Guidance

Section 3: measuring surface condition using manual visual surveys

Published 17 December 2024

Applies to England

CHART

CHART was developed in the 1970s at TRL to provide a system of assessing the structural maintenance need of highways and was routinely used by the Highways Agency (now known as National Highways) on the trunk road network until 1999. The CHART system of recording defects was adopted for the NRMCS visual survey when it was established in 1977 and was also used for the Road Conditions in England report in 2007.

The CHART system used manual visual condition surveys to assess the condition of the network, with the sample survey used to measure trends in the visual road condition in England by comparing changes in a ‘defects index’ for the following types of road:

• all purpose trunk roads - until 2002

• principal ‘A’ roads and motorways - until 2006

• other classified roads - until 2006

• unclassified roads

From 2007, the assessment of visual condition was limited to unclassified roads, and it was decided that the condition information on classified roads should be based on that provided by the SCANNER survey for future editions of the publication. Because SCANNER assesses condition using a method very different from CHART, the data provided within ‘Road Conditions in England 2009’ (RCE 2009), which is based on SCANNER surveys, cannot be directly compared with the information provided in RCE 2008 and previous versions of the NRMCS bulletin.

Since 2008, the assessment of condition of unclassified roads has been predominantly provided by the Coarse Visual Inspection (CVI) surveys, described in the following section. Because CVI assesses visual condition using an index calculated using a different approach to that employed in CHART, it is not possible to reliably compare the results of the condition of unclassified roads obtained using CHART in RCE 2008 with the results obtained using CVI in RCE 2009.

Coarse Visual Inspection (CVI)

The Coarse Visual Inspection survey (CVI) is a coarse, rapid survey, usually carried out from a slow moving vehicle, that allows a large part of a highways authority’s road network to be assessed each year. Footways and cycle ways adjacent to the carriageway may also be inspected at the same time as the carriageway itself or can be assessed separately.

The CVI was adopted as one method of reporting best value performance indicators (BVPI) in England on principal roads (local motorways and local ‘A’ roads) in 2000 (BV96), and as the only method on other classified roads in 2001 (BV97 and then BV97(a)), and on unclassified roads in 2002 (BV97b). It was replaced by TTS on principal roads in 2004, and on all classified roads by SCANNER in 2005 (BV223 and BV224a, subsequently replaced by National Indicators in 2008, in turn replaced by Single Data List items 130-01 and 130-02 in 2011). The BVPI for the condition of unclassified roads from CVI surveys was renamed BV224b in 2005, and not required as a National Indicator in 2008. The method of calculating the BVPI changed over the period and there is more detailed information about the changes in Cartwright and Pickett (2004).

CVI survey data are delivered as an HMDIF file, the defined standard format for loading survey data into a UKPMS compliant pavement management system. 

Because CVI is no longer required for the calculation of indicators, there is no centrally defined requirement for Local Authorities in England to carry out CVI surveys on their road networks. Nevertheless, many authorities continue to use the surveys as a routine inspection method on unclassified roads (which are not routinely surveyed by SCANNER), to obtain condition data for maintenance planning.

The UK Roads Board recommends that anyone who undertakes a CVI survey should be accredited to the current nationally accepted standard. A new Visual Survey Inspector accreditation scheme was introduced in April 2007. To be accredited inspectors must have passed through this scheme.  

Detailed Visual Inspection (DVI)

The Detailed Visual Inspection survey (DVI) is a more comprehensive survey than CVI, with defects identified by a larger number of more detailed classifications. The DVI is a walked survey that provides much more detailed information than the CVI and is typically targeted at lengths already identified as defective and potentially in need of treatment either by the CVI, or from other sources of information such as SCANNER.

DVI is not required for the calculation of indicators and there is no centrally defined requirement for Local Authorities to undertake DVI surveys on their road networks. However, some authorities use the DVI survey as the main inspection method on urban roads, particularly where footway condition is important (as these are not surveyed by SCANNER), and some as a scheme level survey to obtain more detailed condition data for maintenance planning.

DVI survey data are delivered as an HMDIF file, the defined standard format for loading survey data into a UKPMS compliant pavement management system. 

Because some authorities prefer to carry out DVI surveys rather than CVI as routine surveys (for example in urban areas), local authorities can opt to undertake DVI surveys as an alternative to CVI and then convert the results to a “CVI equivalent” survey, using UKPMS Conversion Software. However, this is not a straightforward conversion and can lead to subtle changes in the results.

As with CVI surveys, the UK Roads Board recommends that anyone who undertakes DVI surveys should be accredited to the current nationally accepted standard (as noted above).

Carriageway defects measured by CVI and DVI

Both CVI and DVI surveys record a wide range of defects. However, CVI and DVI surveys often take a different approach to recording the presence and extent of a particular defect. Typically, because DVI is a walked survey, it is able to record a greater number of defects and to a higher level of detail. Because the (driven) CVI survey is only able to make a broad estimate of the width of the carriageway affected, the results of CVI surveys are processed to express the defects as “local”, “partial” and “general” for loading into UKPMS.

The following briefly describes the defects that appear on bituminous carriageways (which represent the vast majority of roads in England). Further information on these defects, and those found on concrete carriageways, can be found in the UKPMS user manuals (Volume 2 – Visual data collection for UKPMS). The descriptions are also restricted to defects identified on the road surface and not on the adjacent footway. Both CVI and DVI surveys can be used to assess footway condition, but these defects are not included in the calculation of carriageway condition indices such as those used in the publication to report the condition of unclassified roads.

Wheeltrack cracking

Most vehicles follow a similar path on a road, resulting in the formation of identifiable wheel tracks. The significance of the wheel tracks is that damage to the road structure, as well as the surface, is most serious there. In particular, cracking in the area of the wheel tracks (Figure 1) may indicate structural damage, especially if associated with the presence of a rut along the wheel track. In both CVI and DVI surveys, the length of cracking present is recorded by noting the start and end points of the defect as the survey progresses (as noted above, in the CVI survey the defect is post-processed to “local”, “partial” and “general” for loading into UKPMS).

Figure 1: Examples of wheel track cracking (left) and wheel track rutting (right)

Transverse or reflection cracking

Distinct, single or multiple transverse cracks can often be related to the presence of a defect beneath the surface, such as a crack in the concrete layer of a composite pavement (Figure 2 - this defect is only recorded when the carriageway is of composite or covered concrete construction). These cracks are therefore reported as a specific type of cracking in both CVI and DVI surveys, reported as the number present within each reporting length. In DVI, the defect is separated into two severities, severity 1 and severity 2 as judged by the inspector.

Figure 2: Examples of transverse or reflection cracking (left) and whole carriageway cracking (right)

Whole carriageway cracking

Only DVI surveys record this defect, which is observed as single or multiple cracking present over any part of the road surface. The defect is reported as the area affected and in 2 severities, minor (where the cracking is fine) and major (where the cracking is wide – Figure 2).

Note that, although this defect is only recorded separately in a DVI survey, the presence of this type of cracking would be recorded as Wearing Course Deterioration in a CVI survey.

Wheel track rutting

Wheel track rutting (Figure 1) does not necessarily indicate structural damage if there is no cracking, but it is undesirable because the ruts can hold water which can cause skidding, especially in winter when the roads can become icy. On urban roads in particular, pedestrians may get splashed and there is a hazard to cyclists and motorcyclists, while on roads with higher speeds there is the risk of vehicles aquaplaning. Because it is a driven survey, the CVI is only able to broadly assess the rutting, which is done by recording the lengths of the network where the inspector judges the rutting to be greater than 13mm. In a DVI survey, the depth of the rutting in the nearside wheel path is measured at intervals (for example, every 20m) using an appropriate device (such as a straight edge and wedge). 

Deterioration of the wearing course

Both CVI and DVI surveys collect information on deterioration of the wearing course, again at different levels of detail.

CVI defects in the wearing course are reported as wearing course deterioration or surface deterioration, both of which are reported in terms of the area of the carriageway affected (as “local”, “partial” and “general” deterioration). Whereas wearing course deterioration reports the loss of material from the surface other than surface applied chippings (a defect often referred to as “fretting”, which can in the extreme lead to potholing), surface deterioration assesses the loss of surface applied chippings (such as surface dressing), and reports where bituminous binder is appearing in the surface course (Figure 3) to an extent that the friction material is flush or covered, a defect often referred to as “fatting up”.

Figure 3: Examples of wearing course deterioration (left) and surface deterioration (right)

In DVI, these wearing course defects are reported separately as chip loss (loss of surface applied chippings), fretting (loss of material that has not been surface applied) and fatting, which are classified by the inspector into minor and major defects, depending on their severity. Again, the defects are reported in terms of the area of the carriageway affected.

Settlement and subsidence

Both CVI and DVI record the presence and area of local settlement or subsidence (Figure 4). This will include patches or public utility reinstatements (and areas where the carriageway has heaved, for example due to tree roots. In CVI, this is reported (as “local”, “partial” and “general” deterioration) only where it produces a difference in level to the surroundings of greater than 30mm. In DVI, settlement to such an extent is reported as severe, with settlement between 13mm and 30mm reported as moderate.

Figure 4: Examples of settlement and subsidence (left) and edge deterioration (right)

Edge Deterioration           

Deterioration of the road edge (Figure 15) can present a hazard to users and also lead to more extensive deterioration of the remainder of the carriageway as the edge support becomes eroded. Both CVI and DVI surveys record this defect as the length of deterioration present. In the same approach as the defects described above, the CVI survey reports this defect as “local”, “partial” and “general” deterioration, in a DVI survey the defect is separated into two severities, severity 1 and severity 2, as judged by the inspector.

Calculation of road condition using DVI and CVI data

Processing of CVI and DVI data is carried out in UKPMS to analyse the condition of the network and generate condition indices. UKPMS will also suggest treatments and an indicative budget for the work required. UKPMS processing splits up the network into defect lengths, and then reports the condition, treatment and ranking for each of these defect lengths. There are 4 key steps which together create the defect lengths and enable the calculation of the condition indices.

Chopping to form Rating Lengths

The condition data collected within each section of the network is analysed to identify lengths within the section where the reported level of defects is relatively constant. This is used to chop up each section into subsections of consistent defectiveness. These subsection lengths are referred to as Rating Lengths. 

Calculating Defect Ratings

Each rating length is re-analysed to determine the value or score for each defect within the length; this value is referred to as a defect rating. The defect ratings are obtained from predefined look up tables or curves, with a different rating for each defect. For example, CVI local surface deterioration will have a rating of 10. Therefore, if the chopping process identified an 80m long length with consistent local surface deterioration, this 80m length would be given a defect rating of 10.

Calculating Condition Indices

A condition index (CI) is a measure of the condition of a particular aspect of the defect length. In UKPMS, there are several condition indices. The Edge CI provides information about the condition of the edge of the carriageway and is calculated from defects which measure the deterioration of the edge. Other UKPMS carriageway condition indices include Structural CI, Wearing Course CI, Surface Properties CI and the Overall CI.

A value for each of the condition indices is calculated for each rating length. For some indices this may require combining together the defect ratings for that index. The process is governed by the rules which determine which defects contribute to which condition index, and the precise way in which they are combined together.

Merging to form Defect Lengths

The final stage in the calculation of condition indices is to merge the rating lengths to form defect lengths. This process is controlled by the parameters contained within UKPMS and can be carried out to generate fixed lengths (for example, 100m), pre-defined lengths, or variable lengths (where the system identifies lengths with generally consistent values of condition index). The result is a set of sub-sections (now referred to as defect lengths) each having associated with it a value for the condition index.

Obtaining National Indicators (Best Value Performance Indicators)

As noted above, CVI was adopted as a method of reporting best value performance indicators (BVPI) in England, initially on principal roads in 2000 and later for other classified roads and unclassified roads in 2002. Although the CVI BVPIs have been replaced by SCANNER Single Data List items on classified roads, many authorities undertake CVI surveys on unclassified roads, and the methodology previously used to calculate the unclassified road BVPI (BV224b) is available in current versions of UKPMS.

In the ‘road conditions’ publication, this has been used as the basis for the reporting of the condition of the unclassified road network. BV224b uses the defect lengths calculated above to determine the proportion of the (surveyed) length of the unclassified carriageway for which at least one of the Condition Index thresholds shown in the following table has been equalled or exceeded. The value is reported as the total length exceeding the thresholds (provided in Figure 5) divided by the total length of unclassified carriageway network surveyed, expressed as a percentage.

Figure 5: Threshold values for calculation of BV224b

UKPMS RP6.01 Condition Index	Threshold Value
Structural CI	85
Edge CI	50
Wearing Course CI	60

Further information on CHART, CVI, DVI and the use of data

The above paragraphs provide summary information on the CHART, CVI and DVI surveys and how the data is processed in UKPMS to obtain condition indices. Further information can be obtained from:

• CHART. (1988). A System for Assessing the structural maintenance needs of Highways (HECB/R/16-CHART). Department of Transport, Highways Engineering Computing Branch

• UKPMS User Manual. (2007). The UKPMS User Manual

• UKPMS Technical Note 3 (2009) - UKPMS File Structures and Content

• UKPMS Technical Note 38 (2007): Production of Best Value Performance Indicator report BV224b – Condition of non-principal classified roads

Contact details

Road condition statistics

Email roadmaintenance.stats@dft.gov.uk

Media enquiries 0300 7777 878