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SCOTTISH TRANSPORT STATISTICS No 23: 2004 Edition

Chapter 5 ROAD NETWORK

1. Introduction

1.1 This chapter provides information about public road lengths by local authority, class, type and speed limit. There are also statistics on the amount of trunk road constructed/re-surfaced and information on the residual life of the trunk road network.

1.2 It should be noted that there may occasionally be unusual year to year changes in the reported road lengths due to one or more of the data providers starting to use Geographical Information Systems (GIS) to calculate road lengths - see section 3.4.

1.3 The main change in this edition is the inclusion of table 5.6 which shows the condition of the local authority road network.

2. Main Points

2.1 There were 54,509 kilometres of public road in Scotland at 1 April 2003. The trunk road network accounted for 6% of the total. Other (non-trunk) A roads represented 14% of the total. Minor roads (B and C roads, and unclassified roads) accounted for the remaining 80% of roads. (Table 5.1)

2.2 Over a quarter of the total trunk road network, and about one-seventh of the Scottish road network, is within the area of the Highland Council. Around 10% of the Scottish road network is within the Aberdeenshire Council area and a further 8% is within the Dumfries and Galloway Council area. (Table 5.2)

2.3 There was an increase in 2003-04 of the amount of trunk road that was newly constructed and reconstructed compared with the previous year. However, roads that were either strengthened or surface dressed showed a decrease over the same period. (Table 5.3)

2.4 Nearly half of the trunk road that was newly constructed, reconstructed, strengthened or surface dressed during 2003-04 was in the South West. (Table 5.4)

2.5 In 2003-04, 9.0% of the Motorway network and 5.1% of the dual carriageway trunk road network required close monitoring of the state of the road surface. (Table 5.5 (c))

2.6 In 2003-04 it was determined that about 33% of the local authority A road network may require some kind of maintenance. This is down from 37% in 2002-03. For the whole of the local authority network in 2003-04 about 45% may require some kind of maintenance. (Table 5.6)

3. Notes and Definitions

3.1 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:

a. provide the road user with a coherent and continuous system of routes which serve destinations of importance to industry, commerce, agriculture and tourism;

b. define nationally important routes which will be developed in line with strategic national transport demands; and

c. ensure that those roads which are of predominately local importance are managed locally.

3.2 On 1st 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.

3.3 Major roads: Motorways and A roads.

3.4 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 4.1.3.

3.5 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.

3.6 Trunk road constructed, resurfaced, etc in tables 5.3 and 5.4: Figures up to 1995/96 are 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 Scottish Executive Pavement Management System.

3.7 Local authority road network condition

3.7.1 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".

3.7.2 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 4.3. As with any survey, the nature of the methods used could lead to apparent minor year-to-year variations.

3.7.3 In order to present its results graphically and on maps, the following colour coding has been adopted:

• green - the road is in an acceptable condition;
• amber - further investigation should be taken to establish if treatment is required;
• red - the road has deteriorated to the point at which repairs to prolong its future life should be considered.

3.7.4 The performance indicator therefore 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 differentiate between different categories of defect, give an additional weighting if there is more than one type of defect present, or take account of the difference in the costs of the treatments which may be required to restore the carriageway to an acceptable standard. In addition, the indicator does not currently cover edge deterioration or surface cracking, but it is the intention, subject to further research, to include these in the future.

3.7.5 As it would not be cost-effective (or practical) to maintain roads in their "as built" condition, a degree of maintenance will normally be outstanding. In the light of research in England, SCOTS feels that a figure of between 10% and 15% may be appropriate, provided that public safety is not endangered, and that the extent to which the Scottish average figure exceeds this may be considered as a measure of the "maintenance backlog".

3.7.6 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, but on their future performance to improve the condition of their road networks.

4. Sources

4.1 Road lengths

4.1.1 Information on road lengths is mainly obtained from annual returns made to the Scottish Executive by Councils and by the trunk road management operators. (The figures for motorways are now prepared by the Scottish Executive using a GIS - see section 4.1.4). 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).

4.1.2 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).

4.1.3 Some councils now calculate their road lengths using GISs, 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).

4.1.4 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 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 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.

4.1.5 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.

4.2 Trunk road network - residual life

4.2.1 The physical condition of Motorways and trunk roads is monitored by annual condition surveys which are undertaken for the Scottish Executive 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 5.5), 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.

4.2.2 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".

4.3 Local authority road network condition - the Scottish Road Maintenance Condition Survey

4.3.1 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 specialist contractors using vehicles equipped with lasers and high resolution cameras, which collect data for processing by computer. The vehicles currently record:-

• the road geometry (gradient and shape);
• variations in the longitudinal profile (evenness of ride along the road);
• wheel track rutting / deformation across the road;
• the presence of cracking within the carriageway;
• texture (roughness of the surface of the road).

At present the Performance Indicator is based only on the results from the longitudinal profile, rutting and surface texture. When the results are summarised, each ten metre stretch of road is assigned to one of three categories ("green", "amber" and "red" - see section 3.7), depending on whether one of the "threshold" limits has been breached. Each stretch of road is counted only once, on the basis of the poorest rating.

4.3.2 The survey aims to cover all local authority A roads each year, all B roads every two years, and all C and unclassified roads every four years. Therefore, there may be some year-to-year variation in the results for B, C and unclassified roads (particularly for the smaller local authority areas) which is due simply to the fact that different roads were surveyed in each year. In addition, while the machines used 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).

4.3.3 The 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.

5. Further Information

5.1 Information on GB road network statistics can be found in the Department for Transport annual publications "Road Traffic Statistics" and "Transport Statistics Great Britain".

5.2 Further information on road lengths in Scotland is available from the Scottish Executive Road Network Management and Maintenance Division, contact Mr Stuart Hay (tel: 0131 244 0458).

5.3 Further information on the construction of Scotland's trunk road network, is available from Mr Allan Roberts of the Scottish Executive Road Network Management and Maintenance Division (tel: 0131 244 7166).

5.4 Further information on the maintenance and the condition of Scotland’s trunk road network, is available from Mr David Arran of the Scottish Executive Road Network Management and Maintenance Division (tel: 0131 244 0460).

5.5 Further information on the Scottish Road Maintenance Condition Survey of the local authority road network, which is conducted on behalf of Councils by the Society of Chief Officers of Transportation in Scotland, is available from Alistair Gow, SRMCS Project Manager (tel: 01546 606222).

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