Equipment for Collecting Traffic Load Data (2004) / Chapter Skim
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Pages 46-58
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From page 46... ...
The level will define the amount of truck volume and weight data collected specifically to meet the needs of pavement design efforts. These needs will become requests to collect specific data that are sent to the traffic data collection section of an agency.
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may also provide such a list. If the actual pavement design list is not available when the traffic data collection program needs to be developed, this slightly larger list can be used as a surrogate for the actual list.
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Thus, it makes little sense to design a 5-year WIM site for a pavement that will be repaved in 3 years. For WIM data collection, site failure is often the result of failure of the pavement condition around the site, not just the failure of sensors themselves.
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Similarly, the cost of poor data collection is most likely to be made apparent in costs incurred as a result of poor pavement design. (That is, poor design resulting from bad input data is ultimately more expensive than collecting the data needed to create a good design.)
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Without ongoing attention to equipment performance and data collection site conditions, equipment performance will degrade over time. While vehicle classification equipment tends to be more robust (it is less sensitive to calibration drift)
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reporting similar axle weights? • Has the measured distance between axles for tractor drive tandem axles changed?
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The warranty should specify the expected life of the sensor given specific uses of that sensor. For example, a 5-year warranty on bending-plate weigh pads might be specified given a lane volume of less than 5,000 trucks per day.
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In addition, ASTM5 and the FHWA's Long Term Pavement Performance Project6 have recommended the use of two test trucks of known weight but different vehicle characteristics (different classifications and/or suspension types) for performing WIM scale calibration.
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until the weights estimated by the scale equal the known static weights of the truck and its axles. The problem with the single test truck technique occurs because each truck has a different dynamic motion.
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in order to get a sample of the vehicle dynamic effects at that point in the roadway. • Independent measurements can be used to ensure that the data being collected are not biased as a result of the test truck being used.
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These measurements are used as input to the sensor systems to convert the sensor outputs into the estimates of vehicle speed, length, and axle spacing, which are in turn used to 7 SPS Traffic Site Evaluation -- Pilots Summary and Lessons Learned, May 2, 2002, http://www.tfhrc.gov/pavement/ltpp/reports/lessons/Lessons.pdf (active as of June 20, 2003)
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The accuracy of vehicle speeds should be determined by comparing device output against independent measures of vehicle speed collected using a calibrated radar gun or similar device. Vehicle length and axle spacing computations should be compared by comparing these outputs against independently collected axle spacing and vehicle length data.
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This information can be used both in the budget planning process and as part of sensor deployment planning efforts. For example, if it is known from maintenance activities that pavement at a given site has degraded, data collection staff can plan to move electronics to a new location, where pavement conditions are conducive to accurate data collection, until pavement maintenance activities upgrade conditions at the original site.
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