Technical Overview

Technical Specs/How It Operates

All mainline Caltrans WIM system sensors are bending plates on frames embedded in concrete. 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.