Initial error detection often occurs in the field. The data may be filtered, where pulses or gaps in actuation less than some brief interval, say one-fifteenth of a second, are ignored. These tests usually detect gross errors, but other malfunctions may go unnoticed. More advanced filtering techniques are available to validate loop data. One approach is to compare a detector's on time to the average on-time of all other detectors at that station.
Occupancy is converted to density using a variable g which varies as function of occupancy.
Research has shown that a constant g can introduce significant error into speed estimates. These algorithms achieve good detection rates with low false alarms rates, and often identify malfunctioning detectors overlooked by manual inspection. In practice most TMCs operate in a hybrid fashion, using several elements from the tests described above. This allows the development of analysis software with considerable more robustness. If the base inductance of a loop changes due to mechanical wear or weather induced corrosion, these old threshold detectors must be manually re-calibrated in the filed.