Culway Site Selection Criteria

Road Geometry

The selection of a suitable location for a Culway installation is critical to the accuracy of the information collected.

The following criteria are recommended when assessing the suitability of road geometry for Culway:

  • Straight alignment should have a minimum of 120 metres at each side of the culvert.
  • Longitudinal grade preferably should be less than 1% but not exceed 2%.
  • Crossfall should be between 2% and 3% with a minimum of 2%. The designer needs to balance the needs of Culway (i.e. as flat and smooth as possible) against the engineering necessity of adequately draining the road.
  • Good lane discipline, with no merging or turning vehicles. The Culway should be located on a straight and level road section with good lane discipline. Avoid placing Culway too close to curves since vehicles travelling at speed may swing wide taking the curve and consequently affect the Culway results by invalid vehicle detections.
  • Speed limit >= 80 km/h, with vehicles maintaining a constant speed. The speed of the traffic may also impact Culway results; normal highway speeds are desirable for optimal operation. Slow speed, in particular speeds of less than 30 km/h may cause problems with classification accuracy if vehicles are not travelling at a constant speed. The presence of nearby intersecting roads or any other road geometry that causes traffic acceleration or deceleration over any vehicle weighing system is highly undesirable.

Pavement Conditions

The following criteria are to be used when assessing the suitability of pavement conditions for Culway:

  • The road surface must be well drained with no possibility of water accumulating over the road surface or the shoulders. A dry culvert (one not intended for drainage purposes) is highly desirable, but not essential. The Culway should be located in a section of road where the fill is sufficient to achieve a dry culvert or at least ensure any water can drain away quickly from the culvert.
  • The fill over the culvert is to be between 600 mm and 1 000 mm (providing adequate cover for the culvert).
  • The culvert is of a suitable height between 1 200 mm and 2 100 mm (providing comfortable access).
  • The international roughness index (IRI) of the pavement surface should be less than 4 m/km.
  • Rutting in the pavement should be no more than 10 mm.


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