Weigh-in-motion

Weigh-in-motion (WIM) systems measure the weight of trucks while they are moving in order to identify which vehicles are overweight, so that weight overload penalties can be enforced. The underweight vehicles can bypass the weigh station, while overweight vehicles are directed to another location for additional weighing.

Key characteristics

Setting/Location

Corridor, Urban, Suburban, Rural

Technology

Collaboration

WSDOT regions

Eastern, Northwest, Olympic, South Central, Southwest

Strategy description

Weigh-in-motion (WIM) systems measure the overall weight of vehicles, the per-axle weight, axle-spacing, and vehicle speed so agencies can monitor and enforce overweight vehicles on the highways. Using sensors embedded in the pavement, WIM systems collect data for each vehicle that passes over the sensors and allows underweight vehicles to bypass the weigh station. By measuring vehicles while they are moving, WIM systems also help to reduce congestion and keep goods moving.  

WIM systems data includes:

  • Single or dual tire loads
  • Per-axle load
  • Gross vehicle weights

WIM systems also collect other vehicle and traffic data that can be useful for planning, pavement design, freight studies, enforcement, and truck weight and size studies.

Traffic and vehicle data includes:

  • The amount of vehicles using the roadway or traffic volumes
  • Axle spacing
  • Vehicle size
  • Vehicle speed

Several sensor types exist to weigh and classify the vehicles, and each has different advantages and disadvantages. See the Weigh-in-motion pocket guide for further information (1).

When to use this strategy

Weigh-in-motion makes sense for agencies looking to streamline the administrative requirements for monitoring and enforcing overweight vehicles. The data collected from WIM systems can also be used for planning, freight studies, bridge and pavement design, and highway performance measurement.

Strategy benefits:

  • Weighing vehicles at highway speeds saves time because drivers can bypass stopping at permanent or portable weigh stations
  • The data collected can be used for planning future highway improvements

What you need in order to implement

Planning needs:

  • Document the necessary user requirements (data type, storage, days to collect)
  • Evaluate and select WIM sites and detector technology
  • Site design for the WIM infrastructure and to achieve needed pavement smoothness

Coordination needs:

  • Coordination with the trucking industry to provide WIM information and transponders to communicate to some WIM systems

Equipment needs:

Several WIM technologies can be used to detect and measure vehicle weight.

  • WIM detectors embedded in the pavement.
  • Transponders for trucks to communicate with WIM stations
  • A roadside cabinet, poles, junction boxes, conduit for power and communications
  • A WIM roadside controller and software to process detector outputs, analyze and temporarily store the collected data
  • A central WIM software and user interface to view the processed data from the WIM roadside controller

Maintenance needs:

  • Routine maintenance and repairs for the WIM system including detectors, controller, and communications equipment
  • Annual calibration of the WIM detectors
  • Software upgrades
  • Costs to improve pavement smoothness

Agency resources needs:

  • WIM data processing and reporting, which could include an annual software use fee for access to the analytics.
  • Data quality assurance/quality control
  • Personnel at the weigh station to manage when a truck needs further inspection

Learn more about this strategy

Works cited:

(1) Federal Highway Administration (FHWA) Office of Highway Policy Information, Weigh-in-motion pocket guide
https://www.fhwa.dot.gov/policyinformation/knowledgecenter/wim_guide/

About key characteristics

Location notes:

Applies for highways in urban, suburban, and rural areas because weigh-in-motion is often a statewide deployment to reduce congestion and weigh stations and keep goods moving.

Cost notes:

Requires a statewide system to detect trucks, screen and verify the weight, and detector violators.

Technology notes:

Requires detection systems that can measure truck speed, the distance between axles, and calculate total vehicle weight. Requires notification systems that can notify drivers of potential violations.

Collaboration notes:

Requires collaboration with the trucking industry.

Conditions this strategy addresses