Weigh-in-motion screening explained

Weigh-in-motion (WIM) systems estimate axle and vehicle weights while vehicles travel at normal speed. Combined with cameras and software, they form a "virtual weighbridge".

1. The components

• **Sensors** embedded in the road surface measure the force of each axle as it passes.
• **Loops or detectors** measure speed and axle spacing.
• **Cameras** capture the vehicle and its number plate.
• **A roadside processor** combines the measurements into a vehicle record.
• **Software** compares the record with legal limits and sends alerts.

2. What affects accuracy

1. Road surface smoothness before and after the sensors — bumps make vehicles bounce.
2. Correct installation and regular calibration.
3. Vehicle speed and behaviour, such as braking or lane changes over the sensors.
4. Temperature effects on some sensor types.

3. Screening, not final weighing

High-speed WIM is typically used to screen traffic and select vehicles for static weighing, rather than as the measurement for a penalty. Enforcement uses the method the authority's rules require.

4. Site selection

Choose a straight, level section with a good pavement, power and connectivity, and space for a safe pull-in to the static weighbridge downstream.

See virtual weighbridges and axle load systems.