1 Introduction
1.1 This chapter provides information about road traffic on major roads, such as the total volume of traffic by type of road and type of vehicle, the volume of traffic on major roads by council area, and some figures on traffic flows at selected points on the road network.
1.2 A number of changes have been made in this edition.
Table 6.1 is a new table. It provides estimates for all types of road broken down by road class, speed limit and year.
Table 6.2 provides the time series of traffic on major roads by vehicle type that was previously included in the former Table 6.1.
Table 6.3 gives major road traffic by road class, speed limit and vehicle type (from the former Table 6.2) and provides minor road traffic by vehicle type for the first time.
Table 6.4 provides the same breakdowns as the former Table 6.3.
2 Main Points
2.1 The estimated total volume of traffic on Scotland's roads in 1999 was about 43.0 billion (thousand million) vehicle kilometres. This is apparently slightly more than in 1998. However, it must be noted that these estimates indicate only the broad level of traffic, and may be unable to provide reliable information about year-to-year changes. This is because (as is explained in the "Sources" section) they are based on information from a very small cross-section of the roads in Scotland: "12 hours in one day" traffic counts taken at around 500 sites per year and data from automatic traffic counters at fewer than 100 sites. This limits their precision. For example, last year's estimate of the volume of traffic on major roads (Motorways and A roads) in 1998 was 24.6 billion vehicle-kilometres. Since then, more information for 1998 has become available for use in the calculations, and the method of producing the estimates has been improved. This year's estimate for 1998 is 25.6 billion. The size of the revision (1.0 billion vehicle-kilometres) indicates the scale of the uncertainty around these estimates. The estimate for traffic on major roads in 1999 is 25.1 billion, which is below the revised estimate for 1998 (25.6 billion), but the apparent fall is not great in the context of the uncertainty of the estimates. As the estimates for the four years from 1996 to 1999 have fluctuated between roughly 24.5 and 25.6 billion vehicle kilometres per year, it appears that the total volume of traffic on major roads has remained broadly unchanged between 1996 and 1999. However, the longer-term picture is different: the estimated total volume of traffic on major roads has grown by over a sixth since 1989. (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 nearly half since 1989. (Table 6.1)
2.2 In 1999, traffic on Motorways totalled an estimated 4.6 billion vehicle kilometres (11% of all traffic). This was less than the estimated 8.9 billion vehicle kilometres on trunk A roads (21% of the total), and the 11.7 billion on non-trunk A roads (27%). Most of the total traffic on A roads was on roads with speed limits of over 40 mph (which are sometimes referred to as "non built up" roads), which accounted for 14.1 billion out of the A roads total of 20.5 billion vehicle kilometres. (Table 6.1)
2.3 Minor roads (B, C and unclassified roads) account for the remaining 42% of traffic: an estimated 17.9 billion vehicle kilometres in 1999, most of which was on unclassified roads (9.9 billion). Over two-thirds of minor road traffic (12.2 billion vehicle-kilometres in 1999) is on roads with a speed limit of 40 mph or less (sometimes referred to as "built-up" roads). As is explained in section 3.2, reliable estimates of the volume of traffic on minor roads in Scotland are not available prior to 1998. (Table 6.1)
2.4 Since 1989, the volume of car traffic on major roads has increased, as has the volume of light goods vehicle traffic. As is explained in section 3.4, the apparent fall in the volume of heavy goods vehicle traffic on major roads is due to the clarification of the definition of one of the types of HGV for the traffic counts: in earlier years, vans were wrongly counted as HGVs. (Table 6.2)
2.5 Cars account for over four-fifths of the total volume of traffic, and light goods vehicles for 11%. Heavy goods vehicles contribute only slightly over 5%. (Table 6.3)
2.6 Just under 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 6.4)
2.7 The monthly average daily traffic flows recorded at a selection of Automated Traffic Classifier (ATC) sites are given in Table 6.5. The levels of traffic varied greatly. Taking the total of traffic in both directions, the average flow at the M8 Harthill site was lowest in December (averaging about 45,000 vehicles per day) and highest in November (averaging over 52,000 vehicles per day). In contrast, the average daily flow at the A920 Huntly site was less than 2,000 vehicles in every month. Traffic levels can also vary considerably within the year: for example, the A82 Spean Bridge ATC site in August averaged 7,200 vehicles per day - more than two and a half times the average of 2,800 in January. (Table 6.5)
2.8 A range of information about traffic flows at selected points on the trunk road network is given in Table 6.6. Of the sites listed, the M8 Kingston Bridge was the busiest, with an annual average of over 145,000 vehicles per day in 1999. Its Monday-Friday average was over 155,000 vehicles per day, and its Monday-Friday peak hour average flows were almost 12,500 vehicles in the morning and about 10,500 vehicles in the evening. At the opposite end of the scale, a point on the A9 between Inverness and Wick averaged roughly 1,600 vehicles per day over the year as a whole (2,300 in August: figures are also given for that month to show any effects of the tourist season), and average "peak hour" flows of around 150 vehicles. The A75 between Gretna and Dumfries had, of the sites listed, the highest percentage of traffic accounted for by heavy goods vehicles: in 1999, 23% of vehicles on that road were HGVs (27% on weekdays). (Table 6.6)
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 Estimates of the volume of traffic on minor roads (B roads, C roads and unclassified roads) in Scotland that are suitable for publication are only available for 1998 and 1999. The Department of the Environment, Transport and the Regions (DETR) estimates the volume of traffic on minor roads using information from traffic counts taken at a selection of sites. The sample of sites and the method of estimation were originally designed solely for the purpose of producing estimates for GB as a whole and, for many years, the Scottish component was not suitable to be the basis of reliable estimates of minor road traffic for Scotland. However, as a result of improvements made recently by DETR, estimates of minor road traffic for Scotland as a whole can now be published. Section 4 describes the methods used.
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 There is a discontinuity in the figures for light goods vehicles and heavy goods vehicles between 1997 and 1998. DETR clarified the definition of one of the types of HGV for the traffic counts in 1999. Analysis of the data showed that counts in earlier years had included some light vans as if they were HGVs. Therefore, earlier years' figures over-estimated the volume of HGV traffic, and under-estimated the amount of light goods van traffic. When revising the estimates for 1998, DETR adjusted them to put them onto the new basis.
3.5 The average daily traffic flows at Automated Traffic Classifier Sites are "total past the point" figures: 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 (Motorways and A roads) in Scotland by road type, vehicle type, and area within Scotland were produced by DETR in conjunction with the Scottish Executive Road Network Management and Maintenance Division (SE RNMMD) and, in earlier years, the National Roads Directorate of The Scottish Office (SO NRD).
4.1.2 The method of estimation has two main stages. First, traffic flows are estimated for approximately 1,900 individual road "links" on trunk and principal roads in Scotland. (A link is a section of road between two major intersections). The estimates of the traffic flows on these road "links" are then put together, weighted 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 Scottish Ministers are 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 estimates of traffic flows for the individual major road "links" for each year are derived by a methodology which involves 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, on a rotating basis (on average about once every six years), at each of the approximately 1,900 road links covering nearly all of the major road network in Scotland. These counts take place in "neutral weeks" during late March, April, May, June, September and October (the aim is to avoid counting, for example, during school holidays, and so to obtain counts which are representative of the level of traffic on each link). Roughly one sixth of all the road links on the major road network are counted each year in Scotland (compared with about a third in England and Wales). At one time, the aim was to count each site once every six years. However, in 1999, DETR changed the counting schedule in order to improve the accuracy of the estimates: now, the more important links in Scotland should be counted more often, and the less important should be counted less often. Such "12 hours in one day" counts must be scaled up to estimate the total flow of traffic for the year as a whole, and in order to reflect changes in traffic levels in the years after each count was taken. The "core" counters provide the information that is used in the scaling.
ii. The "core" counters which are used are the SE RNMMD's automatic traffic classifiers, which are located at selected sites on major roads through Scotland. These operate, on the whole, continuously: 24 hours per day, throughout the year, and provide information about traffic flows classified by category of vehicle according to their length and number of axles. Most of the "core" counters are situated primarily to provide information to support the management of the trunk road network, but there are some whose purpose is solely to collect data used for the production of the traffic estimates: their locations were chosen by DETR statisticians so that all the "core" counters, taken together, cover a good cross-section of types of road. There are currently about 68 "core" counters in Scotland, but technical problems may prevent some of them from collecting sufficient information in some years. For example, for the 1995 estimates, information from 54 "core" counters was used: 9 on Motorways, 26 on trunk A roads and 19 on local authority A roads.
4.1.4 The estimated traffic flows for each major road "link" for the latest year are then derived by a series of calculations of which the following provides only a broad outline. The "core" traffic counters are used to derive two sets of factors, which are then applied to each of the 1,900 "link" counts:-
a. "Expansion Factors" for road type and vehicle type are used 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 are used to scale estimated traffic flows in the previous year forward to the latest year, for those links which were not counted in the latest 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 latest 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.
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 SO 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 1998, because the starting point for the estimates for 1993 was the traffic estimates for the individual road links which DOT had already derived for the year 1992 using the previous methods and, in particular, using GB "expansion" and "growth" factors derived from the GB manual "core" counts.
4.4.3 As a result, when the 1993 estimates were prepared, 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.
4.4.4 The earliest year for which there are estimates of the total volume of traffic on major roads in Scotland is 1983.
4.5 Method used to estimate traffic on minor roads for 1999 onwards
4.5.1 Estimates of traffic volumes on minor roads (B roads, C roads and unclassified roads) in Scotland by road type and vehicle type were produced by DETR in conjunction with the SE RNMMD
4.5.2 The method of estimation has two main stages. First, "12 hours on one day" traffic counts are taken at a "panel" sample of slightly over 200 sites on minor roads across Scotland. Taking the counts at the same sites each year should produce a better estimate of the year to year percentage change in the volume of traffic on minor roads. The sample was picked by DETR from a list of all the sites at which traffic counts had been taken between 1992 and 1997 (as described in a later section). Disproportionate stratified sampling, with a higher sampling fraction for roads which have a greater volume of traffic, was used, as this should produce more accurate results than a simple random sample of minor road sites. Sites with average flows of less than 200 vehicles per day were excluded altogether. As with the major road traffic counts, the minor road "12 hour" traffic counts must be "expanded" to estimate the flows for a whole day, and a whole year. This is done using expansion factors calculated from information recorded by a set of "core" automatic traffic classifiers located on a sample of minor roads across GB (there are not enough of them in Scotland to support the calculation of separate Scottish factors).
4.5.3 The estimates of minor road traffic volumes for the latest year are then calculated from the estimates for the previous year, using the estimated overall percentage change in traffic levels from the previous year (obtained from the results of the panel survey) and information about changes in the length of the road network between the two years.
4.6 Method used to estimate traffic on minor roads for 1998
4.6.1 The estimates of minor road traffic volumes for 1998 were calculated from the full set of traffic count data for the years 1992 to 1997, not from the 1998 panel sample results. The data for the sites counted in 1992 to 1997 were used because they provided a much larger and much more evenly-spread sample of minor roads than the panel sample in 1998 . The relevant growth and expansion factors from the GB-wide "core" automatic traffic classifiers were applied to each of the traffic counts taken in 1992 to 1997 in order to estimate the level of traffic at each site in 1998. The average traffic flows for each type of road were then combined with information about the lengths of roads of each type in Scotland as a whole, to estimate the total traffic volume (in millions of vehicle kilometres) in 1998 for minor roads, by road type and vehicle type, for Scotland as a whole.
4.7 Method used to estimate traffic on minor roads for 1997 and earlier years
4.7.1 In 1997 and earlier years, the sample of sites for the minor road traffic counts was chosen in a different way. Each year, a fresh sample of sites was picked by, in effect, taking a series of random points on a map, looking within a circle with a specified radius around each point, and identifying which (if any) minor road was nearest to the selected point. The number of other minor roads within the circle was used, at a later stage, when the results were grossed-up to produce the overall traffic estimates. This method of sampling was suitable for the production of results for GB as a whole, but not for Scotland: the kinds of minor roads in the Scottish sample could vary greatly from one year to the next, and, as a result, the Scottish component of the GB estimates was not sufficiently reliable to be published in its own right.
4.7.2 Over the years, DETR built up a list of all the minor road sites that had been chosen in this way, and this list, augmented with information about each site (such as the volume of traffic recorded in the count) became the basis for selecting the panel sample of minor road sites to be counted in 1998 and later years.
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", and also in DETR's "Focus on Roads" and "Road Traffic Statistics Report" publications.
5.2 For enquiries about DETR's methods of estimating road traffic, contact Mr Michael Hamblin of the Department of Environment, Transport and the Regions (Tel: 020 7944 6599).
5.3 For further information on average daily traffic flows at selected Automated Traffic Classifier (ATC) Sites and on key routes on the road network contact Mr Stuart Hay of The Scottish Executive Road Network Management and Maintenance Division (Tel: 0131 244 0458).