Individual SCANNER parameters can be used to identify lengths containing particular types of defect. However, the SCANNER Road Condition Index (RCI) was developed through the SCANNER research programme to combine SCANNER defects into a single value to assist in the assessment of road condition. The approach used to combine the SCANNER defects was developed by Cartwright & Pickett (2004). This was used with an initial set of thresholds and weightings to calculate the ‘original RCI’ for 2006 and 2007. Further research (McRobbie, Walter, Read, Viner & Wright, 2007) led to new thresholds giving a ‘revised RCI’ which has been used since 2008.
The revised SCANNER RCI is calculated using a sub-set of the parameters measured by SCANNER (these are referred to as the core parameters), which are:
• Maximum rut depth
• 3m Moving Average Longitudinal Profile Variance
• 10m Moving Average Longitudinal Profile Variance
• Whole carriageway cracking
• Texture depth
To obtain an RCI value each parameter is scored between two thresholds – a lower threshold below which there is no need to consider maintenance, and an upper threshold above which further deterioration does not increase the score. These thresholds were based on engineers’ experience of each parameter. The score increases linearly between the lower and upper threshold from zero at the lower threshold to 100 at the higher.
The score for each parameter is then multiplied by two factors, each having a value between zero and one. One factor reflects the “relevance” or importance of the measurement to the maintenance condition of the road. The other reflects the “reliability” of the method of measurement. The result is a weighted score for each parameter for the 10m subsection.
Note that, to avoid Longitudinal Profile Variance having a disproportionate effect on the reported condition, the weighted scores for 3m Moving Average Longitudinal Profile Variance and 10m Moving Average Longitudinal Profile Variance are compared, and only the largest of these two scores is taken forward to contribute to the calculation of the RCI. The same is true for rutting, with only the maximum of the offside and nearside rut depths being taken forward to the calculation of the RCI.
The weighted scores are summed to give a single RCI value for each 10m subsection length, representing the overall condition. The SCANNER RCI values reported by SCANNER can be used by highway engineers (often by displaying the data on a map background in a GIS) to identify lengths of the network in need of further, more detailed, investigation.
SCANNER National Indicators for local roads
The RCI values for each 10m length can be summed to determine the overall percentage of the 10m lengths within the network falling into three categories:
• "GREEN" - lengths where the carriageway is generally in a good state of repair (low RCI values). Green lengths have an RCI score below 40.
• "AMBER" - lengths where some deterioration is apparent which should be investigated to determine the optimum time for planned maintenance treatment (mid-range RCI values). Amber lengths have an RCI score over 40 and below 100.
• "RED" - lengths in poor overall condition which are likely to require planned maintenance soon (i.e. within a year or so) on a "worst first" basis (high RCI values). Red lengths have an RCI score of 100 or over.
In England Local authorities can use UKPMS to obtain the proportion of the network in the "red" category, which is reported as the Single Data List items 130-01 (‘A’ class roads) and 130-02 (other classified roads) for their network. These were formerly National Indicators 168 and 169 respectively.
The parameters, thresholds and weightings that define the calculation of the RCI are published on the DfT website as a “Weighting Set”, which can be found here.