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SCOTTISH TRANSPORT STATISTICS No 22 - 2003 Edition

Chapter 6 ROAD TRAFFIC

1. Introduction

1.1 This chapter provides information about road traffic, such as the total volume of traffic by type of road, by type of vehicle, and by council area. It also provides some figures on traffic flows at selected points on the road network, and information about petrol and diesel deliveries in Scotland and some atmospheric pollutants.

1.2 The main changes in this edition are major revisions to Tables 6.1 to 6.4, following improvements made by the Department for Transport (DfT) to its method of estimating the volume of road traffic in Scotland and in other parts of Great Britain. DfT has produced revised estimates for traffic on major roads (Motorways and A roads) for each year back to 1993. DfT has also estimated, for the first time ever, traffic on minor roads (B, C and unclassified roads) for individual local authority areas, again producing estimates for each year back to 1993. All the figures which were published previously for 1993 onwards have been superseded by DfT's new estimates. In some cases, the new figures differ greatly from the previous ones.

1.3 Many changes have been made to the tables which show DfT's traffic estimates:

• Table 6.1 now provides estimates of minor road traffic volumes for the years from 1993 to 1997, for the first time ever;
• Table 6.2 is an expanded version of the former Table 6.3. It provides extra breakdowns of the figures for "minor roads" and "all roads";
• Table 6.3 is an expanded version of the former Table 6.2. It provides time-series of traffic by vehicle type for minor roads and for all roads, for the first time ever;
• Tables 6.4 (a) and (b) have been expanded to provide figures for minor roads and for all roads for each local authority, for the first time ever;
• Also, DfT's estimates now use an "urban / rural" classification, rather than the previous "built-up / non-built-up" classification. As the two classifications are not the same (see sections 3.3 and 3.4), one should not use, say, figures for the numbers of road accidents on "built-up" roads in conjunction with figures for the volume of traffic on "urban" roads.

2. Main Points

2.1 The estimated total volume of traffic on Scotland's roads in 2002 was about 41 billion (thousand million) vehicle kilometres: about 4% more than the estimate for 2001. These figures are on a different basis from those published in previous years, following DfT's introduction of an improved method of producing the estimates. As a result, the figures published this year should not be compared with those in previous editions. DfT has produced revised traffic estimates for each year back to 1993 using its new method, and these are shown in some of the tables in this edition. In particular, it should be noted that DfT's revised estimates for minor roads differ greatly from the figures which were published previously.

2.2 When using the traffic estimates, it must be remembered that they 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 about two dozen sites in Scotland (which are combined with data from automatic counters at similar sites in England and Wales). This limits their precision. The previous edition discussed how the extent of the revisions to the estimates, following more information becoming available and improvements in methodology, indicated the uncertainty in the estimates. The introduction of DfT's new estimation method this year has resulted in particularly large revisions to the previously-published estimates for the total volume of traffic on minor roads in Scotland.

2.3 The total volume of traffic on major roads (Motorways and A roads) in 2002 was estimated to be 27.0 billion vehicle-kilometres. Traffic on Motorways totalled an estimated 5.6 billion vehicle kilometres (14% of all traffic). This was less than the estimated 9.6 billion vehicle kilometres on trunk A roads (23% of the total), and the 11.8 billion on non-trunk A roads (29%). Most of the traffic on A roads was on roads in rural areas, which accounted for 16.1 billion out of the A roads total of 21.4 billion vehicle kilometres. ( Table 6.1)

2.4 Minor roads (B, C and unclassified roads) accounted for the remaining 35% of traffic in 2002: an estimated 14.3 billion vehicle kilometres, most of which was on unclassified roads (8.1 billion). Most minor road traffic (8.0 billion vehicle-kilometres in 2002) is on roads in urban areas. ( Table 6.1)

2.5 The total volume of traffic on major roads (Motorways and A roads) in 2002 was 4% more than in the previous year, due to 5% growth in traffic on trunk A roads, a 3% increase on Motorways and a 2% rise in traffic on non-trunk A roads. There were increases in traffic on rural A roads and urban A roads of 4% and 1% respectively. Minor road traffic was also 4% higher than in 2001. ( Table 6.1)

2.6 When looking at the figures for earlier years, it must be remembered that the volume of traffic in 2000 was affected by the fuel protests in September. DfT's estimates for Scotland, like those for GB as a whole, show a slight fall in the total volume of traffic on major roads, and a slight rise in the total volume of traffic on minor roads, between 1999 and 2000.

2.7 The revised DfT estimates suggest that the volume of traffic on major roads in Scotland has increased in every year since 1993 (which is the first year for which DfT has produced revised estimates) apart from 2000, which was affected by the fuel protests. The estimated total volume of traffic on major roads in Scotland in 2002 was 20% higher than DfT's revised estimate for 1993. Motorway traffic was estimated to have increased by 42% since 1993 - representing much more rapid growth than the rises over the same period in the estimated volumes of traffic on trunk A roads (18%) and non-trunk A roads (13%). ( Table 6.1)

2.8 It is estimated that traffic on minor roads rose by 14% in the period between 1993 and 2002. As a result, the overall total volume of traffic on all roads in Scotland in 2002 was estimated to be 18% higher than in 1993. ( Table 6.1)

2.9 Cars account for four-fifths of the total volume of traffic on the roads (i.e. of the total for major roads and minor roads combined), and light goods vehicles for 12%. Heavy goods vehicles contribute under 6%. ( Table 6.2)

2.10 Since 1993, the volume of car traffic has increased on both major and minor roads, as has the volume of light goods vehicle traffic. Heavy goods vehicle traffic rose less rapidly. ( Table 6.3)

2.11 Over a fifth of motorway traffic was within the City of Glasgow, and Highland was the Council area with the greatest total volume of trunk A road traffic (measured in vehicle kilometres). Other Council areas with large volumes of traffic on major roads (over a thousand million vehicle-kilometres, in total) were Aberdeenshire, Dumfries & Galloway, Edinburgh, Fife, North Lanarkshire, Perth & Kinross and South Lanarkshire. The areas with more than a thousand million vehicle-kilometres of traffic on minor roads (B, C and unclassified roads taken together) were Aberdeenshire, Edinburgh, Glasgow and North Lanarkshire ( Tables 6.4(a) and (b))

2.12 The monthly average daily traffic flows recorded at a selection of Automated Traffic Classifier (ATC) sites are given in Table 6.5. Unfortunately problems with the equipment at some sites meant that figures were not available for some months or, in a few cases, for any of the months of the year. The levels of traffic varied greatly. Taking the total of traffic in both directions, the average flows at the M80 and A720 sites were around 60-70,000 vehicles per day. In contrast, the average daily flow at the A920 Huntly site was less than 1,700 vehicles in every month. Traffic levels can also vary considerably within the year: for example, the A82 Ballachulish site in August averaged 6,899 vehicles per day - more than double the average of 2,550 in January. ( Table 6.5)

2.13 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 A720 Edinburgh City Bypass was the busiest, with an annual average of over 67,900 vehicles per day in 2002. Its Monday-Friday average was over 73,000 vehicles per day, and its Monday-Friday peak hourly flows were almost 6,400 vehicles in the morning and about 6,700 vehicles in the evening. At the opposite end of the scale, a point on the A9 between Inverness and Wick (just North of Helmsdale) averaged roughly 1,700 vehicles per day over the year as a whole (over 2,400 in August: figures are also given for that month to show any effects of the tourist season), and "peak hour" flows of around 150 vehicles. Of the sites listed, the points on the A1 between the Borders and Dunbar and on the A76 Mennock had the highest percentage of traffic accounted for by heavy goods vehicles: in 2002, 19% of vehicles on these roads were HGVs (23% on weekdays). ( Table 6.6)

2.14 The Department of Trade and Industry's figure for the overall total amount of petrol and diesel delivered in Scotland fell by 27% between 1992 and 2002: it was 2.76 million tonnes in 1992, then fell to 2.03 million tonnes in 2002. However, there have been some large changes within that overall total. Deliveries of leaded petrol fell sharply from over 890,000 tonnes in 1992 to only 30,500 tonnes in 2002. Over the same period, deliveries of unleaded petrol grew from 880,000 tonnes in 1992 to 954,000 tonnes in 2002. Overall, the DTI's figure for the total amount of petrol delivered declined by 44% from 1.77 million tonnes in 1992 to 0.98 million tonnes in 2002: a drop of 0.79 million tonnes. At the same time, there was a 6% increase in deliveries of diesel, from about 0.99 million tonnes in 1992 to 1.05 million tonnes in 2002: a rise of around 56,000 tonnes. However, during the period 1992 to 2002, the DTI figure for diesel deliveries reached a peak of 1.19 million tonnes in 1998. ( Table 6.7)

2.15 At the selected monitoring sites, carbon monoxide concentrations have been below the level of the air quality strategy objective for December 2003 in every year from 1993, and the lead concentrations have been below the value set as the objective for December 2008 for longer. However, nitrogen dioxide concentrations at both the Edinburgh and Glasgow monitoring sites have been above the level set as an objective for December 2005 in every year from 1992 to 2002. The air quality strategy objective for ground level ozone states that by the end of 2005 the maximum daily concentrations should not exceed 100 m g/m ³ on more than ten days per year. While ozone concentrations at the selected monitoring sites have fluctuated over the years, the target value was exceeded on more than ten days a year at the Strath Vaich site in most of the years from 1992 to 2002. However, Eskdalemuir has met this target since 2000, although it had been above the target in several years between 1992 to 1999; and Edinburgh met the target in every year from 1992 to 2002. ( Table 6.8)

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 DfT used to produce the estimates for 1993 onwards, and how the figures for 1992 and earlier years were calculated.

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 from 1993. The Department for Transport (DfT) 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, DfT has improved its methodology for estimating traffic volumes, so that it can produce, for the first time ever, estimates of minor road traffic for individual local authority areas for each year back to 1993. There are also major revisions to the figures previously published for Scotland as a whole for 1998 onwards. Section 4 describes the methods used.

3.3 The new DfT methodology for estimating traffic volumes distinguishes between Motorways, "urban roads" (i.e. roads, other than Motorways, which are in urban areas) and "rural roads" (i.e. roads, other than Motorways, which are in rural areas). For the purposes of the DfT traffic estimates, the general rule is that an "urban" road is a road (other than a Motorway) that lies within the boundaries of an urban area which had a population of 10,000 or more in 2001 (DfT identified such areas using the Population Census boundaries for settlements); a "rural" road is one located in an area with a smaller population. However, there are exceptions. DfT adjusted the "urban/rural" classification of stretches of major road which are on the outskirts of urban areas, in some cases where it was not possible to break them at a junction with another major or minor road. For example, a stretch of road which is part of a trunk road bypass will usually be classified by DfT as "rural" (even the part of it which runs through an "urban" area) whereas a relatively short road between two urban areas that are close to each other will normally be classified by DfT as "urban" (even the stretch which is in a "rural" area). DfT's view is that the effects of such adjustments on the overall traffic estimates are likely to be small.

3.4 DfT's "urban / rural" classification differs from the previous "built-up" / "non-built-up" classification, which is based on speed limits (roads with a speed limit of 40 mph or less being classed as "built-up"; those with a higher speed limit being "non-built-up"). For example, a dual carriageway with a 50 mph speed limit in an urban area is counted as an "urban" road on the basis of its location, but as a "non-built-up road" on the basis of its speed limit. In contrast, a road with a 40 mph speed limit in a small town (population under 10,000) is classed as a "rural" road on the basis of its location, but as a "built-up" road on the basis of its speed limit. While most roads in urban areas have speed limits of 40 mph or less (so are "built-up"), there are many roads in small towns and villages in rural areas which also have speed limits of 40 mph or less (so are also "built-up"). Therefore, the "urban / rural" traffic figures are not comparable to the previous "built-up / non-built-up" data: the figures on the two bases could differ noticeably for some local authority areas. It will not be possible to quantify this over time, because DfT's revised estimates are produced only using the "urban / rural" breakdown. Another point is that urban boundaries tend to change slowly over time, whilst there has been a trend for more roads in rural areas to be assigned speed limits of 40 mph or less. So, a time series for traffic on "urban" roads may show a different trend from a time series for "built-up" roads.

3.5 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. Section 4.3 of the previous edition described how this affected the traffic estimates in two ways, and caused discontinuities in the series of figures for traffic volumes on major roads. The revised DfT traffic estimates are not affected by such discontinuities, because they count major roads on the basis of their "trunk road status" at a recent date, rather than on the basis of their trunk road status in the year in question. As a result, there is no discontinuity in the figures between 1995 and 1996. DfT's use of a new estimation method for the period back to 1993 has also removed some other discontinuities from the figures. For example, paragraphs 3.4 and 3.5 of the previous edition referred to discontinuities in the figures for light goods vehicles and heavy goods vehicles between 1997 and 1998, and to the figures for 1999 and 2000 being on different bases because of the use of different sources of data on road lengths. Such discontinuities do not affect DfT's revised estimates.

3.6. 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.

3.7 The atmospheric pollutants listed in Table 6.8 have been selected because they are considered to be a threat to human health, and transport is understood to be a significant contributor to emissions of the pollutants. In February 2003, the Scottish Executive, UK Government and the other devolved administrations published an Addendum to the Air Quality Strategy containing air quality objectives for eight pollutants (benzene, carbon monoxide, lead, nitrogen dioxide, ozone, particulates (PM10), sulphur dioxide and 1,3-butadiene). The objectives are policy targets expressed as a maximum ambient concentration to be achieved, either without exception or with a permitted number of exceedences, within a specified timescale. The table below sets out the agreed air quality objectives for the first six of these (the ones to which transport is understood to contribute significantly).

3.8 Carbon monoxide produced from road transport, particularly petrol-fuelled vehicles, accounted for 62% of UK emissions in 2001. Emissions have been declining and this is attributed to the installation of catalytic converters in vehicles. Additives in petrol have been the primary source of atmospheric lead. Significant reductions in urban lead levels have occurred since the introduction of unleaded fuel in 1986. The general sale of leaded petrol was banned in the UK from 1 January 2000. All combustion processes in air produc e oxides of nitrogen (NO _x). Road transport accounts for about half of all NO _x emissions in the UK; this contribution is greatly increased in urban areas. The greatest source of PM ₁₀ ( particulate pollution less than 10m m in diameter) is combustion. In particular, this includes fuel from road transport, which accounts for around 20% of UK emissions of PM ₁₀ . Ozone is not emitted directly from any man-made source in significant quantities but arises from the chemical reaction of other pollutants such as NOx and volatile organic compounds produced by road transport, industrial processes and solvent uses. Benzene is a recognised genotoxic human carcinogen and in the UK the main atmospheric source is the combustion and distribution of petrol. Road transport accounted for 36% of UK benzene emissions in 2001, compared with around 65% prior to 2000, due to a reduction in the benzene content of petrol.

AIR QUALITY OBJECTIVES FOR SCOTLAND

4.1. Sources

4.1. The method of estimating major road traffic volumes for 1993 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 are produced by DfT in conjunction with the Scottish Executive Road Network Management and Maintenance Division (SE RNMMD).

4.1.2. The method of estimation has two main stages. First, traffic flows (which represent the numbers of vehicles flowing past particular points in a specified period) are estimated for approximately 1,900 individual road "links" on Motorway and A roads in Scotland. (A link is normally a section of road between two major intersections). The estimates of the traffic flows on these road "links" are then combined with information about the lengths of the links, 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 it is in an "urban" area or a "rural" area (see Section 3.3). 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 40% in England and Wales). At one time, the aim was to count each Scottish site once every six years. However, in 1999, the counting schedule was changed 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 are automatic traffic classifiers, which are located at selected sites on major roads through Great Britain. 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. The locations of the "core" counters, taken together, cover a good cross-section of types of road. There are currently roughly 130 "core" counters on major roads at sites across Great Britain, of which about 17 are in Scotland.

4.1.4. For the purpose of combining the data from the manual counts and the automatic counters, DfT allocates each road link, and each "core" counter, to one of 22 groupings of road type. These were based on a detailed analysis of the results from all the individual automatic counter sites, and take into account traffic flow levels, (GB) regional groupings, and the road's "category", which is a combination of its class (e.g. Motorway, A road, etc) and its urban/rural classification. The groupings range from lightly-trafficked roads in holiday areas, such as Devon and Cornwall, to major roads in Central London. There are no groupings which consist solely of Scottish roads, because there are not enough "core" counters on roads in Scotland which are in the same category, and have similar levels of traffic flow, to form any separate Scottish groupings.

4.1.5. 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.

4.1.6. DfT estimates the total traffic volume (in vehicle-kilometres) on each major road link by multiplying together the estimated traffic flow for the link and the length of the link. DfT obtains the length of each major road link, and identifies the Council(s) in which it is located, using a Geographic Information System (GIS). When a link lies completely within the area of one Council, its estimated traffic volume is counted wholly against that Council. In a case where a link crosses a boundary between Councils, its traffic volume is (in effect) split between the Councils pro rata to the lengths of the parts of the link which are in each local authority. DFT obtains the length of each part of the link which is in each Council's area from the GIS. Splitting the estimated traffic volume for a link pro rata to the lengths of the parts involves an implicit assumption that each part of the link has the same average traffic flow as the part on which the traffic count was taken. This assumption is reasonable, because a link is defined as a section of road between major intersections, and so the traffic flow should not vary much along its length .

4.1.7. DfT compared its new estimates for some motorway and trunk road links with the information that was available from the "volumetric" automatic traffic counters which are operated on motorway and trunk road links by RNMMD, its equivalent in Wales, and the Highways Agency in England. In general, there was a much closer correlation between the two sets of data than before. DfT noted that its estimates were slightly lower, and thought that there might be a number of reasons for this (e.g. the manual counters might miss some vehicles, the fact that the DfT "core" counters cannot be positioned on the most congested roads, etc). DfT therefore adjusted its expansion factors in order to eliminate the apparent slight bias in its overall estimates. DfT did not attempt to make its estimate for each individual link agree exactly with the total from any "volumetric" counter on that link because, for example, the volumetric counters on some links did not provide information for the whole of the year.

4.1.8. These calculations produce estimates of traffic volumes for each road link (or part of a road link) which is within the area of each Council. The estimated traffic volume for each Council is then obtained by adding up the estimates for the relevant links (or parts of links), and the estimates for Scotland as a whole are then produced by adding up the estimates for each Council. As indicated earlier, DfT produced the figures for trunk roads by counting each major road link on the basis of its "trunk road status" at a recent date.

4.2. The method of estimating major road traffic volumes for 1992 and earlier years,

4.2.1. The method that was used to produce the estimates for 1992 and earlier years differed significantly, in several respects, from the current method.

4.2.2. Estimates for 1992 and earlier years were produced by the then Department of Transport (DoT) alone. There were significant differences in the kinds of data that were available for use. DoT did not have GIS-based information about the lengths and locations of individual major road links. Instead, it used information about the total length of roads of each type in each of the nine former Scottish Regions, and the three Island Areas, which was obtained from the road lengths returns (see Chapter 5; the lengths of Motorway slip roads were excluded from the calculations). In addition, because automatic counters had not then been introduced, the scaling factors were calculated from manual "core" traffic counts at about 130 fixed sites throughout GB (including about 20 in Scotland). These manual "core" counts were taken on three days in each month of the year (a weekday, a Saturday and a Sunday) for 16 hours each day.

4.2.3. The calculations were performed for each road type, for each Region (and Island Area). DoT first calculated the average traffic flow for each road type and area for the latest year by weighting the estimated traffic flow for each individual road link of that road type in that area (calculated as described above) by the total length of the link (as supplied to DoT by the then Scottish Office National Roads Directorate). Not having the GIS-based information required to split links which crossed boundaries, DoT counted each link as being in the Region which included the location at which the link's traffic count was taken. Therefore, each link contributed to the estimated average traffic flow for only one Region.

4.2.4. DoT then estimated the total traffic volume (vehicle kilometres) for each type of road in an area by multiplying the estimated average traffic flow for the road type and area (calculated as described above) by the total length of roads of that type in that area (as had been reported in the road length returns). The figures for the total road lengths for each area took proper account of links which crossed boundaries, because the people making the returns had to include only the length of each link that was within an area in the calculation of the total road length for that area. Therefore, the figures for an area's total road lengths could cover a somewhat different road network from that used to estimate its average flows (remember that the latter were calculated using data for only those road links for which the locations of their traffic counts were within the area).

4.2.5. It follows that old method of estimation was likely to be less precise than that used to produce the revised estimates. For example, suppose that there were only two major road links in a particular Region: a short low-flow link whose traffic count was taken at a point within the Region, and a long high-flow link, which crosses the boundary into another local authority, whose traffic count was taken at a point in the other area. Using the old method of estimation, the average traffic flow for the Region would be calculated using only the data for the low-flow link, and then multiplied by the total road length for the Region (including the length of the part of the high flow link that was within its boundaries). The total traffic volume for the Region would therefore be under-estimated: the method could not take account of the high traffic flow on the long link, because its traffic count took place in another local authority.

4.2.6. The estimates produced using the previous methods were also affected by a number of discontinuities, which were caused by changes in local government and trunk road organisation, changes in the availability of data and changes in methodology over the past ten or so years. Some of these discontinuities have been referred to earlier, and others are described in the previous edition. The introduction of DfT's revised method of estimation has removed all the discontinuities that previously affected the estimates for 1993 and subsequent years.

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

4.3. Method used to estimate traffic on minor roads for 1993 onwards

4.3.1 Estimates of traffic volumes on minor roads (B roads, C roads and unclassified roads) in Scotland by road type and vehicle type are produced by DfT in conjunction with the SE RNMMD.

4.3.2. The method used differs from that used for the major roads, because far fewer data are available for minor roads: only 200 or so "12 hours on one day" manual traffic counts per year are taken at Scottish minor road sites. In each of the years up to 1997, 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.

4.3.3. Over the years, a list of all the minor road sites that had been chosen in this way built up, and became the basis for selecting a "panel" sample of minor road sites to be counted in 1998 and later years. 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 from a list of all the sites at which traffic counts had been taken between 1992 and 1997. Disproportionate stratified sampling was used, with a higher sampling fraction for roads which had had a greater volume of traffic, 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. Some of the sites chosen for the panel for 1998 were found to be unsuitable, and were replaced by substitute sites in the panel for 1999. Since then, there has been little change in the composition of the panel of sites.

4.3.4. 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 roughly 40 minor roads across GB, of which about 6 are in Scotland.

4.3.5. The data from the GB-wide "core" automatic traffic classifiers were used to calculate growth and expansion factors for minor roads outwith London (with separate sets of factors for "urban" and "rural" roads of each class). There are too few "core" classifiers in Scotland for there to be any separate Scottish groupings.

4.3.6. The 200 or so manual counts per year at minor road sites across Scotland represent an average of only 6-7 per local authority area per year - clearly, too few to be the basis for reliable estimates of minor road traffic for individual local authority areas calculated solely from the data collected in each year. DfT had therefore to estimate the volume of traffic on minor roads in individual local authority areas in other ways. DfT started by producing estimates of the volume of traffic on minor roads in each local authority area in 1999 (as that is the new base year for its panel of minor road manual traffic count sites). The "information base" for these estimates was widened to include manual counts taken in other years by "uprating" them to 1999 using the growth factors produced from the "core" counters. DfT used different methods for "B" roads and for other minor roads ("C" roads and unclassified roads).

4.3.7. "B" roads : DfT looked at the location and traffic levels of all the "B" road manual traffic count sites, including ones counted in the past that were not included in the panel sample, identified gaps in coverage and initiated extra counts where necessary. Using its knowledge of the variation in B road traffic by type of location, and the length of B roads in each area, DfT produced estimates of B road traffic for each local authority area.

4.3.8. "C" and unclassified roads : Estimating traffic on other minor roads was more difficult, and had to be done in another way. First, DfT estimated the average levels of traffic flow on each type of these roads across GB (e.g. "urban C roads", etc), using the information from the minor road manual counts and "core" counters. Second, DfT compared the average levels of traffic flow on the non-trunk A roads in each local authority area with the GB average traffic flows for such roads. Third, DfT made the assumption that an area which has non-trunk A road flows that are above the GB averages will also have minor road flows that are proportionately greater than the corresponding GB averages, and that an area whose non-trunk A road flows are below the GB averages will have proportionately lower flows on its minor roads. DfT then estimated the flows for each type of minor road in a local authority by applying to the GB average flows for each type of minor road the relevant ratios (of its non-trunk A road flows to the corresponding GB averages). The resulting estimates were multiplied by the length of minor road of each type in that local authority to give the estimated minor road traffic volumes for the area. This produced what DfT considered to be sensible results for many local authorities. However, there were some areas for which DfT felt the results were odd in relation to those for nearby areas or similar areas. For these local authority areas, DfT undertook a more detailed study. This involved looking at the minor road traffic count data for different parts of the local authority, deriving a "traffic intensity" value for each part, and comparing the results with the "traffic intensities" of other local authorities for which DfT was confident about the minor road traffic estimates, in order to produce what DfT considered to be more credible estimates for some parts of the local authority. The resulting estimates were then added together to produce totals for the local authority as a whole, and the results for all the local authorities in Scotland were then added together to produce minor road totals for each area and for Scotland as a whole.

4.3.9. DfT used its estimates for 1999 as the basis for the estimates for earlier years and for later years. The minor road traffic volumes for the years prior to 1999 were estimated by applying year to year change factors, which were calculated from the information produced by the "core" counters. The estimates for later years were calculated using the estimated overall percentage changes in traffic flow levels (obtained from the results of the panel survey) and information about changes in the length of the minor road network.

4.3.10. Given the assumptions that DfT has to make, it is clear that these estimates can only provide a broad indication of the likely volume of traffic on minor roads in each local authority area, which is why figures for individual minor road types are not published for local authority area: only the total volume of minor road traffic for each area appears in Table 6.4, with no breakdown by type of minor road within local authority. The introduction of DfT's new method of estimating the volume of traffic on minor roads resulted in large revisions to the previously-published estimates of the total for Scotland as a whole. DfT feels that the new estimates are more reliable than the previous ones, having been produced using a better method.

4.3.11. 1993 is the first year for which there are estimates of the volume of traffic on minor roads for individual local authority areas, and also is the first year for which there are estimates for Scotland as a whole. There are no reliable estimates of the total volume of minor road traffic in Scotland for 1992 or any earlier year.

4.4. Pollutants and air quality objectives

4.4.1 The information on pollutants is taken from the Scottish Executive National Statistics publication " Key Scottish Environment Statistics". The air quality objectives are taken from "The Air Quality Strategy for England, Scotland, Wales and Northern Ireland: Addendum".

5. Further Information

5.1 Further information on GB road traffic statistics can be found in the annual DfT publications " Road Traffic Statistics" and " Transport Statistics Great Britain", and also in the former DETR's "Focus on Roads" publication.

5.2 For enquiries about DfT's methods of estimating road traffic, contact Mr Ashley Pottier of the Department for Transport (Tel: 020 7944 6396).

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).

5.4 For further information on petrol and diesel deliveries contact Mr Ian Corrie of The Department of Trade and Industry, Energy Policy and Analysis Unit (Tel: 020 7215 2714).

5.5 For further information on pollutants see " Key Scottish Environment Statistics", alternatively contact Harvey Snowling of The Scottish Executive, Environment Statistics branch (Tel: 0131 244 0158).

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