Weigh in Motion
Weigh in Motion

Weigh-in-Motion (WIM) is the term used to describe the various methods  used to measure vehicle axle weights while the vehicle is moving at highway speeds. WIM systems are increasingly used to collect data on bridge loading, which is essential in any calculation to determine bridge safety in service.  There are primarily two categories of WIM systems;


  • Pavement  based systems, which consist of a system of sensors embedded in the road surface

  •  Bridge WIM systems, which consist of sensors underneath the bridge.


ROD-IS has extensive expertise in WIM systems, from procurement advice through to the analysis of the output.  Professor OBrien, Chairman of the ROD-IS Board, has been involved in the development of European best practise in the calibration, validation and testing of WIM systems. He is part of the group that recently submitted a draft CEN (European) standard on WIM, served as President of the International Society of WIM and has worked for nearly 30 years in the field.​


A typical pavement WIM installation consists of inductive loops, with either plate or strip sensors, which measure vehicle speed, detect the length of vehicle and the number of axles. While plate sensors are typically more accurate than strip sensors, they generally take longer to install, resulting in extended periods of road closures.


Bridge WIM is the process of using an existing instrumented bridge or culvert to weigh trucks while they are travelling at full highway speeds. These systems provide equivalent results to the pavement WIM systems. However, as the sensors are installed on the soffit of the bridge, road closure in most cases is not required. Equally, the system is portable and so can be moved from bridge to bridge relatively easily. The system can also provide additional results over and above what the road pavement systems provides and therefore the data is suitable for advanced bridge assessment.