1. Introduction

1.1. This chapter provides information about traffic on major roads, such as the types of vehicle, types of road, council areas, and average daily flows at selected sites.

1.2. Due to the inclusion of two new tables in this chapter, the table numbers differ from those in previous editions of "Scottish Transport Statistics". The new tables are:-

Table 6.1 Traffic on major roads by vehicle type and type of road.

Table 6.4 Average daily traffic flow at selected Automated Traffic Classifier Sites in Scotland by month

2. Main Points

2.1 The volume of traffic on major roads (Motorways and A roads) in 1997 is estimated to have totalled almost 25 thousand million vehicle kilometres. It increased by an estimated 0.8% between 1996 and 1997, and has grown by about a third since 1987. (The apparent fall between 1995 and 1996 is thought to be due to the effect of local government reorganisation on the method of estimating traffic volumes - see section 4.2 below). Traffic on Motorways has risen faster - by over two-thirds since 1987. (Table 6.1)

2.2 Cars account for almost four-fifths of the total volume of traffic on major roads, and heavy goods vehicles for under 9%. About a sixth of the traffic on major roads is on Motorways, and a further 36% is on trunk A roads. (Table 6.2)

2.3 Over a quarter of motorway traffic is within the City of Glasgow, and Highland is the Council area with the greatest total volume of trunk A road traffic (measured in vehicle kilometres). (Table 6.3)

2.4. In 1997, the levels of traffic varied greatly between roads: taking the total of traffic in both directions, the average daily flows at the A720 Dreghorn and M8 Harthill Automated Traffic Classifier Sites (ATCS) were in excess of 60,000 and 40,000 vehicles respectively, whereas the average daily flows at the A9 Berridale and A920 Huntly ATCS sites were less than 2,000. The levels of traffic also varied considerably within the year: for example traffic at the A82 Ballachulish and A82 Spean Bridge ATCS sites in July/August was more than double that for January/February. (Table 6.4)

3. Notes and Definitions

3.1. The methods that have been used to estimate the volume of traffic on major roads (Motorways and A roads) in Scotland have changed over the years. Section 4 describes the method which is currently used, and then indicates how the methods which were used previously differed from the current method.

3.2. At present, there are no reliable estimates of the volume of traffic on minor roads (B roads, C roads and unclassified roads) in Scotland. The Department of the Environment, Transport and the Regions (DETR) estimates the volume of traffic on minor roads for Great Britain as a whole, using information from traffic counts taken at a selection of sites across GB. The sample was designed for the purposes of producing estimates for GB as a whole and, unfortunately, the Scottish component is too small to provide reliable estimates of minor road traffic for Scotland. The average of some recent years' estimates of minor road traffic volumes is about 14-16,000 million vehicle kilometres per year for Scotland as a whole (in comparison, the estimated Scottish major road traffic volumes average around 25,000 million vehicle kilometres per year).

3.3. 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: some former non-trunk roads became trunk roads, and some former trunk roads ceased to be trunk roads. This affected the calculation of the traffic estimates, as is described in Section 4.2. The apparent fall in the estimate of the total traffic volume for major roads between 1995 and 1996 is assumed to be due to the discontinuities caused by the above factors: in effect, the figures for 1995 and 1996 are on different bases, and are not directly comparable.

3.4. The average daily traffic flows at Automated Traffic Classifier Sites are "total past the point" i.e. traffic is counted in both directions. The estimated traffic flows are based on 7-day averages which include both weekdays and weekends. On occasion, the ATCS counters are not in operation for enough of the month to provide a reliable estimate: in these cases, ".." is used to indicate that no estimate is available.

4. Sources

4.1. Method used to estimate traffic on major roads for 1996 onwards

4.1.1. Estimates of traffic volumes on major roads in Scotland by road type, vehicle type, and area within Scotland were produced by DETR in conjunction with The Scottish Office's National Roads Directorate (SO NRD).

4.1.2. The method of estimation has two main stages. First, the SO NRD estimates traffic flows for approximately 1,800 individual road "links" on trunk and principal roads in Scotland. (A link is a section of road between two major intersections). Then, DETR summarises and weights the estimates of the traffic flows on these road "links" by the lengths of roads of the relevant types to derive total traffic volume estimates (measured in millions of vehicle kilometres) for major roads by road type, vehicle type and Council area. The type of a road is determined by its class (Motorway or A road), by whether or not it is a trunk road (trunk roads are those roads for whose upkeep the Secretary of State is responsible), and by whether or not it is in a "built-up area" (roads in built-up areas have speed limits of 40 mph or less). The steps involved in each of these stages are described in subsequent paragraphs.

4.1.3. The SO NRD's estimates of traffic flows for the individual major road "links" for each year were derived by a methodology, comparable to that used by DETR for the rest of GB, which involved the use of two different types of traffic counts: "link" and "core":

i. The road "link" traffic counts are taken manually, for 12 hours in one day, once every six years, on a rotating basis at each of the approximately 1,800 road links covering nearly all of the major road network in Scotland. Thus one sixth of the road links of the major road network covered are counted for one 12 hour period each year, during the months of April, May, June, September or October. (In England and Wales "link" counts are taken once every three years). These counts must then be scaled up to estimate the total flow of traffic for the year as a whole, and in order to take account of any changes in traffic flows in the years after the count was taken. The "core" counters provide the information that is used in the scaling.

ii. The "core" counters which were used were the SO NRD's automatic traffic counters at statistically random selected sites on major roads through Scotland. (For example, for the 1995 estimates, 54 "core" counters were used: 9 on Motorways, 26 on trunk A roads and 19 on local authority A roads). These automatic "core" counters classify the counts by category of vehicle according to their length and number of axles. The counters operate, on the whole, continuously: 24 hours per day, throughout the year.

4.1.4. The estimated traffic flows for each major road "link" for the "current" year were then derived from a series of calculations of which the following provides only a broad outline. The "core" traffic counters were used to derive two sets of scaling factors, which were then applied to each of the 1,800 "link" counts:-

a. "Whole year" factors for road type and vehicle type were derived from the "core" counters, to scale the single day 12 hour "link" counts to provide estimated traffic flows for the whole year in which the counts were taken.

b. "Growth" factors for each road and vehicle type were also derived from the "core" counters, to scale estimated traffic flows in the previous year forward to the current year, for those links which were not counted in the current year.

4.1.5. DETR then produces the major road traffic volume estimates for Scotland from these individual "link" traffic flow estimates, and from information about the lengths of roads of various types in different areas. The calculations described below are performed for each road type, for the area of each Council.

i Average traffic flows for each road type and area for the "current" year are obtained by weighting the estimated traffic flows for the individual road links of that road type in that area by the lengths of those links.

ii. Estimates of the total traffic volume (vehicle kilometres) for each type of road in an area are then derived by multiplying the estimated average traffic flow for the road type and area by the total length of roads of that type in that area (from the road lengths returns - see Chapter 5: the lengths of Motorway slip roads are excluded from the calculations).

4.1.6. These calculations produce estimates of traffic volumes for the area of each Council. Estimates of traffic volumes for Scotland as a whole are then produced by adding up the estimates for the individual Councils. (Estimated traffic volumes for the areas of the former Regions and Island Areas can be produced by adding up the estimates for the areas of the relevant Councils.)

4.2. The discontinuity in the estimates between 1995 and 1996

4.2.1. 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: some former non-trunk roads became trunk roads, and some former trunk roads ceased to be trunk roads. This affected the calculation of the traffic estimates in two ways:

a. the areas that were used in the calculations changed, from Regions/Islands to Councils. This introduced a discontinuity. The method of estimation (described above) multiplies the average traffic flows (for those "links" for which figures are available) for each type of road in an area by the total lengths of road of the same type in that area. It can be demonstrated that the result of such calculations for a Region as a whole may well differ from the result of performing the calculations for each of the relevant Councils separately, and then adding up the results. Therefore, the change in the kind of area used in the calculations introduced a discontinuity into the results.

b. the re-classification of some trunk roads as non-trunk roads, and vice versa, meant that the total lengths of certain types of road changed in some areas. In addition, some "links" which formerly contributed to the average flows for the "trunk" road types now contributed to the "non-trunk" averages, and vice versa, and so the averages changed. These changes in road lengths and in average flows introduced further discontinuities.

4.2.2. The apparent fall in the estimate of the total traffic volume for major roads between 1995 and 1996 is assumed to be due to the discontinuities caused by the above factors: in effect, the figures for 1995 and 1996 are on different bases, and are not directly comparable.

4.3. Method used to estimate traffic on major roads for 1993 to 1995 inclusive

4.3.1. The estimates for these years were produced by applying a very similar method, the only difference being that the calculations were performed for the areas of the nine former Regions and three Island Areas, rather than for the present Councils.

4.4. Method used to estimate traffic on major roads for 1992 and earlier years

4.4.1. Estimates for 1992 and earlier years were produced by the then Department of Transport (DOT) alone. Because the NRD automatic counters had not then been introduced, the scaling factors used in the DOT's traffic estimates for Scotland for years prior to 1993 were calculated from manual "core" traffic counts at about 129 fixed sites throughout GB. (Although about 20 were in Scotland, it was the core counts for the whole of GB that were used.) These manual "core" counts were taken on three days each month of the year (a weekday, a Saturday and a Sunday) for 16 hours each day.

4.4.2. This change of method has implications for the traffic estimates for 1993 and later years, up to and including 1997, because the starting point for the SO NRD estimates for 1993 was the traffic estimates for the 1800 individual road links which DOT had already derived for the year 1992 using the previous methods and, in particular, using GB "whole year" and "growth" factors derived from the GB manual "core" counts.

4.4.3. As a result, when SO NRD prepared the 1993 estimates, one-sixth of the major road links had traffic counts in 1993, and so required no growth factoring, leaving five-sixths of the links to have their 1992 traffic estimates (which had been derived by DOT) scaled forward by the "growth" factors for Scotland from the new NRD automatic "core" counters. It will not be until the 1998 traffic estimates that the traffic estimates for Scotland will be wholly based on Scottish counters.

4.4.4. The earliest year for which there are estimates of the total volume of traffic on major roads in Scotland is 1983.

5. Further Information

5.1. Further information on GB road traffic statistics can be found in the annual DETR publications "Road Traffic Statistics Great Britain" and "Transport Statistics Great Britain".

5.2. For enquiries about DETR methods of estimating road traffic, contact Mr Michael Dark of the Department of Environment, Transport and the Regions (tel: 0171 890 6574)

5.3. For further information on average daily traffic flows at selected Automated Traffic Classifier Sites (ATCS) contact Mr Stuart Hay of The Scottish Office National Roads Directorate (tel: 0131 244 0458).

List of Tables/Figures