Accuracy in terms of WIM refers to the closeness between a quantity measured or estimated by a WIM system and a known, accepted value.
It is best practice to decide the necessary accuracy needed before deciding the type of WIM to purchase. The ASTM establishes functional requirements for WIM accuracy.
Austroads Guide to Traffic Management Part 3: Traffic Studies and Analysis provides broad guidelines on accuracy requirements for different WIM applications (as detailed in the table below).
The quality of the data obtained depends on the survey method selected and the amount of quality control performed. More precise survey techniques with higher quality control generally consume more survey resources. A trade-off must be made on the basis of time, money and people so that a feasible survey method and sample size can be found for the successful completion of a survey.
Table: WIM data accuracy specification
|WIM applications||Maximum GVM error||Confidence level and comments|
|Economic analysis, transport studies, vehicle classification||±20%||GVM for 95% of vehicles|
|Road and bridge management, overload warning, road safety||±15%|
Source: Austroads 2013
A summary of WIM systems and products is provided in the table below. These technologies include load cell, bending plate, piezoelectric cable and strain gauge. Of great importance is the level of accuracy in WIM measurements. Vendors claim an accuracy of ±10 -15%. Note that the maximum error for enforcement compliance should be set at ±5% for GVM for 95% of vehicles weighed.
Table: Main WIM systems used in Australia and New Zealand
|Feature||Strain gauge||Bending plate||Piezoelectric cable||Capacitance pad|
|WIM system name||ARRB Culway||PAT DAW 100||ARRB Express-weigh
|Mikros Systems capacitance HSWIM system|
|Installation mode||Semi-permanent within pavement culvert installed||Semi-permanent flush mounted||Permanent flush mounted||Permanent flush mounted||Permanent flush mounted||Semi-permanent flush mounted|
|Max no. of lanes||1-4||4||2||8||8||8|
|Measurement of axle load up to (tonnes)||50||20||–||20||20||20|
|Ambient temperature (C)||-10 to +70||-40 to +75||-50 to +80||-20 to +65||-20 to +65||-20 to +65|
|Sensor life span (years)||10||15||–||6||6||20|
(±10% at 95%)
(±10% at 95%)
(±10 at 95%)
(±10 at 95%)
(±10 at 95%)
Not better than ± 300 kg
|ASTM WIM type||Type I||Type I||Type II||Type I||Type I||Type I|
* GVM (±10% at 95%) means that the accuracy of measuring GVM is ±10% for 95% of vehicles weighed. For Culway, a higher accuracy of ±7% is possible.
Source: Adapted from Austroads 2013.
As with all component and modular based technologies, WIM systems have a life expectancy corresponding to the life span of each of the components. It is no coincidence that the most important and readily damaged component of a WIM system is the mass sensor. For this reason, when determining overall life cycle costs, the mass sensor is the component used to quantify the ongoing costs of the WIM system.
Little quantitative or field test information exists about the life span or long-term performance of different mass sensors. According to vendors and suppliers, the life span of weigh sensors ranges from three to 12 years (Austroads 2000).
There is no doubt that there is a degree of uncertainty regarding any equipment or system left unattended on a road, undergoing a multitude of vehicle axle passes.
The potential for vandalism, in addition to the volume of traffic and, more specifically, the volume of heavy vehicles, must be taken into account when considering the life span of mass sensors.
Austroads 2013, Guide to Traffic Management Part 3: Traffic Studies and Analysis, Appendix G, AGTM03-13, Austroads, Sydney, NSW.
Austroads 2000, Weigh-in-motion technology, AP–R168-00, Austroads, Sydney, NSW.