Center for Transportation Studies University of Minnesota
Weigh-In-Motion (WIM) system tends to go out of calibration from time to time, as a result generate biased and
inaccurate measurements. Several external factors such as vehicle speed, weather, pavement conditions, etc. can be
attributed to such anomaly. To overcome this problem, a statistical quality control technique is warranted that
would provide the WIM operator with some guidelines whenever the system tends to go out of calibration. A
mixture modeling technique using Expectation Maximization (EM) algorithm was implemented to divide the Gross
Vehicle Weight (GVW) measurements of vehicle class 9 into three components, (unloaded, partially loaded, and
fully loaded). Cumulative Sum (CUSUM) statistical process technique was used to identify any abrupt change in
mean level of GVW measurements. Special attention was given to the presence of auto-correlation in the data by
fitting an auto-regressive time series model and then performing CUSUM analysis on the fitted residuals. A data
analysis software tool was developed to perform EM Fitting and CUSUM analyses. The EM analysis takes monthly
WIM raw data and estimates the mean and deviations of GVW of class 9 fully loaded trucks. Results of the EM
analyses are stored in a file directory for CUSUM analysis. Output from the CUSUM analysis will indicate whether
there is any sensor drift during the analysis period. Results from the analysis suggest that the proposed
methodology is able to estimate a shift in the WIM sensor accurately and also indicate the time point when the
WIM system went out-of-calibration. A data analysis software tool, WIM Data Analyst, was developed using the
Microsoft Visual Studio software development package based on the Microsoft Windows .NET framework. An
open source software tool called R.NET was integrated into the Microsoft .NET framework to interface with the R
software which is another open source software package for statistical computing and analysis.
Liao, Chen-Fu; Chatterjee, Indrajit; Davis, Gary A..
Implementation of Traffic Data Quality Verification for WIM Sites.
Center for Transportation Studies University of Minnesota.
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