This paper presents the results of an extensive study to identify the source of the discrepancy, and formulating a procedure for adjusting the historical measurements to ensure continuity. The paper discusses the original calibration procedures that were based on precise rod and level measurements, the inherent variability and the current procedures that use a Dipstick. Furthermore, historically the response type roughness measuring devices were calibrated whereas profilometers measure the actual road profile from which the riding quality statistics are developed. These issues are also discussed.
It was found that the riding quality on the calibration sections were consistent over time, which permitted conversion between measuring systems. With the changed technology a historical IRI of 2 would be an IRI of 1.59 with the dipstick, and a historic measurement of 4.5 would now give a value of 4.21 (all IRI values are in m/km). These conclusions hold important implications for long-term monitoring as well as for international road user cost relations developed prior to 2000 when profilometers became the norm for roughness measurements.