Laser profilometers are high precision instruments used by road agencies to assess pavement roughness, one of the main parameters of road condition assessment.
Roughness, which can impact traffic flow, fuel consumption and vehicle wear, is characterised by the International Roughness Index (IRI). This is an internationally recognised value of ride comfort and is measured by calculating the response of a quarter car model to the longitudinal profile of the road section travelled.
The traditional method for validating a roughness profiler has been to compare its roughness metric, usually the IRI (m/km), against that measured by a reference device, such as a Walking Profiler. This is achieved through a least squares regression and is the basis of the validation method documented by Austroads.
This type of validation has generally proved to be adequate. However some road agencies have noted significant changes in roughness levels across their networks when different contractors were engaged to undertake roughness surveys. This occurred despite the contractor meeting the requirements of the existing validation criteria.
To address this problem, Hans Prem developed the profiler gain validation technique which assesses how a profiler amplifies, or attenuates, specific wavelengths in a road profile across all IRI wavebands, especially in those of greatest sensitivity. Prem’s method was included in the data collection contracts of several road agencies, but had only limited success, particularly on pavements with high values of texture.
The Austroads Asset Management Program contracted ARRB to assess the profiler gain validation technique as an alternative method for validating laser profilers. In addition, the study sought to quantify the influence of roughness and texture on the validation technique and identify a suitable reference profiler against which other profilers could be assessed.
A series of field tests were undertaken where a Walking Profiler was used as the reference device as a comparison to laser profilers in measuring IRI roughness over five test sites with varying roughness and texture.
These field tests did not conclusively confirm the profiler gain validation technique. Both laser profilers failed to meet the specified gain limits at most of the sites.
The field tests suggest that the profiler gain validation technique should be conducted under a standard test speed for the profiler which can be maintained and allows repeatable tracking of the profiler to minimise variation in the auto-spectral density plots. The test speed should be 60 km/h in urban areas and 80 km/h in rural areas.
At this stage a suitable reference profiler against which other profilers are assessed has not been identified because of the inconclusive outcome of the field tests.