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SCOTTISH TRANSPORT STATISTICS: No 21

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 condition of road surfaces (referred to as "pavements" by engineers) and the amount of new roads which have been constructed, re-surfaced, strengthened or surface dressed.

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.

2. Main Points

2.1 There were over 54,000 kilometres of public road in Scotland at 1 April 2001. 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 Trunk road area constructed/opened in 2000-01 was more than in the previous year, but remained lower than in all the years from 1990-91 up to and including 1998-99. There were decreases in the amounts of trunk road that were reconstructed and roads surface dressed. There was an increase in the amount of strengthened roads compared with the previous year. (Table 5.3)

2.4 The new trunk road constructed in 2000-01 was in the North West and South East areas. (Table 5.4)

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: in 1999-2000, for the purpose of maintenance, the trunk road network was sub-divided into 8 operating units. On the 3 Premium Units, covering Tay, Forth and Clyde, and comprising mainly the motorways and major inter-urban dual carriageway routes, Operating Companies provided a complete management and maintenance service. On the 5 All Purpose Units covering the North West, North East, Central, South East and South West and comprising primarily rural single carriageway routes, Management Agents were appointed to undertake management and professional services, and to co-ordinate the work of the local Councils who provided the front-line routine and winter maintenance service. Details of the areas that were covered by these Units can be found in the Annex. New arrangements were introduced with effect from 1 April 2001.

3.6 Areas of trunk road constructed, resurfaced, etc: in tables 5.3 and 5.4 the area of road resurfaced, strengthened, etc are expressed as ‘million square metres’. The reason for this is that the data represents the aggregate value of the area of work undertaken on the whole network including single carriageway roads, three or more lane single carriageways, dual two lane roads and dual three or more lane roads including motorways. It also includes work done on slip roads and junction areas. In addition, work is not always undertaken on the whole carriageway width at specific sites. As a result, it is not possible to say what the total lengths of trunk road resurfaced etc were for each year. However, by assuming a standard lane width of 3.5 metres, an equivalent road lane length has been estimated, in order to provide figures which, to the user of this publication, may be a more comprehensible overall measure of the amount of work done.

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 for 2001 were 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 road at roundabout 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 2001-02 represents the combination of the condition in 2001-02 of the roads which were surveyed in 2001-02, the condition in 2000-01 of the roads which were surveyed most recently in 2000-01, 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".

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 Mr Stuart Hay of the Scottish Executive Road Network Management and Maintenance Division (tel: 0131 244 0458).

5.3 For further information on maintenance and the condition of Scotland’s trunk road network, contact Mr David Arran of the Scottish Executive Road Network Management and Maintenance Division (tel: 0131 244 0460).

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