Weigh-in-motion (WIM) captures and records a vehicle's weight, speed, length, class and axle count with per-axle load while it is moving. Compared with a static weighbridge its value is not better accuracy but the fact that the vehicle never has to stop — no deceleration, no queueing, no disruption to traffic flow.
Deployment scenario
Ahead of toll plazas, on bridge approaches and in over-limit inspection lanes, piezo cable is embedded in transverse slots (several in parallel to cover lane width), alongside inductive loops or ANPR cameras. As a vehicle passes, each axle presses the cable in turn, generating charge pulses whose amplitude and timing give per-axle load and axle spacing — hence gross weight, axle count, speed and class.
In China it is used mainly for screening overloaded trucks on expressways, bridge overload warning and freight statistics; in Europe and North America there is a larger installed base.
Achievable performance
The coaxial cable is self-shielding and works reliably in electrically noisy gantry environments; its rugged structure withstands repeated truck axles. Practice generally uses a two-stage strategy: WIM screens suspect vehicles, then a low-speed accurate system determines overload for enforcement — preserving throughput while avoiding disputes over a single dynamic measurement.
Honestly stated: WIM accuracy depends heavily on pavement flatness, speed distribution and suspension dynamics, and absolute accuracy is lower than a static weighbridge. The industry therefore treats it as a screening tool rather than a direct enforcement basis.
Comparison of technical routes
| Aspect | Piezo cable WIM | Static weighbridge | Bending-plate / load-cell WIM |
|---|---|---|---|
| Vehicle must stop | No — passes at normal speed | Yes | No |
| Traffic impact | None | Slowing and queueing | None |
| Construction | Slots cut and refilled quickly | Large foundation works | Large pre-embedded frames |
| Environment | Coaxial self-shielded, EMI resistant | — | Sensitive to temperature and moisture |
| Accuracy | Medium — screening and preselection | High (≈ ±0.1%) | Medium-high |
