In order to provide an improved measure of the maintenance condition of the road network the Department for Transport (DfT) has introduced SCANNER surveys on local roads in England and Wales. These surveys are performed using vehicle based automatic monitoring equipment, and monitor the condition of the roads by making measurements of a range of parameters such as cracking, rutting, longitudinal profile and surface texture.
There is a need for a consistent analysis method for SCANNER survey results, such as a condition indicator or defects index. This will facilitate a reliable quantitative comparison between the condition of roads in different parts of the country, and enable the change in condition over time to be monitored.
A preliminary definition for such a condition indicator was developed by a group of experts. This group identified which parameters should be included in the Defects Index and defined the initial method of analysing and combining them to provide a measure of the condition of the road.
TRL has taken the recommended Defects Index model as initially proposed, and has used real data to test it. These tests have examined different methods for combining and aggregating the data, different threshold levels and weightings for the individual contributory parameters, and different ways of reporting the condition of a given 10m subsection of road.
This has been done by obtaining Engineering assessments of a number of sites from a number of Local Authorities. These opinions were then used as a reference data set in a series of comparisons between the output of the Defects Index model in its various forms. Using crude statistical approaches, the work identified the parameters which showed the strongest relationship with the engineers’ opinions.

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