General Principles

According to the ASTM-E1318, WIM users must provide and maintain an adequate operating environment for the system to perform properly.

Factors that influence the performance of a WIM system are the road geometry and pavement condition of the roadway and the overall site location.

These factors influence the dynamic behaviour of the vehicle and thus influence the accuracy of the estimate of the static weight made by the WIM system.

WIM site selection principles are summarised in the table below.

Table: WIM site selection checklist
WIM site selection principles
1 Select the site based on the required site design life and accuracy performance level.
2 Evaluate the road geometric conditions based on the following qualities:
  • determine if horizontal curvature is acceptable
  • determine if roadway grade is acceptable
  • determine if cross slope is acceptable
  • determine if the lane is wide enough and marked properly.
3 Determine if the pavement is adequate or if the pavement needs to be replaced.
4 Evaluate the site based on the following qualities:
  • determine availability of the power and phone
  • determine if there is adequate location for the controller cabinet
  • determine if the site provides adequate drainage
  • determine traffic conditions at the site.

Source: Adapted from McCall and Vodrazka 1997

COST 323 has identified desirable pavement characteristics for virtually all types and classes of WIM installations (see table below).

These European pavement specifications can be used as a guide in Australia and New Zealand in the absence of similar specifications being set for Australian and New Zealand pavements. However the accuracy of WIM sites in relation to their pavements should always be verified. Further research is required to produce a similar table specifically for Australian and New Zealand pavements.

Surface sensors need to be well supported by reasonably stiff pavements. Culway systems do not require such stiff pavements and hence some relaxation of the deflection values may be possible.

Table: European desirable pavement characteristics for WIM sites
Characteristic Pavement type Excellent Good Acceptable
Longitudinal slope All < 1% < 2% < 2%
Lateral slope All < 3% < 3% < 3%
Radius of curvature All > 1 000 m > 1 000 m > 1 000 m
Rutting All ≤ 4 mm ≤ 7 mm ≤ 10 mm
Surface roughness All International roughness index  (IRI) ≤ 1.3 m/km IRI 1.3 – 2.6 m/km IRI 2.6 – 4 m/km
Deflection (dynamic) FWD with 5 t test load Semi-rigid Deflection ≤ 0.10 mm,  left/right difference ± 0.02 mm Deflection ≤ 0.15 mm, left/right difference ± 0.04 mm Deflection ≤ 0.20 mm, left/right difference ± 0.07 mm
Deflection (dynamic) FWD with 5 t test load All bitumen Deflection ≤ 0.15 mm, left/right difference ± 0.03 mm Deflection ≤ 0.25 mm, left/right difference ± 0.06 mm Deflection ≤ 0.35 mm, left/right difference ± 0.09 mm
Deflection (dynamic) FWD with 5 t test load Flexible Deflection ≤ 0.20 mm, left/right difference ± 0.05 mm Deflection ≤ 0.35 mm, left/right difference ± 0.07 mm Deflection ≤ 0.55 mm, left/right difference ± 0.1 mm

Source: FEHRL-1999

 


Further reading

McCall, B and Vodrazka, WC 1997, State’s successful practices weigh-in-motion handbook, Federal Highway Administration, Washington, DC.

Forum of European National Highway Research Laboratories, 1999, COST 323: Weigh-in-motion of road vehicles, Final Report, Appendix 1 European WIM Specification, Laboratoire Central des Ponts et Chaussees (LCPC), Paris, France


 

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