The location on the bending plate affects the accuracy of the data recorded.
At the site of the bending plate, trucks should not be encouraged to straddle the WIM scale by partially driving on the shoulder. To prevent straddling, the plate should be installed at a guardrailed section or one immediately after. If guardrail is not available an additional weigh pad can be installed in the shoulder.
The bending plate should avoid locations with regular stop and go traffic. Bending plates are typically installed at road sections with little or no bypassing by heavy vehicles to capture all vehicles.
Some basic factors for determining locations are:
- Typically the bending plate can be installed at road sections with little or no bypassing by heavy vehicles.
- The bending plate can be installed at some locations that can use historical data from nearby counters.
- Locations that need to monitor the impact of different climatic conditions on pavement design, such as locations discussed in the long-term pavement performance (LTPP) program, can also be considered.
- Industrial areas where there is a need for traffic load monitoring can also be considered.
Once the general section/node of highway for the WIM installation is determined the following road geometric factors need to be considered for two-direction operation:
- Curvature radius should be no less than 1 700 m. In one-direction traffic, a curve still allowing maximum speed can start 20 m past the scale. Trucks should not have to slow down for a curve or any other obstacle while driving over the scale.
- Longitudinal slope should be not more than ± 2%. Ideally the longitudinal slope should be 0%. If there is any slope, the site location should be selected at about 1 km on either side as the calibration trucks want to reach the maximum speed loaded.
- Places with regular stop and go traffic should be avoided.
- Cross slope is not critical but should not exceed 3% and should be consistent across the lane to minimise swaying of vehicles and allow level installation of plates.
The following pavement conditions should be considered for two-direction operation:
- The pavement base should be well drained. Avoid places where flooding can occur.
- Ideally the aprons should be built new and made of concrete or hard wearing, well compacted asphalt of at least 200 mm thickness.
- The pavement levels have to meet the ASTM specification (ASTM 2002) of maximum deviation of 3 mm under 6 m straight edge for 60 m before and 20 m after the scale. This is usually not achieved at construction and allowance for grinding should be made.
Other Considerations for Site Selection
- For the electronics a concrete cabinet should be chosen at a (concealed) location at least 5 m off the travelled lane.
- Power and phone services should be close by. Cell phone and solar are options, however, it is recognised that the highest reliability and continuity of data collection are achieved with permanent power and landline.
- Trucks should not be encouraged to straddle the WIM scale by partially driving on the shoulder. Generally choose guardrailed section or immediately after. If it is not available an additional weigh pad can be installed in the shoulder.
- There should be safe parking for the maintenance vehicles.