Challenges for direct WIM enforcement in South Africa

Abstract

The South African National Roads Agency Ltd (SANRAL) embarked on a research project to investigate the possibility of direct weigh-in-motion (WIM) enforcement, WIMe, as a possible method to enhance the overload control system in South Africa. The initial research project was concluded in 2023, and an implementation project for testing of the WIMe concept commenced in 2025. The use of high-speed WIM for direct enforcement is not common. It requires the most advanced of WIM systems to meet the accuracy requirements as well as an excellent organisational structure. One of the key challenges of the implementation of WIMe is to achieve legal acceptance of a WIM system of which the guaranteed maximum measurement error on total vehicle mass is typically ±5%. The spread of WIM error on axle units is wider than for total vehicles mass. A further complication is that steering and driving axles are mostly affected by load transfer that causes a systematic measurement error on these groups and make them less suitable for prosecution. The typical metrological requirement for full repeatability and traceability of reference measurements to certified mass pieces means that a selection of pre-weighed test vehicles is mostly used for calibration or verification of WIMs. These vehicles and their load dynamics are not necessarily representative of the heavy vehicle population, and the limited test samples that can be achieved using this process provide low statistical confidence that the tested WIM indeed operates within the Maximum Permissible Error (MPE) limits. While test vehicles are still used for the purpose of System Certification, research showed that a statistical interpretation of WIM measurement error, sample size and confidence intervals is preferable in the Data Quality Management process to ensure compliance with MPE. The last big hurdle in the implementation of WIMe is the anticipated low strike rate. To eliminate the possibility of unfair prosecution, the prosecution grace needs to be increased by a margin equal to MPE. Few vehicles In South Africa are overloaded so severely (i.e. by a margin exceeding MPE) that they can be successfully prosecuted with WIM. The proportion of vehicles that might produce measurements that are suitable for prosecution is further reduced by errors in vehicle identification using licence plates, and exceeding of on-site quality parameters such as changes in speed or poor lane discipline that result in automatic discarding of the measurement. For this reason, WIMe in South Africa is not considered as a standalone solution but should be integrated with self-regulation and a weighbridge bypass system.