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Consistency of TTS results on local roads

Consistency of survey data

The current SCANNER survey machines collect a range of data about the condition of roads.; The overall objective of the SCANNER implementation project is to introduce automated road condition surveys that can meet local and national government's needs for consistent road condition information.; As part of the development and implementation programme to support the introduction of SCANNER surveys on all local roads in England, this research investigated how SCANNER surveys could be reported with statistically reliable confidence limits. The primary objective of this project was to develop a reliable systematic method for estimating the overall repeatability, reproducibility and reliability of results obtained from accredited survey machines, to provide an objective assessment of survey consistency.

This research was carried out by the Chris Britton Consultancy in two stages.

The interim report describes the development of a method for assessing the consistency of results derived from TTS (TRACS Type Surveys) and SCANNER surveys. The interim report is available here: Consistency of TTS Results on Local Roads Interim Report

The method is based on an analysis of the differences of the results derived from two independent surveys of a sample network carried out close together in time. As it may be assumed that there is little real change in the condition of the sample network between the two surveys, any differences in the results must be due to differences in the performance of the survey equipment, including the normal operating variability due to changes in environmental conditions, variability in location referencing, etc.

This information can be used to derive confidence limits for the survey results obtained over the whole of a road network. Provided the sample is typical of the network, this can provide confidence limits for all results obtained from surveys undertaken by a particular survey machine collecting data in similar circumstances on road networks similar to the sample network.

The final report illustrates how the method can be used to obtain statistically meaningful confidence limits by calculating confidence limits for BV223; BV96 and the SRMCS indicators from real survey data. The final report is available here: Consistency of TTS Results on Local Roads Final Report

This describes the potential application of the method for national condition monitoring, local performance monitoring and local maintenance management and makes recommendations for the implementation of the method. These include the need for a pilot study or phased implementation, to confirm the minimum size and constitution of the sample network, and the incorporation of the sample survey as part of the formal Accreditation, Quality Assurance and Audit procedures in the SCANNER specification.

These results have been implemented in the SCANNER User Guide and Specification for 2007.