Weigh in Motion (WiM) Technology and Design Traffic loading Variables

Weigh in Motion, commonly referred to as a WiM stations are reliable instruments used to determine the traffic load distribution and design traffic loading characteristics of moving heavy vehicles on a particular road section.

The Weigh in Motion (WiM) technology has been developed to develop a greater understanding of in service axle weights of heavy vehicles and has historically been done so using static measurement – requiring the vehicle to be stationary.

Australia currently has over 170 WiM stations proportioned across the states and territories and has trialled a number of types, encompassing strain gauges, load cells, compactive strips and bending plates to determine dynamic axle loads and configurations for input into design traffic loading calculations for mechanistic pavement design purposes.

The use of Weigh in Motion (WiM) stations has developed a reliable method to calculate:

1. a) SAR Values: Standard Axle Repetition factors used to characterise the performance of different pavement materials under axle configurations and loads. These are calculated using the ratio of measured axle configuration to the equivalent standard axle configuration raised to a nominal integer (Asphalt = 5, Subgrade = 7, Cemented Material = 12)
2. b) NHVAG: The average number of heavy vehicle axle groups per vehicle, including:

SAST – Single Axle Single Tyre

SADT – Single Axle Dual Tyre

TAST – Tandem Axle Single Tyre

TADT – Tandem Axle Dual Tyre

QADT – Quad Axle Dual Tyre

The NHVAG can be calculated by proportioning the sum of the %SAST and %TAST over the remaining heavy vehicle axle groups.

1. c) ESA/HV: The loading variable used to determine the number of ‘equivalent standard axles’ per heavy vehicle. This is calculated using the ratio of measured axle configuration to the equivalent standard axle configuration raised to the fourth integer.

WiM technology is considered one of the most important tools (in addition to traffic count information) to accurately determine design traffic loading characteristics. These stations are typically located on high priority road sections and are often integrated into box culverts (for single and multi-lane configurations) to reduce the impact during maintenance.

Information and historical summaries of each WiM station within the Austroads database is continually updated to reflect the relevant characteristics and is displayed in Austroads Guide to Pavement Technology: Part 2.

For more information on Global Road Technology or Weigh in Motion (WiM) Technology and Design Traffic loading Variables please contact GRT