treatment at Interchanges
Main Roads practice is to use a treatment similar to that shown in Figure 14.2 passing through the solid median far enough behind the ramp nose to provide protection for cyclists. Typical drawings should be developed in consultation with cycling groups.
Signing and pavement marking of interchanges shall be designed in accordance with:
Interchange lighting shall be designed in accordance with the Lighting Design Guideline for Roadway and Public Spaces.
Landscaping shall be designed in accordance with the Handbook of Environmental Practice for Road Construction and Maintenance Works
Current Main Roads practice is to retain all existing vegetation outside the toe of batter. Horizontal clearances should allow for future services, future widening or run-out areas.
Fencing shall be designed in accordance with the Design of Fencing/Walls.
The need for safety barriers shall be assessed using Main Roads Supplement to Austroads GRD - Part 6 - Roadside Design, Safety and Barriers.
Safety barriers shall be designed in accordance with:
Main Roads Supplement to Austroads GRD - Part 6 - Roadside Design, Safety and Barriers
(Main Roads) - Roadside Items
17.1 Access in the Vicinity of Interchanges
17.1.1 Access to the Freeway
On roads currently at, or planned to be upgraded to a freeway standard, control of access shall be enforced over the full length of the freeway including the interchange ramps. Special cases for direct access may exist to accomodate Freeway Service Centres, public transport facilities, enforcement sites or in extenuating circumstances where suitable access is unavailable.
17.2 Service Centres
Any proposals for development of new services centres shall be referred to the Manager Road Planning for consideration.
17.3 Intelligent Transport Systems (ITS) Infrastructure
ITS requirements such as traffic cameras, variable message signs, etc should be determined in accordance with Road Network Services requirements.
17.3.1 Designing for Future Ramp Signals
The following design features should also be considered to facilitate the future retrofitting of ramp signals:
17.4 Oversized Loads and High Wide Load Corridors
Where roads are to be grade separated the oversized load/HWL corridor route should desirably pass over cross roads to reduce clearance requirements and costs.
Where the oversized load/HWL corridor route passes under the crossroad and the crossroad links to the oversized load/HWL corridor with an interchange, a diamond shape interchange parallel to the corridor should be installed to minimise clearance requirements at the interchange. If there is no connection between the oversized load/HWL route and the crossroad or if there is an existing non-diamond interchange, an alternative route with appropriate clearances should be considered. Options that may be considered include a detour only for the use of oversized load/HWL, or a cross over to the opposite carriageway.
For oversized load requirements refer to Main Roads, Heavy Vehicles Operations.
For minimum design requirements HWL corridors refer to Main Roads - Guide to Design and Operation of High Wide Load Corridors: Guideline Section 4.6 Clearances.
17.5 Emergency Provisions
Emergency telephones shall be designed in accordance with (Main Roads) - Design and Installation of Roadside Help Phones.
17.6 Planning Considerations
17.6.1 Lane Balance
Lane configuration, including number of lanes and lane balance, shall be designed in accordance with:
Right turning traffic from an exit ramp should always be directed into through lanes on the minor road and not into shared through/right turn lanes or sole right turn lanes on the minor road.
17.6.2 Traffic Considerations
Levels of Service
Level of service of all freeway/highway segments shall be designed in accordance with; (Transportation Research Board 2000) - Highway Capacity Manual.
The lane configuration shall generally provide level-of-service C or better for all segments of the freeway/highway and interchange during the worst 15-minute period of the day for the predicted future traffic volumes. An ultimate interchange should be designed to meet LOS C based on 20-30 year traffic horizon and an interim stage LOS C based on 10 year traffic forecast. Isolated segments with level-of-service D may be acceptable where it is not cost effective to provide additional lanes.
The level-of-service shall remain constant or nearly constant between adjacent freeway segments. For example C-C-C-D-C-C is acceptable but C-D-C-D-C-D or C-C-C-E-C-C is not. The LOS change between segments must not be greater than one increment e.g. A-B-A-A-B-D is not acceptable. Refer to: (AASHTO 2004) - A Policy on Geometric Design of Highways and Streets, p85.
In inner Perth areas with high traffic volumes it may not be possible to achieve a high LOS due to significant site constraints. In these circumstances, the interchange should be designed to achieve the highest LOS possible.
APPENDIX A EXAMPLE OF RAMP SPEED ANALYSIS
For loop ramp details refer to Main Roads drawing 201131-0026.
APPENDIX B RAMP TERMINAL LOCATION
Main Roads has no supplementary comments for this section.
APPENDIX C EXAMPLES OF RAMP SIGNAL LAYOUTS
Refer to Main Roads guideline drawings for ramp metering
Step out line markings shall only be used at the start of exit ramps with the approval of the Manager Road and Traffic Engineering.