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

Document No:  D17#491152
Revision:  3A
Date amended:  14-Nov-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 Dave Nicholls by e-mail or on  (08) 9323 4325.

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 4.1.1, 5 & 6 Drawings 200331-197, 200331-198, 200331-201 & 200331-202 amended. 26-Jan-2012
1B 6 Drawing 200831-0016 amended. 01-Feb-2012
1C 4.1.1, 4.5.3, 5 & 6 Drawings 200331-197, 200331-198, 200331-201, 200331-202 & 200331-203 amended. 12-Feb-2012
1B 6 Drawing 200831-0016 amended. 01-Feb-2012
​1D​4.1.1, 4.6.2, 6, Fig D1 & Fig C3 2​Drawings 200331-194, 200331-196 to 200331-202 amended. Carriageway width note added. Notes added to Appendix D & Commentary 3.​26-Jun-2012
1E 4.1.1 & 6

Drawings 200331-197 and 200331-198 amended.


Guideline amended.

​2A​4.1.2​Clause 4.1.2 added.​10-Feb-2016
​2B​4.1.1 & 6​Drawings 200331-197, 200331-198 and 200331-0202 amended.​28-Jun-2016
​3​All​Guideline amended.​26-Jun-2017
​3A​4.5.2Drawing 200331-0203 amended.​14-Nov-2017

Table of Content

MRWA Supplement to the Austroads Guide to Road Design
Part 4B: Roundabouts

This Supplement has been developed to be read as a supplement to the Austroads Guide to Road Design (GRD) Part 4B: Roundabouts (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.


The purpose of this document is to detail the requirements for the design of roundabouts in Western Australia and to provide guidance in the application of those principles.

The design of roundabouts 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.


Main Roads has no supplementary comments for this section.


2.3 Design Procedure

In addition to the procedure detailed in Austroads Figure 2.2 and Table 2.1, Main Roads has included a series of Guideline drawings at 4.1.1 below.


Main Roads has no supplementary comments for this section.


4.1.1 Design Steps

The initial steps in the geometric design of a roundabout are detailed in Sections 4.2 to 4.4.

These consist of selecting the:

    • Number of legs
    • Number of entry, circulating and exit lanes
    • Central island radius

Once these initial parameters have been considered the drawings listed below describe a method for the design of typical single and dual lane roundabouts where the approach speed is less than 80 km/h.

The dimensions and radii shown in the drawings are a guide for initial selection only.


Single Lane Roundabout


Step 1

Drawing 200331-194

Step 2

Drawing 200331-195

Step 3

Drawing 200331-196

Step 4

Drawing 200331-197


Dual Lane Roundabout


Step 1

Drawing 200331-199

Step 2

Drawing 200331-200

Step 3

Drawing 200331-201

Step 4

Drawing 200331-202


The geometric design process is an iterative one and consists of:

    • Preparing a geometric layout.
    • Construct and review the vehicle entry path for each leg.
    • Check the swept path for the design vehicle for all movements.
    • Adjust as required.

This process is detailed in Sections 4.5 to 4.9.

As an alternative to the manual method described above, consideration could be given to the use of Roundabout design programs such as ARNDT and TORUS.

Drawing 200331-198 is an example of a single lane approach to a dual lane roundabout.


4.1.2 Roundabout Speeds and Vehicle Path Radii

Table 4.1a below is based on AASHTO research and provides an estimate of vehicle speeds when negotiating roundabouts using appropriate side friction values.


Table 4.1a:      Recommended Friction Factors for Predicting Speed and Vehicle Transit Path Radii for a Roundabout


Side Friction FactorSpeed (KPH) ​Vehicle Transit Path Radii
​Entry and Exit Curve (m) ​Circulation (m)


Note: vehicle transit path radii for entry and exit curve based on superelevation of +3% and vehicle transit path radii circulation based on superelevation of -2%.

​Values for vehicle transit path radii were calculated using the adjacent formula, rearranged to solve for R.Image: Side friction formula.PNG

4.5.2 Approach and Entry Treatments

Approach Treatments for High Speed Areas

Rural and high speed roundabouts have high average approach speeds in the range of 80 to 110 km/h. They require supplementary geometric and traffic control device treatments on the approaches to encourage drivers to slow to an appropriate speed before entering the roundabout. The use of reverse curves on approaches may also be considered for lower speed environments.

Drawing 200331-0203 shows an example of the reverse curve approach treatment.

4.6.2 Width of Circulating Carriageway

On roundabouts where the circulating carriageway width exceeds 7m (single lane) or the lane width exceeds 5.5m (2 and 3 lane) consideration should be given to providing encroachment areas.

Design vehicle selection for determining swept paths and carriageway width on two lane roundabouts should be based on the design vehicle + car

Design vehicle selection for determining swept paths and carriageway width on three lane roundabouts should be based on the design vehicle + car + single unit.

The exception to this would be where the percentage of heavy vehicles in each lane is known and there may be a need to allow for 2 design vehicles in adjacent lanes

Note the total carriageway width will depend on whether the design vehicle is in the inside or outside lane.

Generally lane widths should be equal, assuming some encroachment of the design vehicle into the adjacent lane. Individual lane widths should desirably be limited to 5m with an absolute maximum of width of 6m.

4.6.3 Encroachment Areas

For confined locations where a smaller roundabout needs to accomodate heavy vehicles (trucks over 19m long) raised encroachment areas may be used to maintain entry curvature to a kerb line. Refer to drawing 201731-0006.
Main Roads supports the use of raised encroachment areas around the central island for permit vehicles only.
Refer to Austroads Part 4B Fig. 4.11 Sections AA for encroachment areas. Encroachment areas should be in a different colour to the road surface, either in coloured asphalt, couloured concrete, or a long life coloured material with a skid resistance to match the adjacent road surface. Mountable kerb type A should be painted with a retro reflective paint. Raised aprons are not desirable on roundabouts where it is used by trucks carrying animals or fuel.

4.10 Superelevation, Gradient and Drainage

Reference: Use of a crown in the circulating carriageway

The use of crowns in the circulating carriageway is not Main Roads preferred practice. 


Roundabout radius, crossfall and heavy vehicle stability

Consider vehicle stability for turning movements by providing radii appropriate for the turning speeds and providing a satisfactory crossfall and a uniform rate of change of crossfall.  This is particularly important for multi-combinational vehicles where the prime mover and trailer(s) may be on different crossfalls at the same time.  Where possible, this needs to be minimised and should be checked using software such as HVE (Human, Vehicle, Environment) developed by Engineering Dynamics Corporation, USA, and used by Queensland Main Roads.


The use of on-road cycle lanes within the circulating carriageway and approaches is not Main Roads preferred practice. The preferred practice is to allow for cyclists to exit the roadway prior to entering the roundabout using off-road cycle paths located outside the circulating carriageway as shown in drawings 200331-196, 200331-202.


For roundabout pavement marking and signing refer to the following Main Roads drawings:

 Single Lane roundabout  200331-197
 Dual lane roundabout  200331-202
 Dual lane roundabout - Single lane approach 200331-198
 Local area traffic management  200831-0016
 Multi combination vehicles  201231-0014


Main Roads has no supplementary comments for this section.



Main Roads has no supplementary comments for this section.


Main Roads has no supplementary comments for this section.


Main Roads has no supplementary comments for this section.


The figures shown in Appendix D are indicative only.

Figure D1: shows the hold line at the end of the painted island.
Main Roads WA practice is for the hold line to be located at the end of the solid island,
Not at the end of the painted median as shown.

Refer to Main Roads on-line standards and guidelines for signs and pavement marking details.


Main Roads has no supplementary comments for this section.


Commentary 3 - Figure C3 2:
Shows the hold line at the end of the painted island.
Main Roads WA practice is for the hold line to be located at the end of the solid island,
not at the end of the painted median as shown.