MRWA Supplement to 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, Main Roads' policies, guidelines and standards take 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 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.

1. Introduction

Main Roads has no supplementary comments for this section.

2. Design Principles and Procedure

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.

3. Sight Distance

Main Roads has no supplementary comments for this section.

4. Geometric Design

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 200331-0194
Step 2	Drawing 200331-0195
Step 3	Drawing 200331-0196
Step 4	Drawing 200331-0197

 

Dual Lane Roundabout
Step 1	Drawing 200331-0199
Step 2	Drawing 200331-0200
Step 3	Drawing 200331-0201
Step 4	Drawing 200331-0202

 

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 200331-0198 is an example of a single lane approach to a dual lane roundabout.

Drawing 201831-0002 is an example of a dual lane roundabout with a single lane exit.

Drawing 201831-0031 is an example of a 3 lane roundabout with double right turn.

Spiral marking is typically required where there are more circulating lanes than exit lanes.

Refer to Austroads part 4B Appendix D.2.

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 Factor ​	​Speed (KPH) ​	​Vehicle Transit Path Radii ​
		​Entry and Exit Curve (m) Based on 3% Superelevation	​Circulation (m) Based on -2% Superelevation
0.3	15	5	6
0.3	20	10	11
0.3	25	15	18
0.3	30	21	25
0.3	35	29	34
0.3	40	38	45
0.3	45	48	57
0.3	50	60	70
0.3	55	72	85
0.3	60	86	101

 

Notes:

1. The above table is not to be used for design purposes, and is only intended to be used to predict speeds.
2. The Horizontal Curve Equation (Austroads GRD Part 3, Section 7.4.1) is used to calculate the tabulated radii values.

4.5.2 Approach and Entry Treatments

Approach Treatments for High Speed Areas

For rural and high speed roundabouts which have posted approach speeds of 80 km/h and higher it is mandatory to include supplementary geometric or traffic control device treatments on the approaches to encourage drivers to slow to an appropriate speed before entering the roundabout.

Main Roads preferred treatment is successive reverse curves.  This treatment can also be considered for lower speed environments.  Drawing 200331-0203 shows an example of the reverse curve approach treatment.

In constrained situations, where successive reverse curves would not be suitable (e.g., road reserve constraints or driveways within the proposed curves) the following treatments (one or more) are acceptable:

• Long median islands as described in Austroads GRD Part4B.
• Narrowing of the cross section (only on horizontal straights).
• Diagonal pavement marking in the shoulders to give the impression of narrowing of the carriageway.  For this treatment consideration should be given to cyclists as this treatment may encourage them to use the traffic lanes instead of the shoulders.
• Appropriate speed limit signs. Main Roads practice with respect to speed zoning roundabouts is as follows: 
  • Where the approach road is speed zoned at 90 km/h and above, the roundabout approach shall be a speed zoned at 80 km/h or based on the 85th percentile speed, whichever is the lesser value, with a length not less than 300 metres before the intervening feature.
  • Where there is a reduction in the speed limit of more than 30 km/h, a buffer zone may be required.
• Advanced warning signs, with or without appropriate advisory speed limits.
• Electronic Vehicle Activated Warning signs as described in Austroads GTM Part 10. 
• Rumble strips. This treatment is required to meet specific criteria and requires approval as described in the Main Roads Rumble Strip Guideline. Due to the noise generated by these elements it is also necessary to consider impacts to the surrounding environment (e.g., nearby residents, nearby businesses, or in sensitive habitat areas).

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 accommodate heavy vehicles (trucks over 19 m 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 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 coloured asphalt, coloured 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

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

Roundabouts are generally adopted as a form of intersection due to their car safety benefits (i.e., low speeds and improved impact angles).  To achieve the desired safety improvements offered by a roundabout, speeds need to be controlled through geometry, however at the same time designers need to ensure stability of high centres of gravity (HCoG) trucks is maintained.  This is particularly relevant where approach speeds would be high.  The geometry and movements in a roundabout include series of reverse curves which also often include changes in crossfall direction, these features induce destabilising effects on trucks with HCoG.

Below is a table of features that have the potential to cause instability for HCoG trucks.  Also listed are methodologies to deal with these risks.

HCoG Destabilising Design Features	Risk Reduction Methodology
Approach speed	Manage approach speeds in accordance with Main Roads supplement to Austroads GRD Part 4B,section 4.5.2
Approach reverse curves	Ensure the curves are adequate for the operating speed with speed reduction limited to 20 km/h per curve Ensure that the curves are long enough to allow for a comfortable deceleration rate (2.5 ms-2). An important design element in the approach reverse curves is to provide a short tangent between curves, this reduces the dynamic effect caused by truck suspension.
Changes in superelevation direction	Rates of rotation should not exceed the maximums.  Also by adding short tangents between curves improves superelevation rotation rates, and superelevation can then be located correctly. Having the circulating carriageway as adverse superelevation avoids the need for a superelevation rollover at the entry curve.
Fold or ridge through the centre of the roundabout	Avoid folds or ridgelines in roundabouts, or if unavoidable then limit the changes in grade to 4% and make the vertical curve joining the grades as long as possible.
Kerbing	Ensure that the correct design vehicles are adopted for tracking and ensure that adequate clearance to kerbing is achieved. Consider mountable Type A kerbing for the central island to reduce destabilising affect produced by semi-mountable kerbing, if mounted by HCoG trucks.

Table 4.10.1: Design features affecting HCoG truck stability

Notes:

1. During design development it is appropriate to use Austroads GRD Part 4A, Appendix B to establish truck operating speeds.

Due to the variability of roundabout geometry including approach geometry and the reaction of truck suspension to this geometry, it is difficult to predict the outcome for trucks with HCoG, without modelling.  Listed below is the minimum criteria for when roundabouts should be assessed using simulation software (e.g.,TruckSim, HVE or UM Truck and Trailer).

• Freeway and highway exit ramp approaches to roundabouts where the approach posted speed is 80km/h or greater, especially where the crossing road is under the main alignment.  For these assessments the high speed roundabout approach treatment must also be assessed if adopted.
• Rural roundabouts where the approach posted speed is 80km/h or greater. The approach treatment should also be included in the assessment if adopted.
• All roundabout approaches where the posted speed is 70 km/h and no approach treatment has been provided.

For the assessed movements designers must at least allow HCoG trucks to negotiate the roundabout at 30 km/h without exceeding a Load Transfer Ration (LTR) of 0.6.  Operating speed should be used to assess the roundabout approach treatment, if adopted.

The modelling assessment must incorporate the following design vehicle characteristics:

• Vehicle assessment configurations to be Prime Mover and Semi-Trailer (19 m), and turning movement design vehicle as required for the project.
• Payload height to be 4.3 m.
• Payload to be uniform density.
• Vehicle deck height of 1.4 m to be adopted.
• Tyres and suspension should match widely used values.

Simulation outputs should include a vehicle path profile (distance or chainage on horizontal axis) with LTR plotted along the vertical axis.  In addition it is also useful to graph the vehicle path profile with the superelevation listed. This way designers and reviewers can identify the geometric features that govern the design.

Once simulation has been undertaken designers should consider making geometric changes if the following results are produced:

• The criteria as described above has not been achieved.
• High speeds (Approximately 40 km/h and above) through the roundabout for HCoG vehicles, as this will result in other vehicles being able to negotiate the roundabout at much high speeds which will create safety risks.
• An isolated geometric feature with an LTR of 0.6 governs the design (single significant LTR spike). This has safety risks as drivers will negotiate the approach geometry at a comfortable speed and may not anticipate the isolated feature and therefore may become unstable.  Good design should have a spread of LTR spikes of 0.6 to provide a consistent feel when negotiating the roundabout.

If geometric changes are made simulation should confirm the outcome.

Main Roads has developed an internal process (between RTE Branch and HVS) for the assessment of heavy vehicle stability -  “Guidelines for Vehicle Stability Analysis – Main Roads Internal Process”

5. Pedestrian and Cyclist Treatments

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 200331-0196 & 200331-0202.

6. Pavement Markings and Signing

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

Single Lane roundabout	Drawing 200331-0197
Dual lane roundabout	Drawing 200331-0202
Dual lane roundabout – Single lane approach	Drawing 200331-0198
Local area traffic management	Drawing 200831-0016
Multi combination vehicles	Drawing 201231-0014

 

7. Landscaping and Street Furniture

Main Roads has no supplementary comments for this section.

8. Appendix

Appendix A - Crash Types

Main Roads has no supplementary comments for this section.

Appendix B - Roundabout Study and Program

Main Roads has no supplementary comments for this section.

Appendix C - Methods of Improving Roundabout Entries

Main Roads has no supplementary comments for this section.

Appendix D - Linemarking of Multi-lane Roundabouts

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.

Appendix E - Extended Design Domain (EDD) for Entry Path RADII

Main Roads has no supplementary comments for this section.

Commentaries

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.