Once the determination has been made that WIM data is needed from a specified route segment, the entire roadway section within the limits of such segment should be reviewed to locate the site best meeting the selection criteria. Depending upon the locations of interchanges and the route’s truck operating characteristics, a “specified route segment” may be a very short stretch of roadway, as in urban areas, or many miles long, as in desert areas.
WIM systems operate best and provide data that is the easiest to validate when all of the vehicles are traveling at a “cruising” speed and are staying in the middle of each lane. The “desirable” conditions are typically found on rural “wide open” highway segments. The “undesirable” conditions are typically found on urban highway segments subject to traffic congestion. In that a system must be calibrated for the range of speeds typically encountered by a majority of the trucks in the traffic stream, it is much more difficult to calibrate a system at a site that routinely experiences slow moving traffic.
“Side by side”, or “in-line”, weighpads require 12’ wide lanes. For lanes with less than 12’ width, a “staggered” weighpad configuration is necessary (complicates installation; more costly). Interchanges can cause “undesirable” traffic conditions and can greatly increase cost and effort for traffic control during system installation.
Roadway grade affects calibration problems in two ways: 1. Only a highly powered test vehicle, when loaded, can attain the needed higher speeds, and, 2. The WIM steer axle weights will read low and the driver weights will read high due to weight transfer. Roadway grade can also result in lane changes due to vehicles (including lighter trucks) passing the slower moving trucks.
If a specific segment of roadway absolutely “must have” a WIM system but no location within the segment meets the selection criteria, the agency will have to consider what “trade-offs” are to be made and then live with the decision when the installation or the traffic data is less than perfect.
Blanket grinding can remove the surface roughness which causes axle bounce (short wavelength) but not the pavement smoothness conditions which cause body bounce (long wavelength).
“SW” = Short Wavelength Initial Calcs presented to Traffic ETG by Dr. David Cebon.
The 325’ and 75’ lengths provide enough distance from the PCC slab’s “cold joints” such that if the pavement approaching and departing the slab fails or is rehabbed the bump or dip at the cold joint will not affect WIM accuracy.
The approach roadway profile should be free of dips, humps, undulations, or any other condition that might create body motion in approaching vehicles. For vehicles such as logging trucks, where the logs are part of the vehicles “body”, much more distance than 500’ is necessary for the body motion to “dampen out” due to the flexing of the logs. It is quite common for roadway embankments adjacent to structures, such as bridges and major culverts, to experience settlement over time. As such, it is recommended that when performing site selection for a WIM system for a newly constructed roadway that the system is kept far downstream of any structures even though the roadway surface profile might appear smooth at the time.
LTPP SPS has specific requirements for pavement roughness (“short wavelength”). Generally, trucks passing through a potential WIM site can be “eyeballed” for both body bounce as well as axle bounce as they approach the planned scale location. With experience, a “drive-through” using a passenger vehicle will also reveal both body motion and axle chatter. Pavement grinding can correct axle bounce but not body motion.
As noted previously, blanket grinding can remove the surface roughness which causes axle bounce (short wavelength) but not the pavement smoothness conditions which cause body motion (long wavelength).
Pavement replacement is difficult and costly. For sites with high truck volumes that are on the upper end of the “strategical importance” scale, the pavement should be improved to the highest quality that is affordable. If the structural stability of the existing pavement approaching and through the WIM site is marginal or less, the replacement section should be long enough such that approaching trucks will not experience body or axle bounce if the existing approach roadway either fails or is rehabilitated. For a low volume truck site on the lower end of the “strategical importance” scale, the minimum pavement preparation effort is probably in order.
The new PCC pavement slab must be “blanket” ground as well as the “cold joints” between the new slab and the existing pavement on both ends.
