Office of Operations Freight Management and Operations

3.0 WIM Technology Overview

A typical WIM system consists of the following hardware:

  • A scale or set of sensors on the mainline or installed on a ramp that records the impact of the passing vehicle;
  • A roadside cabinet containing a processor that converts the downward force readings of the vehicle on the scale into data estimating the vehicle’s gross weight and axle weights; and
  • A communication system that transmits the weight data to the computers of enforcement personnel or to an enterprise-level WIM database management system.  Typically, dial-up communication is used to transmit data from WIM systems to users, but in order to support enforcement functions, high-speed wireless or digital subscriber line (DSL) technology is necessary to transmit real-time data and/or vehicle images. 

The accuracy of the gross vehicle weight or axle weight estimate as computed by the roadside processor can be affected by the WIM scale technology in use.  When installed on the mainline, more expensive WIM systems have less variance in their readings and may better compensate for filtering external factors that affect vehicle weight calculations.  Conversely, less expensive WIM scales or sensors may not be as accurate at highway speed.  When installed on a ramp, however, the quality of weight data uniformly improves and the difference in accuracy between less and more expensive WIM devices is not as great as under high speed conditions.

An example of a low-cost WIM device that has been deployed in the United States, especially for traffic monitoring purposes, is the piezoelectric sensor.  More recently, several states have begun including quartz piezo WIM systems in their programs because of their usefulness in truck enforcement activities.  Quartz piezo WIM devices, like electric piezos, are relatively inexpensive and not difficult to install.  More expensive systems like load cell and bending plate offer more accurate weight estimates but are more expensive to procure and are significantly more intrusive to the pavement structure.

3.1 High-Speed WIM for Fixed Weigh Station Operations

WIM systems, as indicated above, are commonly used to screen vehicles on the mainline for weight compliance as they approach a weigh station.  The WIM scale or sensor embedded in the pavement automatically weighs vehicles and provides an estimate of the vehicle’s weight to station personnel for sorting purposes.  Typically, weight screening is based on estimates from WIM sensors or scales on the mainline that are compared to a weight pass/fail threshold set to a percent of the legal weight.  Thresholds are adjustable by station personnel.  Trucks that exceed the threshold are directed into the weigh station to be weighed on more accurate static scales used to write citations.  High WIM device accuracy results in low probability of incorrectly sorting a truck.

The data generated by a commercial vehicle passing over a WIM site includes information on number of axles and axle weights.  In some states, a digital photograph of the weighed vehicle is automatically recorded and transmitted to enforcement personnel, before the truck arrives at the static scale.  Many WIM sites in Europe are equipped with WIM-VID (video) digital imaging systems, which allow for direct enforcement of commercial vehicles by producing real-time vehicle images associated with vehicle data.  However, in the United States, digital cameras are not standard on WIM systems.

When used as part of an electronic screening or bypass system such as PrePass or NORPASS, WIM provides real-time weight verification concurrent with safety and credentials verification for bypass eligibility.  Vehicles cleared for bypass are not generally directed to pull into the weigh station.  DSRC readers and WIM sensors are located far enough ahead of the station ramp to allow the screening system time to complete the necessary processing of the vehicle as it approaches.  The driver is signaled to pull in or bypass via their transponder before it reaches the ramp. 

Mainline weight screening produces a number of benefits.  Weigh-in-motion significantly increases the capacity of weigh stations.  In the absence of mainline WIM, queues may form and cause closure of weigh stations; as a result, compliance checks are not performed on the bypassed vehicles.  WIM also reduces congestion within the fixed weigh station facility, focuses enforcement on high-risk operators, and provides time savings for safe and legal carriers, supporting more efficient movement of freight.  For example, inspectors at Washington State weigh stations that are instrumented for CVISN electronic screening, which includes weigh-in-motion at mainline speed, reported considerable improvements in traffic flow and a reduction in congestion since electronic and mainline weight screening was deployed. 

The use of mainline weight screening using WIM is increasing in the United States.  There are approximately 550 WIM sites in operation nationwide from which the Federal Highway Administration receives weight-based highway data.  Among PrePass sites alone, 52 percent have WIM.  However, not all WIM sites nationwide are currently used for enforcement purposes.

Two of the states participating in this study offer value as examples.  California supports mainline WIM for screening at 34 weigh station locations, all of which are instrumented for PrePass, and is currently constructing two new weigh stations where mainline WIM will be deployed.  Washington State includes WIM technology at 14 of its weigh stations.  These weigh stations weigh over 80 percent of the State’s commercial vehicles.  The mainline WIM system is linked to a camera that takes a picture of a vehicle as it crosses the WIM sensors; the image is recorded along with the vehicle’s weight data.  The integrated WIM and camera system records all vehicles regardless of whether they are equipped with a transponder (for participation in Washington’s electronic screening system, NORPASS) or not.  A changeable message sign located before the exit ramp directs transpondered trucks into the weigh station if their weights exceed established thresholds.  All trucks without transponders must enter an open weigh station.

Washington’s Commercial-vehicle Roadside Information Sorting System (CRISS) automatically identifies transpondered vehicles and queries the Commercial Vehicle Information Exchange Window (CVIEW) for up-to-date safety and credentials information about the carrier and vehicle.  Generally, if the vehicle’s weight and other information meet the State’s defined screening criteria, the driver is signaled via the transponder to bypass the weigh station.  Non-transponder equipped vehicles are handled differently because they are not automatically identified as they approach the weigh station, and all of them must exit the main highway to be weighed on the static scale.  The CRISS software displays a picture and weight information for each vehicle as it approaches the weigh station.  An algorithm determines if there are any potential axle weight violations, which are highlighted on the computer screen at the scale house.  The system helps inspectors better identify vehicles with problems.  CRISS was the first system in the U.S. to associate digital photos of trucks with their vehicle data on a weigh station computer to aid in visual identification and enforcement.

3.2 Low-Speed WIM for Ramp Sorting

WIM scales are also installed on weigh station ramps to weigh and sort vehicles at low speeds.  Vehicles that have left the main highway move to the approach ramp where they are weighed by a ramp, or sorter, WIM.  The ramp WIM sorts the arriving trucks based on a weight threshold set by weigh station personnel.  Axle spacing, vehicle height, and vehicle classification also may be determined.  Vehicles that do not exceed the threshold are signaled by a message sign to move to the bypass lane for return to the main highway.  Remaining vehicles are directed to the static scale for weighing.  Compared to mainline WIM systems, ramp WIM systems weigh vehicles moving at lower speeds and provide a more accurate measure of a vehicle’s weight.

The deployment of ramp WIM, like mainline WIM, has increased recently.  Most of the states participating in this study have installed or plan to install ramp WIM systems at their weigh stations.  Kentucky, Michigan, Mississippi, and Indiana utilize ramp WIM at five or more weigh stations.  Ramp sorting combined with a bypass lane can process more vehicles than could be supported by static weighing alone.  Several states reported that additional sorter WIM systems are desired in order to increase throughput.

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