Explanation of Austroads Traffic Multipliers

The references below are to the Austroads Guide to Pavement Technology – Part 2: Pavement Structural Design (AGPT02-12), February 2012.
For pavement design purposes, the following (heavy vehicle) axle group types are identified:

•    single axle with single tyres (SAST)
•    single axle with dual tyres (SADT)
•    tandem axle with single tyres (TAST)
•    tandem axle with dual tyres (TADT)
•    triaxle with dual tyres (TRDT)
•    quad-axle with dual tyres (QADT).

Design traffic loading is commonly described in terms of the number of Equivalent Standard Axles (ESA). Design traffic so described does not, by itself, contain enough information for the design of pavements containing one or more bound layers.

The design traffic loading for flexible pavement design is – for each relevant damage type – the total number of Standard Axle Repetitions (SAR) during the design period which cause the same damage as the cumulative traffic.

In general, 4 parameters quantify the overall traffic load distribution. These are:

  • DESA – the Design number of Equivalent Standard Axles
  • Traffic multiplier for Asphalt (SAR5/ESA)
  • Traffic multiplier for Cemented Material (SAR12/ESA)
  • Traffic multiplier for Subgrade (SAR7/ESA)

Full details are in Chapter 7 of AGPT02-12.

The design traffic calculated in terms of ESA of loading is converted to SAR of loading using the average SAR/ESA value for each damage type.  These factors are determined from the Traffic Load Distributions (TLDs) that characterise actual or presumptive traffic data.

Damage Type Damage Exponent Traffic Multiplier
for
Sample Traffic Load Distribution
(Appendix F)
Traffic Multiplier
for
Different Traffic Load Distribution

Asphalt

5.0

SAR5/ESA = 1.1

?

Cemented Material

12.0

SAR12/ESA = 12.0

?

Subgrade

7.0

SAR7/ESA = 1.6

?

The key point is that the Traffic Multipliers depend on the assumed Traffic Load Distribution.  APPENDIX D (CHARACTERISTICS OF TRAFFIC AT SELECTED WIM SITES) has many examples.

Here is the left hand side of the first part of APPENDIX D:

image001

Here is the right hand side of the first part of APPENDIX D:image002

Details of the procedure to estimate ESA/HVAG and SAR/ESA from a selected TLD are given in Appendix H.

This screendump shows where DESA and the Traffic Multipliers are entered in CIRCLY 6.0:

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