Technical Overview

System Manufacturer:

Caltrans weigh-in-motion (WIM) system components are provided by either PAT or IRD (International Road Dynamics, Both companies recently consolidated under the IRD brand name, but PAT components are still available from IRD. There are only a handful of other WIM component manufacturers and WIM system integrators worldwide, including Massload, Mettler Toledo, and some European companies. Although Caltrans WIM installation projects are subject to a public bidding process, IRD/PAT is the only manufacturer bidding on new installations to the Caltrans specification.

Technical Specs/How It Operates:

All Caltrans WIM system sensors are either bending plates on frames embedded in concrete or piezo sensors epoxied into the pavement. Another alternative sensor is the load-cell. (Caltrans typically does not use load cells due to the long installation times that require extended traffic lane closures.) Inductive loops are placed before and after the WIM sensor array. Loops measure vehicle speed and overall length. A smooth pavement surface is the key to accurate weight readings. This helps dissipate any stored energy in the form of vertical displacement resident in the vehicle suspension while driving over the WIM sensors. Smooth pavement and proper calibration ensures quality and consistency in weight data. Caltrans specifies 200 feet of approach and 100 feet of departure concrete ground to a +/- 3mm tolerance in 12 feet. Before final acceptance, a new WIM system must be calibrated to +/- 5% accuracy with a test vehicle of known static weight driven at various highway speeds over the WIM instrumentation. Routine maintenance and calibration are extremely important, and especially so if a WIM system is used in critical weight and balance calculations, such as for loading military hardware on aircraft or ships.

WIM Highway System Capabilities:

Caltrans WIM systems are configured to calculate GVW (gross vehicle weight), individual axle weights, weight violations, vehicle speed, overall length, axle spacing, and vehicle classification (such as passenger vehicle, bus, or truck-tractor/semitrailer).

Office Automation & Personnel:

WIM field systems gather and store data 24/7/365 automatically in roadside cabinets. Data collection is via telemetry using an office file server and PC. WIM data downloads are both automated and manually performed according to a preset schedule. Data collected must be screened and sorted on a historical and operational basis to validate its quality before archiving or distributing. Data that exceeds control limits needs further investigation into possible field site problems. Routine maintenance and calibration of WIM systems is vital for consistent and accurate archive data. There is no substitute for experienced personnel with adequate equipment and diligence to maintain the systems and access trouble situations early. Electronic sensors can help expedite a proper and timely response to equipment or system health issues by sending some form of notification.


Caltrans WIM systems are not portable. Caltrans experience with portable systems reveals shortcomings concerning accuracy and service life due to the extraordinarily high and heavy truck volumes on California highways.

Contact Information

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