n Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese - Weigh-in-motion : years of South African experience : technical paper

Volume 49, Issue 1
  • ISSN : 1021-2019



Weigh-in-motion (WIM) applications in South Africa - a country with 50 years of weighin-motion history - include routine traffic load monitoring for purposes of pavementdesign and maintenance, selection of heavy vehicles for accurate weighing and possibleprosecution, guidance of traffic police to roads with a serious overloading problem,periodic auditing of operation of static weighbridges, and the estimation of pavementdamage caused to roads by overloaded heavy vehicles.

WIM is a scale that measures dynamic forces applied to road surface by passingwheels. Yet, most users expect WIM to imitate static weights of vehicle axles. Thisexpectation causes a number of problems related to accuracy, calibration andinterpretation of measurements. Because of economic reasons half-lane high-speed WIMsare frequently used in this country. Pavement deterioration, poor installation and sitemaintenance dominate the list of reasons when these do not perform well.
Several measures have been devised and implemented to alleviate the difficultiesregarding WIM accuracy. Extensive WIM data checks and validations are undertakenmonthly. A software correction method has been proposed to suppress systematic andrandom WIM errors. Continuous monitoring of the so-called truck-tractor tonnage trendshas been implemented and used as a diagnostic tool. Recently, the responsibility forthe installation and maintenance of WIM sites has often been transferred from the userto specialised and highly competent WIM service providers - their customers pay forgood information rather than WIM equipment, which they do not know how to operateand service.
Notwithstanding all the above, WIM users should accept that WIM measurementshave limited accuracy and may occasionally be burdened by considerable errors.Although being ideal to establish the order of magnitude and facilitate comparison ofloads applied, the WIM measurements are not suitable for litigation purposes.
In the future, changes in the Road Traffic Act allowing dynamic vehicle weighing forprosecution purposes could initiate further WIM proliferation and development. Also, WIMwould play a prominent role should comprehensive road pricing become a reality.

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