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

Document No:  D11#308735
 
Revision:  4
 
Date amended:  20-Oct-2017

 

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 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.

This Supplement has been developed to be read as a supplement to the Austroads Guide to Road Design (GRD) Part 4C: Interchanges(2015), 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.

Revision Register

 

Ed/Version Number Clause Number Description of Revision Date

1

All

Guideline Developed. 09-Dec-2011 
​2​AllGuideline Revised.​07-May-2015
​3​AllGuideline Amended.​08-Jun-2015
​3A​All​Links to ramp guideline drawings updated.​26-May-2016
​3B​Header​Contact person changed to Kyle Smith.​25-Jan-2017
​3C​8​Horizontal Curve Tables updated to rev 3A​16-Oct-2017
​4​All​Guideline Reviewed.​20-Oct-2017

Table of Content


MRWA Supplement to Austroads Guide to Road Design Part 4C - Interchanges

 

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:

    • Maximise safety
    • Minimise costs associated with construction, maintenance and use of the route
    • Minimise adverse impacts on the environment
    • Maximise operational efficiency - ie the ability to carry the required volume of traffic at a speed acceptable to the road user
    • Be aesthetically pleasing and in harmony with the surrounding environment
    • Consider the planned ultimate layout in the vicinity of the works and ensure that it can be accommodated with a minimum of reconstruction in the future

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. INTRODUCTION

Main Roads has no supplementary comments for this section.


2. DESIGN CONSIDERATIONS, PROCESSES AND PRINCIPLES

2.4 Principles

A minimum length equivalent to 4 seconds of travel time at the respective design speed is to be used between consecutive decision making points (eg. Merge from two lanes to one after an intersection, lane changes after an intersection, a merge followed by a diverge). This length should be extended if:

  • A traffic analysis and/or operating experience shows the need for greater separation.
  • The lane has to go over a crest and suitable sight distance to the next decision making point has to be achieved (at least ASD).

3. FORMS OF INTERCHANGE

3.2.1 General

Further guidance on the different types of interchanges can be found in:

  • Austroads  - Guide to Traffic Management - Part 6: Intersections, Interchanges and Crossings (2007)
  • AASHTO 2004 - A Policy on Geometric Design of Highways and Streets (2004), p743- 7, p802-804 and p822.

 

Parclo Interchanges

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.


4. STRUCTURES

4.1 General

Structural Clearances

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.

Horizontal Clearances

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. 3912/02-12, Clearances and High Load Routes.

 

4.3 Cross-sections on Bridges

Shoulder Widths

A reduction to shoulder widths may be made where the structure length is more than 50m.

Approvals

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.

 

4.9 Safety Screens

Refer to Main Roads Structures Engineering Branch Technical Guideline “Assessment of Overpass Structures with Pedestrian Access for the Risk of Thrown Projectiles" for their policy of safety screens on bridges over roads.

 

For safety screens over railways refer to Public Transport Authority for their policy.

 

5. CROSS-SECTION

5.2 Ramp Cross Section

 

5.2.1 Number of Lanes on Ramp

Exit Ramps

Refer to Main Roads drawing 201131-0028 for the transition from one lane at the nose to two for exit ramps.

 

Entry Ramps

Refer to Main Roads drawing 201131-0029 for the transition from two lanes to one at the nose for entry ramps.

  

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.

 

Kerbs

For extent of kerbing on exit and entry ramps refer to the Main Roads drawings listed in Section 6.4.1.

 

Where a concrete barrier is required, the absolute minimum offset from the edge of the traffic lane shall be:

  • 0.3m for a turn pocket or road speed zoned under 70km/h
  • 0.6m for a road speed zoned between 70km/h and 90km/h
  • 1.0m elsewhere

 

6. DESIGN SPEED

6.1 General

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 Ramps

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

201131-0020

Dual Lane Exit Ramp - Typical Tapered Design

201131-0022

Single Lane Exit Ramp- Typical Parallel Design

201131-0023

Dual Lane Exit Ramp - Typical Parallel Design

201131-0024

​Single Lane Entry-Ramp – Typical Parallel Design (New Installation)

201431-0053

​Single Lane Entry-Ramp – Typical Parallel Design (Retrofit Installation)

201431-0017

​Dual Lane Entry-Ramp – Typical Parallel Design (Dual Lane Merge)

201431-0055

​Dual Lane Entry-Ramp – Typical Parallel Design (Merge and Added Lane)

201731-0001

​Dual Lane Entry-Ramp – Alternative Parallel Design (Merge and Added Lane)

201731-0002

Single Lane Loop Exit Ramp

201131-0026

​Single Lane Loop Entry Ramp - Typical Parallel Design201531-0020

Exit Ramp Shoulder Tapers

201131-0028

Entry Ramp Shoulder Tapers

201131-0029

 


7. SIGHT DISTANCE

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.

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.

For the purpose of this document "ramps terminals" are defined as the ramp intersection with the minor road.

 

7.5 Safe Intersection Sight Distance

Main Roads has not adopted Minimum Gap Sight Distance (MGSD).


8. HORIZONTAL ALIGNMENT

Interchange geometry including major and minor roadways, ramps and turning roadways shall be designed in accordance with:

 

8.3.1 General

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 201431-0053 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. VERTICAL ALIGNMENT

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. RAMP TERMINALS AT MINOR ROADS

10.3 Ramp Terminal at Minor Road

It is not Main Roads preferred practice to install concrete aprons at ramp terminals with minor roads.


11. RAMP TERMINALS AT THE MAJOR ROAD

11.1 General

Ramp terminals shall generally be configured as follows:

  • Parallel type entry ramp terminals shall be used on all entry ramps
  • If the exit ramp adjoins a horizontal curve with a radius less than or equal to 900m on the main alignment, the exit ramp should be a parallel design rather than a tapered design.
  • Parallel type exit ramp terminals shall be used when traffic analysis determines their use is appropriate.
  • Tapered exit ramp terminals established from horizontal curves should be developed using the same taper rates as used for tapered ramp terminals on tangents.

 

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:

  • AUSTROADS - Guide to Traffic Management - Part 6: Intersections, Interchanges and Crossings (2013), Section 6.6.6 and Table 6.34;
  • AASHTO - A Policy on Geometric Design of Highways and Streets (2004), p843 and Exhibit 10-68, p844; and
  • The minimum lengths measured between successive ramp terminals must be checked in accordance with the procedure outlined in the Highway Capacity Manual (2000) referred to in Clause 1.3(a) (ii) (I).

 

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.

 

11.3 Entry Ramps

11.3.1 General

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 320' 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 (2015); 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 drawings 201431-0055, 201731-0001 and 201731-0002.

 

11.4.3 Geometric and layout design

Merge geometry

Any merging of lanes on the ramp beyond the ramp signals should be completed before the ramp nose.

 

11.3.5 Loop Ramps

For loop ramp entry details refer to Main Roads drawing 201531-0020.

 

11.4 Ramp Traffic Signals

Ramp traffic signals shall be designed in accordance with:

 

12. RAMPS ON TWO-LANE TWO-WAY FREEWAYS

Main Roads has no supplementary comments for this section.

13. PEDESTRIANS

Main Roads policy is to provide grade separation for Principal Shared Paths at grade separated interchanges. (Document No. 37/09/01)  Shared path facilities for cyclists and pedestrians through interchanges shall be designed in accordance with Pedestrian and Cyclist Facilities.

14. CYCLISTS

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.


15. PAVEMENT MARKINGS, SIGNS AND LIGHTING

Signs

Signing and pavement marking of interchanges shall be designed in accordance with:

 

Lighting

Interchange lighting shall be designed in accordance with the Lighting Design Guideline for Roadway and Public Spaces.


16. LANDSCAPING AND STREET FURNITURE

16.1 General

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.

 

Fences

Fencing shall be designed in accordance with the Design of Fencing/Walls.

 

Safety barriers

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

 

Additional items

Main Roads - Roadside Items


17. OTHER CONSIDERATIONS

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 Oversized Loads and High Wide Load Corridors

For oversized load requirements refer to Main Roads, Heavy Vehicle Services.

For minimum design requirements HWL corridors refer to Main Roads - Guide to Design and Operation of High Wide Load Corridors: Section 4.6 Clearances.

17.4 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.

 

17.4.1 Designing for Future Ramp Signals

The following design features should also be considered to facilitate the future retrofitting of ramp signals:

  • For drawings indicating on-ramp requirements for ramp metering signals (including heavy vehicle by-pass lanes), refer to the Ramp Metering Guideline Drawings  on the Main Roads website.
  • Entry ramp lengths to provide for future storage being a minimum of 420m from the ramp entrance to the ramp nose (storage for flows up to 1,200 vehicles/hour).
  • Vehicle detector locations on the entry ramp to suit future stop line location.
  • The position and spacing of stormwater pits should be based on the future allowable spread width, assuming that the shoulder is used as a traffic lane.  If the pit spacing becomes uneconomically close, it may be necessary to allow for a nominal future shoulder width to accommodate some of the flow width.
  • Consideration should be given to not providing a shoulder on the ramp: the ramp would be line marked as a two-lane ramp with the “Form 1 Lane" sign and merge in its future position.
  • If the implementation of ramp metering is likely to occur within a short timeframe (eg. the next two years or so) and it is considered undesirable to provide the pavement markings in their future position, consider using an approved temporary line marking tape, which meets Main Roads Specification 604 - Pavement Marking.
  • Consideration should be given to the future installation of road safety barriers to protect against crashes with the future ramp signal poles.  The depth and / or positions of pipes and gullies needs to be considered in relation to the depth and spacing of barrier posts.
  • Consideration should be given for verge width requirements for ramp signals and other required road side furniture, including an allowance for an appropriate pull-off area for maintenance parking.

The above must be adhered to if ramp signals are within 5 years of ramp works being completed. Serious consideration should also be given if signals are within 10 years of completion.

 

17.5 Emergency Provisions

Emergency telephones shall be designed in accordance with 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:

  • (AASHTO 2004) - A Policy on Geometric Design of Highways and Streets; p810 onwards
  • (Transportation Research Board 2000) Highway Capacity Manual

 

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

Managed Freeways - Typical Entry Freeway Ramp Signals for Two Lanes Metered
Managed Freeways - Typical Entry Freeway Ramp Signals for Two Lanes Metered Plus Free Flow Priority Lane
Managed Freeways - Typical Entry Freeway Ramp Signals for Two Lanes Metered Plus Metered Priority Lane
Managed Freeways - Merge Layouts Freeway Ramp Signals Three Lanes Metered to One Lane at the Nose
Managed Freeways - Merge Layouts Freeway Ramp Signals Four Lanes Metered to Two Lanes at the Nose
​Managed Freeways - Merge Layouts Freeway Ramp Signals Three Lanes Metered to Two Lanes at the Nose
​Managed Freeways - Typical Freeway Ramp Signals Layout Freeway to Freeway Interchange


COMMENTARY 1

Main Roads has no supplementary comments for this section.


COMMENTARY 2

Main Roads has no supplementary comments for this section.


COMMENTARY 3

Step out line markings shall only be used at the start of exit ramps with the approval of the Manager Road and Traffic Engineering.


COMMENTARY 4

Main Roads has no supplementary comments for this section.


COMMENTARY 5

Main Roads has no supplementary comments for this section.