WIM systems

A whole WIM system includes:

(1) a set of weighing sensors, either mounted in the pavement (road sensors), or attached to a bridge part (B-WIM). For on-board WIM the sensors are installed on the vehicle. Additional sensors (e.g. induction loops) are mostly used to measure vehicle speed, length, wheel transverse location, etc.
Road sensors are either scales or strips/bars mounted inside of the pavement, and sometime mats attached to the pavement surface. Transducers, extensometers and strain gauges are used for B-WIM. Accelerometers, displacement sensors, strain gauges, etc. are used on-board.

(2) a road side unit contains all the electronics incl. data acquisition tool, A/D converter, weight calculation device and the related software and (self-)calibration tool, and the storage and communication devices. It may also include some display device and energy supply or storage unit (e.g. solar cells, battery).

(3) other measuring tools: depending on the WIM technology and application, various additional sensors may be added and linked to a WIM system, such as temperature sensors to compensate sensor response variations, or cameras and AVI for enforcement…

system1, system2

WIM output

Detailed individual data (per vehicle):
  • Unique identification (registration) number
  • Date and time stamp (yy-mm-dd + hh:mn:ss:cc)
  • Location : road, direction, traffic lane
  • Type of vehicle (truck, van, car, 2 power wheel, etc., and class according to a given classification)
  • Speed
  • Total vehicle length and wheel base
  • Number of axles
  • Gross vehicle weight and axle loads
  • Distances between axles
And in some cases
  • Weights per sensor (if more than one sensor per lane)
  • Vehicle width or wheel spacing on an axle
  • Wheel lateral location
  • Calibration coefficient (of the WIM sensors)
  • License plate number
  • Error code (to validate or eliminate measurements)
Aggregated data (per time period):
These data consist of part or all of the individual measurements summed or averaged over a given period of time (e.g. 6 mn, 1 hr, 1 day, 1 week, 1 month, 1 yr).
Among others:
  • Traffic classification (% per types of vehicles over a given time period)
  • Classified flow: mean number of vehicles or axles of a given type per time unit
  • Histograms of any of the detailed data (type/class of vehicles, speed, vehicle length or axle spacing, GVW or axle load, etc.)
  • Means, standard deviations, fractiles of any of the detailed data
Some WIM systems provide only aggregated data, which limits the storage capacity. The aggregated data may be calculated afterwards.

Short time aggregated data (e.g. 6 mn, 1 hr) are used for real time traffic information and management. Long time aggregated data (1 week to 1 yr) are used for statistics, traffic monitoring and road operation or maintenance.