Weigh-in-Motion (WIM) Systems

Weigh-in-motion (WIM) devices are designed to capture and record truck axle weights and gross vehicle weights as they drive over a sensor. Unlike older static weigh stations, current WIM systems do not require the subject trucks to stop making them much more efficient. Gross vehicle and axle weight monitoring is useful in an array of applications including:

• Pavement design, monitoring, and research
• Bridge design, monitoring, and research
• Size and weight enforcement
• Legislation and regulation
• Administration and planning

Strengths of WIM

1. Processing Rate.  Trucks can be weighed as they as they travel at highway speeds, resulting in a significantly greater number of counted vehicles in a short period of time compared to static weight stations.
2. Safety.  The minimization of static weighing will significantly decrease vehicle accumulation at highway lanes leading to weight stations.
3. Continuous data processing.  WIM can be performed continuously rather than static weighing, which uses traffic streams samples.  This can eliminate any inherent data bias in static weighing.
4. Increased coverage and lower cost.  More sites may be monitored with WIM at the same cost.
5. Minimized scale avoidance.  WIM can monitor truck traffic without alerting truck drivers.  This results in more truthful data as overweight trucks are less likely to avoid weighing stations.
6. Dynamic loading data.  Unlike static weight stations, WIM can record dynamic axle load information, which can be significantly greater than static load information.

Shortcomings of WIM

1. Less accurate.  WIM systems are less accurate than static scales. According to the National Bureau of Standards, wheel load scales are required to have an accuracy of ±1% when tested for certification and must be maintained thereafter at ±2%.  The best accuracy obtained with the most expensive commonly used WIM devices is 6% of actual vehicle weights for 95% of measured trucks.
2. Reduced information.  Truck information that is easily collected at static weight stations such as fuel type, state of registry, year model, loaded or unloaded status, origin, and destination cannot be obtained with typical WIM systems.
3. Susceptibility to damage from electromagnetic transients.  WIM systems are sensitive to electromagnetic disturbances caused mostly by lightning strikes in the vicinity of the equipment.

Contemporary WIM Technology

The most widely accepted and utilized WIM devices in North America are:

• Piezoelectric Sensor.  The most common WIM device.  The sensor is embedded in the pavement and produces a charge that is equivalent to the deformation induced by the tire loads on the pavement’s surface.  It is common to install two inductive loops and two piezoelectric sensors in each monitored lane.  A properly installed and calibrated Piezoelectric WIM system can provide gross vehicle weights that are within 15% of the actual vehicle weight for 95% of the measured trucks.
• Bending Plate.  The bending scale consists of two steel platforms that are 0.6 x 2 m (2 ft. x 6 ft.), adjacently placed to cover a 3.65 m (12 ft.) lane.  The plates are instrumented with strain gages, which measures tire load induced plate strains. The measured strains are then analyzed to determine the tire load.  A properly installed and calibrated bending plate WIM system can provide gross vehicle weights that are within 10% of the actual vehicle weight for 95% of the measured trucks.
• Single Load Cell.  This device consists of two 3 x 3 m (6 ft. x 6 ft.) platforms placed adjacently to cover the 3.65 m (12 ft.) monitored lane.  A single hydraulic load cell is installed at the center of each platform to measure the tire load induced forces that are then transformed into tire loads.  A properly installed and calibrated single load cell WIM system can provide gross vehicle weights that are within 6% of the actual vehicle weight for 95% of the measured trucks.