Different types of Weighing-In-Motion.
There are several different methods to measure the gross vehicle weight, axle group loads and axle loads of a vehicle:
1. Static weighing, even though by definition not weighing in motion, the static weighing of road vehicles has an important relationship to WIM. The static weighing results are, in most cases, used as the reference values when testing and calibrating a WIM system. Static weighing systems are in many countries around the world legally approved for direct enforcement or trade applications.
2. Low speed WIM (LS-WIM), where the weighing takes place in a dedicated controlled area, mostly outside the main traffic lane, on a flat and smooth platform (generally made of concrete) that is longer than 30 m. In the weighing area the velocity and transverse movement of the passing vehicles are controlled in order to eliminate the dynamic effects of the vehicle.
3. High speed WIM (HS-WIM); here the weighing is carried out in the open traffic lanes at normal speed and under free flow conditions. The measurements are affected by the vehicle dynamics that depend on a combination of the geometry of the road, the driving behaviour of the driver and the reaction of the vehicle suspension on the influences mentioned previously. In general, good high speed WIM systems on smooth roads have an inaccuracy of between ±5 to ± 10% for GVW measurements. More and less accurate systems are also available.
4. Bridge WIM (B-WIM) is a special type of dynamic weighing system where the sensors are attached to the soffit (bottom side of beams or deck) of a bridge, viaduct or culvert. The sensors typically measure strains due to the bending of the bridge caused by the passing vehicles. In addition to the same vehicle information as provided by the pavement WIM systems, the B-WIM systems can also collect valuable data about bridge behaviour that can be used for safety assessment of the bridge.
5. Dynamic On-Board WIM (OBW) systems are fitted to vehicles, rather than to the infrastructure. An OBW system will constantly measure the GVW, axle and wheel loads of the vehicle while it is moving. The typical measurement inaccuracy is between ±1 and ±3 %, depending on the sensing technology. The measured weight data of the moving vehicle may be combined with location (GPS) data and stored during the entire travel. Such combined OBW systems can be used to manage heavy vehicle operation and to monitor compliance to access to certain parts of the road network.