Defects Index thresholds and weightings

The TTS scoping study produced a plan of the activities needed to implement TTS on local roads. This included the development of a 'defects index'. A preliminary study was carried out by the Chris Britton Consultancy (CBC) and reported as TTS Defects Index Preliminary Analysis in October 2004.

The Roads Board set up a Defects Index Working Group with representatives from the Road Performance Management Group (RPMG), the TTS / SCANNER Project Management Group (PMG) and the UKPMS Steering Group (UKPMS SG) to review the recommendations of the report, the proposed methodology and the thresholds and weightings.

From the preliminary analysis report, the Defects Index Working Group developed an initial definition for an overall defects index.  This identified which parameters should be included in the index, the initial settings for the thresholds and weightings and suggested some possible variations in the model.  The Group commissioned this study to assess the suitability of the proposed threshold and weightings.

The project was carried out by TRL Ltd.  The final project report is here. 

The study developed a database with sample data from local authority principal roads and tested the effects of different methods for combining and aggregating the data, different threshold levels and weightings and different ways of reporting the condition of each 10m sub section of carriageway.

Members of the SCANNER Implementation Group carried out an independent engineering assessment of the sample roads lengths and TRL used simple statistical techniques to compare the engineering assessments with the individual parameters measured and the overall defects index.

The results were reported to the Defects Index Working Group, which recommended the initial settings for the Defects Index to be used with SCANNER data.