The essential characteristics of a road suitable for site selection are the surface condition, road materials, road thickness, latitudinal and longitudinal slope, road curvature, rutting and cracking. Therefore not only the longitudinal evenness but also the deteriorations (such as rutting, deformation, cracking etc.) limit the accuracy of a measurement, and may reduce the sensor durability or affect its response. The deflection and the transverse evenness may also affect the reliability and durability of the sensors.
It is strongly recommended that the road section, between 50 m upstream and 25 m downstream of the system, meets the following geometric characteristics:
- longitudinal slope < 1% (class I site) or < 2% (other site classes) and as far as possible must be constant (site classes according to European specification COST 323).
- transverse slope < 3%
- radius of curvature >1 000 m. Ideally a straight road would be preferred.
The pavement must not have rutting or deformations exceeding 4 mm over the whole lane width. Each 40 cm before and after the sensors the roughness of the pavement must not exceed 1 to 2 mm.
In order to get high quality data, the following criteria must be met:
- class I (excellent) according to European specification COST 323 and ASTM specification 1318-02 (ASTM 2002).
- minimal quasi-static and dynamic deflection
- minimum ruts
- even pavement; International Roughness Index (IRI) less than 2 m/km.
Jacob, B, Jehaes, S & O’Brien, E, 2002, COST 323: Weigh-in-motion of road vehicles (WIM-load), final report, Laboratoire Central des Ponts et Chaussees (LCPC), Paris, France