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:
|3||Determine if the pavement is adequate or if the pavement needs to be replaced.|
|4||Evaluate the site based on the following qualities:
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
|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|
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