Capacitance WIM Site Selection Criteria

There are two main applications for capacitance WIM: statistical and pre-selection (screening).

Statistical applications are for normal WIM measurement in a traffic stream.

Pre-selection applications usually take place on specially constructed screening lanes.


Road Geometry

For statistical WIM sites within the traffic stream

  • A minimum lane width of 3.0 m with at least 1.0 m hard shoulder is required.
  • Maximum crossfall allowed is 2%.

For pre-selection WIM sites on specially constructed screening lanes

  • Lane width recommended is 3.5 m plus a 1.0 m shoulder each side. Speed restricted to 40 km/h. No rutting must be evident. Maximum cross fall allowed is 1%.
  • Flatness allowed in the sensor area (5 m either side) is defined as: using a 3 m straight edge at any point in any direction a 150 mm diameter disc of 1 mm thickness may not pass under the straight edge.

Pavement Conditions

It is recommended that capacitance pad securing frames be installed into a road wear surface structure of dense grade asphalt or concrete. Open grade (popcorn) asphalt can be used but achieving the above accuracies is much more difficult and cannot be guaranteed.

For statistical WIM sites within the traffic stream

The pavement conditions should be in accordance with ASTM-E1318. For an area 50 m upstream and 25 m downstream a 150 mm diameter disc 3 mm thick shall not pass under a 6 m straight edge.

Flatness allowed in the sensor area (5 m either side) is defined as: using a 3 m straight edge at any point in any direction a 20 mm diameter disc of 2 mm thickness may not pass under the straight edge.

At the sensor area a minimum depth of 100 mm good quality pavement must be available for anchoring the sensor frames. On the approaches a maximum rut of 2 mm depth over the lane width can be accommodated.

For pre-selection WIM sites on specially constructed screening lanes

The pavement should have a maximum International Roughness Index (IRI) of 1 for a minimum of 100 m upstream from the sensor area and 50 m downstream from the sensor area.

In the direct area of sensor installation (5 m either side) the pavement thickness should be at least 100 mm.

 


Further Reading

American Society for Testing Materials, Standard specification for highway weigh-in-motion (WIM) systems with user requirements and test methods, E1318-09, American Society for Testing Materials, West Conshohocken, PA, USA


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