User guide – Road network
User guide – Road network
Notes and definitions
Trunk road network
The trunk road network is the responsibility of Scottish Ministers, and comprises all motorways and some of the main A roads (local councils are responsible for non-trunk roads). The Government's view, when it reviewed the trunk road network in 1994, was that the trunk road network should:
- provide the road user with a coherent and continuous system of routes which serve destinations of importance to industry, commerce, agriculture and tourism;
- define nationally important routes which will be developed in line with strategic national transport demands; and
- ensure that those roads which are of predominately local importance are managed locally.
On 1 April 1996, local government was reorganised, and the 32 present Councils replaced the former Regions, Districts and Island Areas. At the same time, changes were made to the trunk road network: about 580 km of former non-trunk roads became trunk roads, and over 340 km of former trunk roads ceased to be trunk roads.
Major roads
Motorways and A roads.
Changes in road lengths
Where there has been a change to the use of a Geographical Information System (GIS) as the basis of the road lengths figures, they may differ significantly from those for the previous year: see section 4.8.4. In 2012 the Trunk road figures were recalculated to include A road slip roads which had been excluded from the totals in previous publications. The time series has been updated to include this data resulting in an increase of 3-4% in Trunk road length and an increase in overall road length of 0.2%. The methodology for calculating the trunk road totals from the database has also changed resulting in some small changes to road lengths from those previously published.
Operating Units
Since 2001-02, the management and maintenance of the trunk road network has been performed by 4 Operating Companies (South West, North East, South East & North West). Details of the areas covered by these Units can be found in the Annex.
Trunk road constructed, resurfaced
Figures up to 1995/96 (which appeared in previous editions) were estimates based on the area that was treated, and an assumed standard lane width of 3.5 metres. From 1996/97 actual figures are produced from the Transport Scotland Trunk Roads Network Management.
Local authority road network condition
The statutory performance indicator for the condition of the local authority road network is defined as the percentage of the road network, derived from a combination of established condition parameters measured at network level, which should be considered for maintenance treatment, i.e. have reached a condition where more detailed monitoring or investigation is required to establish if and when remedial measures are required.
In 2007-08, the indicator changed from the former Scottish SPI, which included data on longitudinal profile, rutting and texture, to the new UK. Standard Road Condition Indicator (RCI), which in addition includes data on carriageway cracking and takes account of the severity of each defect and its relative importance to road users. Further information about the collection of RCI data can be found at:
Information on the condition of local authority roads is collected in the Scottish Road Maintenance Condition Survey, which is co-ordinated by the Society of Chief Officers of Transportation in Scotland (SCOTS), on behalf of Scottish Local Authorities. The survey is described briefly in section 4.9.4. As with any survey, the nature of the methods used could lead to apparent minor year-to-year variations.
Where previously, a breach of any single parameter threshold would result in a 10 m-section being classified as amber or red; from 2007/08 onwards the new RCI each defect is assigned a score, dependent on its severity and relative importance, and the summation of the individual parameter scores is used to define the section category.
In order to present its results graphically and on maps, the following colour coding has been adopted:
- Green - a score less than 40 – the road is considered to be in an acceptable condition;
- Amber - a score of 40 or greater but less than 100 - further investigation should be taken to establish if treatment is required;
- Red - a score of 100 or greater - the road has deteriorated to the point at which repairs are likely to be required to prolong its future life.
The performance indicator covers the amber and red categories, taken together. It represents the percentage of the road network for which some kind of maintenance may be required. It does not take account of the difference in the costs of the treatments which may be required to restore the carriageway to an acceptable standard. The indicator does not currently cover edge deterioration, although it is the intention, subject to further research, to include this.
SCOTS notes that, when examining the results for individual local authorities, it is important to remember that local road networks vary in character, carry different volumes of traffic and serve widely disparate communities. In SCOTS' view, authorities should not be judged on the absolute values of their amber or red proportions in any given year, but on their performance to improve the condition of their road networks.
Sources
Road lengths
Information on road lengths is mainly obtained from annual returns made to the Transport Scotland by Councils and by the trunk road management operators. (The figures for motorways are now prepared by Transport Scotland using a GIS - see section 4.8.3). These returns provide the total lengths of the roads for which the Council or trunk road management operator is responsible. The road lengths are categorised in a number of ways (e.g. by class of road, by type of road and by speed limit).
Because the returns provide only the total lengths of roads of various types (they do not provide any information about any individual roads) they can contain errors which cannot be detected, and, even in cases where an error is suspected, it may not be possible to determine how the figures should be corrected. There are a few cases of apparently unusual changes in the figures between one year and the next, which may be due to errors in the statistical returns (for example, it appears that the figures for dual carriageways may have been affected by the double-counting of some lengths of dual carriageway in some years).
Some councils now calculate their road lengths using GIS, which should reduce the number of errors in the longer term. However, changing to a GIS as the source of the statistics can cause a discontinuity in the figures. They will no longer be affected by any errors inherent in the old method of estimation. There may also be changes in the basis of the figures - for example, in the way in which the lengths of roads at roundabouts are counted. Different methods can give different results: for example, the straight-line distance across a roundabout will differ from the distance around the roundabout; or just half the distance around might be used (to represent the average distance which is travelled on the roundabout).
The effect of a change to a GIS as the source of the data can be seen using the figures for motorways for 2000, which were prepared by the then Scottish Executive using a GIS. The figures for each local authority area (which were published in Table 5.2 of Scottish Transport Statistics no. 20 / 2001 edition) could differ from the figures reported by the trunk road management operators for 1999 (which were published in the previous edition), even in local authority areas where there were no changes to the motorway network between April 1999 and April 2000. The then Scottish Executive derived its figures using particular ways of counting the road lengths for (eg) slip roads and roundabouts. The precise basis of the figures which were reported for earlier years is not known.
The change to the use of a GIS was also the reason why the length of unclassified roads reported by Falkirk Council increased from 400 km in 1999 to 572 km in 2000. In such a case, it must be assumed that the figures produced by the use of the new system are more reliable than those which had been provided previously.
Trunk road network - residual life
The physical condition of Motorways and trunk roads is monitored by annual condition surveys which are undertaken for Transport Scotland by specialist contractors. The surveys are designed to provide information about the structural, surface and safety condition of the road surface (which are referred to as pavements by the engineers). Road condition data is measured by a slow moving vehicle that tests the structural strength by pushing a weight onto the road and measuring how much it deflects. This is then analysed to assess how much life is left in the road pavement. A road network cannot be kept in perfect condition: there will always be some wear and tear, and it is most economic to replace a worn out carriageway at the end of its useful life. When there is no life (which is counted in the residual life <0 column in Table 4.5), the road requires close monitoring to ensure its overall condition does not deteriorate significantly before it is replaced. The data from the surveys is processed annually in a Pavement Management system so as to identify objectively performance and to target the available funds on those areas of greatest need.
The base network includes most motorways and dual carriageway trunk roads. The surveyed network also includes some single carriageway trunk roads. The surveyed network figures are on a cumulative basis – for example, the figure for 2002-03 represents the combination of the condition in 2002-03 of the roads which were surveyed in 2002-03, the condition in 2001-02 of the roads which were surveyed most recently in 2001-02, and so on. Therefore, the surveyed network figures do not represent the current position in each of the specified years: there may have been some improvement or deterioration in the condition of some of the roads since they were surveyed in earlier years. In addition, as the coverage of the surveyed network expands, it includes further roads, whose condition may differ significantly from that of the roads that were already in the surveyed network. Therefore, some of the apparent changes in the figures between years may be due to the expansion of the surveyed network.
Local authority road network condition - the Scottish Road Maintenance Condition Survey
The Scottish Road Maintenance Condition Survey, which is organised by the Society of Chief Officers of Transportation in Scotland (SCOTS) on behalf of Local Authorities, is carried out by a specialist contractor using vehicles accredited annually by the TRL. TRL also undertakes quality assurance checks throughout the year. The vehicles are equipped with lasers and high resolution cameras, to collect data for processing by computer and currently record:-
- The road geometry (gradient and shape);
- Variations in the longitudinal profile (evenness of ride along the road);
- Transverse profile variance (deformation across the road );
- Wheel track rutting / deformation in the wheel path ;
- The presence of cracking within the carriageway;
- Texture (roughness of the surface of the road);
- The extent of edge deterioration (due to over-riding or lack of lateral support).
As indicated previously in section 4.7, the construction of the Scottish road performance indicator was changed in 2007-08 to the new UK Standard Road Condition Indicator (RCI), with each ten metre stretch of road being assigned to one of three categories (Green, Amber or Red) depending on the overall defect score.
The survey currently aims to cover all local authority A roads in both directions every two years, all B and C roads in both directions every four years, and a 10% sample of unclassified roads in one direction each year. In order to minimise the effect of sampling errors on the result, the RCI for unclassified roads is calculated from four years data, as agreed with Audit Scotland and is in effect a rolling four-year indicator. While the survey machines have been calibrated and shown to provide consistent results, variations can occur due to minor differences in machine settings or in the path followed by the survey vehicle (which may well be dictated by, for example, the presence of other vehicles on particular parts of the road).
The SRMCS survey started in the 2002-03 financial year, when it covered all A roads in all local authorities plus a sample of the B, C and unclassified roads in some local authority areas. 2003-04 was the first year for which the survey covers a sample of all road categories in all local authority areas, and is therefore the first year for which results can be produced for Scotland as a whole.
Further information
Within Scottish Transport Statistics:
- Chapter 5 – Road traffic
- Chapter 12 – International comparisons.
The Department for Transport produces a number of related publications:
Eurostat:
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