Temporary Alignments in Urban Areas

1. General Standards and Application

The purpose of this document is to detail Main Roads' standards for the geometric design of temporary alignments in Western Australia and to provide practical guidelines for the application of these standards.

This document shall be applied to temporary alignments in urban and outer urban environments.

This guideline does not apply to sidetracks. Guidance on these can be found in Operational Guideline No 92 and Main Roads Specification 202.

All temporary alignments should have the following primary design objectives:

• Maximise safety for both the road users and road construction workers.
• Minimise costs associated with construction, maintenance and use of the route.
• Consider the planned ultimate layout (road and adjacent developments) in the vicinity of the works and ensure that it can be accommodated with a minimum of reconstruction in the future.
• Maximise operational efficiency, that is the ability to carry the required volume of traffic at a speed acceptable to the road user.
• Maximise opportunities to cater for the needs of all road user groups.

Geometric road design should generally be in accordance with the following documents unless otherwise explained in this guideline:

• Main Roads Standards Website; Roads and Traffic Engineering Design Guidelines.
• Horizontal Curve Tables; Main Roads Western Australia.
• Main Roads Supplements to Austroads Guide to Road Design Series.
• Austroads Guide to Road Design Series. 

If there is a discrepancy between reference documents, the order of precedence should be firstly Main Roads and then Australian Standards and Guidelines.

In general, minimum standards should only be used where they are considered necessary to meet one or more of the design objectives listed above. Generally, if a minimum is used for any particular element it is preferable to avoid using a minimum for another element. This is necessary to allow an appropriate factor of safety to the road user.

1.1 Glossary of Terms

• Detours - An alternative route available to traffic on an existing road network during temporary closure of a road.
• Temporary Alignments - A section of carriageway constructed in a location to enable the future construction of infrastructure in its permanent location.

2. Geometry

For temporary alignments that may be in place for twelve months or more then the design shall, as far as practicable, meet full design standards.

Consideration to the staging of a project should be given, as the design of temporary alignments may be required, and in cases such as staging of a future grade separation, a cross road may be constructed on a temporary alignment that may be in place for many years.

2.1 Design Speed

Wherever practical, traffic should be able to travel through the temporary alignment at the same speed as the adjoining road sections. This is particularly important for temporary alignments of a longer duration.

Where the geometric standards cannot be met for the design speed of the adjoining road sections, then the temporary alignment may need to be designed for a lesser design speed. The incremental change of design speed on the approach to a temporary alignment shall be no more than 20km/h and signed accordingly.

The design speed shall be based on the proposed traffic management plan posted speed plus 10km/h in recognition of the likelihood of high approach speeds.

The adopted design speed shall take into account the safety requirements for the construction site.

2.2 Horizontal and Vertical Alignments

The horizontal and vertical alignments shall be designed to meet the appropriate design speed designated for the temporary alignment.

Wherever possible, horizontal and vertical geometry shall be co-ordinated for temporary alignments in place for twelve months or more. In all cases, crest curves shall not obscure the start of horizontal curves to avoid possible safety problems.

Tie-ins should not be located at sites where a change in environment is not obvious (e.g. on a crest or at a curve). Temporary designs shall include the design of all tie-ins and carriageway edges.

The design of the tie-in must meet all appropriate design standards for the adopted design speed of the temporary alignment.

Design factors such as approach, intersection and stopping sight distances shall not be compromised within the temporary alignment.

In determining a suitable alignment, consideration shall be given to providing sufficient clearances to both proposed worksites e.g. future bridge site, allowance for deep excavation and clearance for machinery. Wherever possible, separate the working zone from the traffic route.

2.3  Superelevation

Superelevation on temporary alignments shall be applied in accordance with Main Roads Horizontal Curve Tables wherever practical.

Temporary alignments in place for up to twelve months that cannot meet the Main Roads Horizontal Curve Tables criteria shall apply superelevation at a rate based on the Main Roads Supplement to Austroads GRD Part 3: Geometric Design using the desirable friction factor but with no distribution of  e or f ie:-

 E = V²/(127R) - f des.

A minimum superelevation and/or crossfall of 2% is required to cater for drainage on sealed surfaces.

Temporary alignments in place for twelve months or more shall apply superelevation at a rate in accordance with Main Roads Horizontal Curve Tables. Where these superelevation rates cannot be achieved it may be acceptable to apply superelevation in accordance with Main Roads Supplement to Austroads GRD Part 3: Geometric Design based on the desirable friction factor and with distribution of the e & f ie:-

Rmin = V²/[127 x (Emax + Fdes)]

For any other radius other than Rmin then

E₁ = (Rmin / R) x Emax  with Emax =5%

If adverse crossfall is to be used then Fdes shall be limited to 50% of Fdes, as recommended by the Main Roads Supplement to Austroads GRD Part 3: Geometric Design.

3. Design Vehicles

When designing temporary alignments, consideration should be given to design vehicles, including construction traffic, that will be using this section of road, as this may have an influence on geometric elements of the design such as lane widths, superelevation etc.

Where the temporary alignment forms part of a permit route, Main Roads' Heavy Vehicle Operations shall be consulted to ensure the design of the temporary alignment makes allowance for over width and over height vehicles.

4. Cross Section

4.1  General

A reduced cross section width may be appropriate when designing temporary alignments to minimise cost and the earthworks footprint. Lane widths for short term temporary alignments may be reduced but design checks shall be made in reference to pavement widening on horizontal curves where applicable. The geometry must enable the design vehicle to remain lane correct through the entire length of the temporary alignment.

For more information relating to minimum lane widths, refer to Table 2 Main Roads Traffic Management for Works on Roads - Code of Practice, except use 3.3m minimum lane width for B Double/Road Trains for design speed < 60km/h, refer to ARRB  Document - Analysis of lane width requirements of Western Australian road trains.

In determining the number of lanes required, the capacity of a lane shall be taken as 1000 vehicles per lane per hour.

Wherever practical, the number of lanes provided for a temporary alignment should be the same as the adjoining road sections. The number of lanes should only be reduced if it can be demonstrated that Main Roads operational requirements can be met. If the number of lanes is to be reduced, then the number of lanes required needs to be determined based on a capacity analysis in accordance with Austroads Guide to Traffic Management Part 3: Traffic Studies and Analysis.

4.2 Median Widths

Bicycle Median widths on proposed temporary alignments, where required shall be based on the following criteria:-

 

 

5. Intersections

Intersections shall be designed to accommodate the appropriate design vehicles. If reduced lane widths have been used, care shall be taken to ensure that there is no encroachment into adjacent lanes, other than as allowed by normal Main Roads design standards.

Minimum sight distance criteria shall be met for all intersection treatments.

The number of lanes at signalised intersections should only be reduced if it can be demonstrated that Main Roads operational requirements can be met.

6. Drainage

Where drainage for the temporary alignment forms part of the permanent drainage network or will be in place for a duration greater than twelve months, it shall be designed to the same standards as a permanent network.

In other situations for temporary alignments in place for up to twelve months the following standards shall apply to temporary drainage systems:

• Where culverts are provided for drainage they shall be designed for an Average Recurrence Interval (ARI) of two (2) years and in accordance with the Main Roads Guide to the Design of Culverts.
• Spread limits; Retain the widths contained in Table 1.2 of the Guide to the Design of Piped Systems, with the ARI event being 2 years rather than 10 years.
• Underground piped network system being designed for a 1 in 2 year event.
• Check on failure mechanisms and impacts on property damage.

Temporary detention basins shall be designed for a 1 in 10 year event and in accordance with the Main Roads Guide to the Design of Drainage Basins.

Drainage systems may require special features for sediment trapping in accordance with environmental requirements.

Refer to Main Roads website for information relating to Drainage and Waterways.

7. Pavement Marking and Signing

Pavement marking and signing shall meet the required standards for the proposed design speed of the temporary alignment and shall be to current Main Roads standards unless this is achieved by the implementation of a temporary Traffic Management Plan approved by Main Roads.

It may be necessary to place additional signs in the vicinity of the temporary alignment to advise road users of the change in road environment.

If the design speed of the temporary alignment is different from that of the existing alignment, then the speed zoning must be altered to reflect the new operating environment.

8. Roadside Hazards

When designing a temporary alignment for the construction of a bridge or structure, open excavation, sand piles or other work areas then horizontal clearances to the structure shall be checked for road safety barrier warrants.

In designing temporary alignments, the Designer must assess both existing roadside hazards, which may be closer to the edge of the travelled way due to the temporary alignment, as well as any hazards created by the Works.

An example of the latter would be formwork required for bridge piers, deep excavations etc.

For details of warrants for various types of roadside hazards at construction sites refer to the Main Roads Traffic Management for Works on Roads - Code of Practice.

Above ground hazards shall be assessed in accordance with Clause 7.4.4 of the Code of Practice.

Clearance from the temporary alignment to excavations - if this is of sufficient depth and slope to require barrier protection, then the width required to accommodate the barrier as well as the deflection of the system and any support to the barrier shall be provided.

9. Road Safety Barriers

Road Safety Barriers if required, are to be designed in accordance with the MRWA Supplement to Austroads Guide to Road Design - Part 6.

10. Street Lighting

If street lighting is present on the existing alignment, then it shall be replicated on the temporary alignment to the existing standards as a minimum.

Where appropriate, consideration should be given to the provision of street lighting for the temporary alignment, even if it is not present on the existing alignment.

11. Examples of Temporary Alignments

LOCATION	DRAWING NUMBER
Mitchell Freeway / Warwick Road	8531-0012
Kwinana Freeway / Farrington Road	8931-0114
Kwinana Freeway / South Street	8931-0124
Albany Highway / Leige Street	9231-0509
Albany Highway / Hamilton Street	9231-0510
Albany Highway / Mallard Way to George Street	9231-0512
Reid Highway / Wanneroo Road	9231-0746

 

12. References

1. Main Roads Specification 202 – Traffic
   
   
2. Main Roads Traffic Management for Works on Roads - Code of Practice
   
   
3. MRWA Supplement to Austroads Guide to Road Design - Part 6
   
   
4. Drainage and Waterways :
   • Design of Piped Systems
   • Design of CulvertsDesign of
   • Drainage Basins

5. AS 1742.2 Manual of uniform traffic control devices; Part 2: Traffic control devices for general use.
6. AS 1742.3 Manual of uniform traffic control devices; Part 3: Traffic control device for works on roads.
7.  ARRB  Document - Analysis of lane width requirements of Western Australian road trains (2005).