Chapter 5 ROAD TRAFFIC
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, some statistics on delays and congestion, and information about petrol and diesel deliveries/consumption in Scotland and some atmospheric pollutants.
1.2 Traffic estimates, indicate only the broad level of traffic, shouldn't be relied upon for year-to-year changes as they are based on a very small cross-section of the roads in Scotland: 12 hours in one day traffic counts taken at around 750 sites per year and data from automatic traffic counters at about two dozen sites in Scotland (then combined with data from automatic counters at similar sites in England and Wales). See Sources section.
2 Main Points
Major & Minor Roads
2.1 The estimated volume of traffic on Scotland's roads in 2010 was around 43.5 billion (thousand million) vehicle kms: 1.7% less than 2009. (Table 5.1)
2.2 The total volume of traffic on major roads (Motorways and A roads) in 2010 was estimated to be 28.5 billion vehicle-kms. Traffic on Motorways accounted for 6.5 billion vehicle kms (15% of all traffic). This was less than the estimated 9.7 billion vehicle kms on trunk A roads (22% of the total), and the 12.3 billion on non-trunk A roads (28%). Most of A road traffic was in rural areas: 16.5 billion out of the A roads total of 22 billion vehicle kms. (Table 5.1)
2.3 Minor roads (B, C and unclassified roads) accounted for the remaining 35% of traffic in 2010: an estimated 15 billion vehicle kms, most of which was on unclassified roads (8.4 billion). Most minor road traffic (8 billion vehicle-kms in 2010) is on roads in urban areas. (Table 5.1)
2.5 The total volume of traffic on major roads (Motorways and A roads) in 2010 was 1.6% lower than in the previous year. Minor road traffic was about 1.7% lower than in 2009. (Table 5.1)
2.6 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. Also, the effects of the foot and mouth outbreak may have affected the volume of traffic in 2001.
2.7 Although slightly lower than the previous year, the DfT estimates suggest a continued rise in traffic on major roads in Scotland since 1993 (first year for estimates on the current basis) except in 2000, (likely due to the fuel protests). The total volume of traffic on major roads in Scotland in 2010 has risen by 10% in the ten year period 2000-2010. Motorway traffic was estimated to have increased by 20% for the ten year period 2000-2010 - representing more rapid growth than the rises in traffic on trunk A roads (9%) and non-trunk A roads (6%). (Table 5.1)
2.8 Traffic on minor roads is estimated to have risen by 10% between 2000 and 2010 and the total volume of traffic on all roads in Scotland in 2010 was also estimated to have risen by 10% since1999. (Table 5.1)
2.9 Cars account for over three quarters (77%) of the total volume of traffic on the roads (i.e. of the total for major roads and minor roads combined), light goods vehicles for 14% and heavy goods vehicles for 6%. (Table 5.2)
2.10 Since 2000, the volume of car traffic has increased by 7%, light goods vehicle traffic by 33%, and heavy goods vehicle traffic by 5%. (Table 5.3)
Local Area volumes
2.11 Over a fifth of motorway traffic was within the City of Glasgow, whereas Highland had the highest volume of trunk A road traffic. Other Council areas with large volumes of traffic on major roads were Aberdeenshire, Dumfries & Galloway, Edinburgh, Fife, North Lanarkshire, Perth & Kinross, South Lanarkshire and West Lothian. Aberdeenshire, Edinburgh, Fife, Glasgow and North Lanarkshire had the highest traffic on minor roads (B, C and unclassified roads). (Table 5.4)
2.12 The monthly average daily traffic flows recorded at a selection of Automated Traffic Classifier (ATC) sites are given in Table 5.6. The average flow (both directions) at the A720 Dreghorn site was around 80,000 vehicles per day. In contrast, the average daily flow at the A835 Aultguish site was 1,300 vehicles, peaking at over 2,100 in its busiest month. Traffic levels also vary considerably depending on the month: e.g. the A9 Tomatin site in August averaged 11,600 vehicles per day - compared to 5,000 in January. (Table 5.6 & 5.7)
2.13 Some trunk road traffic flows are given in Table 5.7. The A720 Dreghorn was the busiest site, with an annual average of 77,735 vehicles per day in 2010. Its Monday-Friday average was 84,269 vehicles per day, and its Monday-Friday peak hourly flows were 7,017 vehicles in the morning and 7,376 vehicles in the evening. At the opposite end of the scale, the A835 Aultguish averaged 1,246 vehicles per day over the year as a whole and its Monday-Friday peak hourly flows were around 130. The A75 Carsluith had the highest percentage of heavy goods vehicle traffic in 2010 at 27% for the week, followed by the A7 Langholm (22%). (Table 5.7)
Delays and Congestion
2.14 Table 5.8 estimates the time lost by traffic due to delays on trunk road routes monitored by Transport Scotland. (See sections 3.3 and 4.4). Causes of delays vary, and include traffic congestion, roadworks, increases in traffic for particular events, and seasonal factors. On average only a few seconds is lost a month, per vehicle per km. Longer routes would be identified as the worst-affected if the total time lost by a vehicle travelling over the whole of the route was used (rather than per km), and heavily-trafficked routes would be identified as the worst-affected if the total delay for all vehicles were used(rather than per vehicle figures). Transport Scotland produces more detailed information (traffic levels, speed, congestion/delays) on its monitored routes, see section 5.4. (Table 5.8)
2.15 The Scottish Household Survey provides estimates of delays attributed to congestion experienced by drivers (on the previous day). In 2010, 11% of journeys made as the driver of a car were said to be delayed due to traffic congestion. This figure is broadly comparable to the 2003 congestion level, with a peak of 14% in 2007. Short delays were more common than longer ones - 4% of car drivers' journeys were delayed by around 5 minutes compared to 1% by 15-20 minutes and under 1% by 25 minutes or longer. Weekday journeys were most likely to suffer congestion delays between 7 and 9 am and 4 and 6pm (21-22% and 20-23% respectively). Fewer delays (5-6%) were experienced by people residing in remote small towns and remote rural areas than those in accessible small towns (11%). Congestion experienced by bus users rose between 2009 and 2010. (Tables 5.9a and 5.9b)
These statistics underpin Scotland's National Indicator on driver congestion. More information on National Indicators can be found on the Scotland Performs website: http://www.scotland.gov.uk/About/scotPerforms/indicators/reduceCongestion
Fuel Deliveries & Consumption
2.16 The Department of Energy and Climate Change (DECC) estimates of road fuel deliveries (petrol and diesel) in Scotland suggest a fall of around a quarter between 1998 and 2008 (from 2.6 mill tonnes to 1.4). However almost a third of that drop occurred between 1999 (2.6 mill tonnes) and 2000 (2.4 mill tonnes), which may be due to inconsistency in reporting and/or changes in the data collection arrangements (see section 4.6). Despite these doubts, it is clear that there have been changes in the types of fuel delivered in Scotland. In 2008, petrol accounted for 42% of all the reported deliveries, compared with 54% per cent in 1998. The decline in petrol's share of the total is partly due to the complete decline of leaded petrol (from 9% in 1998). Unleaded petrol's share rose from 45% in 1998 to 50% in 1999 then back to 42% in 2008 - this last fall due to diesel's rising share (46% in 1998 to 58% in 2008), reflecting increasing dieselisation. Note this table should be treated with caution (as it may present a misleading indication of fuel used) and will not be produced in future. (Table 5.10)
2.17 DECC estimates suggest that the traffic on Scotland's roads consumed a total of 3.1 million tonnes of petrol and diesel in 2009. This total differs markedly from the total figure for petrol and diesel deliveries in Scotland as it includes fuel purchased outwith Scotland which is consumed in Scotland, and excludes fuel purchased in Scotland which is used outwith Scotland. It is also estimated using different information (about average fuel consumption, vehicle emissions and traffic volumes - see section 4.6). As a DECC article (see paragraph 5.7) notes a difference between the estimates of consumption (38.9 million tonnes of fuel) and sales (38.8 million tonnes of fuel) for the UK as a whole for 2007, a proportionately larger difference between the estimates is not surprising. DECC believes that the consumption figures are more reliable than the delivered figures for Scotland, and are promoting the use of sub-national petrol and diesel consumption data. Petrol and diesel consumption increased between 2005 and 2007 and has then fallen back below 2005 levels. There has been a steady fall in petrol consumption in cars over the period and a smaller increase in diesel cars (Table 5.11)
2.18 At the selected monitoring sites, carbon monoxide concentrations were below the level of the air quality strategy objective (see section 3.5.1) in every year from 1998 to 2010. However, annual mean nitrogen dioxide concentrations in the Glasgow Chambers and Glasgow Kerbside monitoring sites exceeded the level set as an objective for December 2005 in every year from 1998 to 2010. Glasgow Centre also exceeded the level in 2009 and 2010. The air quality strategy objective for ground level ozone states that by the end of 2005 the maximum daily concentrations should not exceed 100 g/m3 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 1998 to 2009; however, in 2010 the target value was met at all of the sites including Strath Vaich. Since 1986, Eskdalemuir had not met the objective on 15 occasions, including 2009 but met it this year. Edinburgh St Leonards exceeded the target in 2004, 2005, 2006 and 2008. In 2010, the objective was not met at Peebles. Annual mean particulate concentrations in the four sites were below the December 2004 objective level (40g/m3) in all the years from 1998 to 2010 for which figures are available. The December 2010 objective level (18g/m3) was met by the Aberdeen, Edinburgh St Leonards and Grangemouth sites in 2009 and 2010, it has yet to be met in Glasgow Centre. (Table 5.12)
2.19 In 2009, Transport (including international aviation and shipping) accounted for 26.6% of net greenhouse gas emissions allocated to Scotland in the Greenhouse Gas Inventories. This is a 4.8% fall between 2008 and 2009. Total net emissions from all sources fell by 7.0% between 2008 and 2009. Within Transport's emissions, Road Transportation accounted for approximately 70% of the total, (Passenger Cars contribute 41% alone). Heavy Goods Vehicles and Light Duty Vehicles were the other significant contributors to Road Transportations emissions. National Navigation (incl. international shipping) and Civil Aviation (domestic and international) contributed roughly 15% and 11% of Transports total emissions respectively with Railways contributing roughly 2%. As these are estimates, using methodology designed to produce internationally-comparable estimates, apparent year-to-year fluctuations could be due to limitations in the underlying data. See Section 4.8 for details. (Table 5.13)
2.20 The Greenhouse Gas Inventories include emissions of several types of gases. However, in the case of Transport, the quantities involved are relatively small except for carbon dioxide, which accounts for about 99% of all the emissions of greenhouse gases by Transport which are allocated to Scotland. (Table 5.14).
2.21 Estimates of carbon dioxide emissions per passenger-km for different modes of transport are available only for GB/UK as a whole. The lowest emitting modes of transport per passenger-km are national coaches and national rail - 31 and 57 grams of CO2 respectively. Air travel tends to be the highest emitter per passenger-kilometre, particularly domestic flights, which account for 172 grams of CO2 per passenger kilometre. The basis of the estimates is described in section 4.9 (table 5.15).
3. Notes and Definitions
3.1 The traffic estimates produced by the Department for Transport
3.1.1 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 4.1 describes the method which the Department for Transport (DfT) used to produce the estimates for 1993 onwards, and section 4.2 explains how the figures for 1992 and earlier years were calculated. 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. Section 4.3 describes the methods used.
3.1.2 Please note that the DfT traffic estimates provide only a rough indication of the likely volume of traffic on the roads in each local authority area, and that the DfT traffic estimates for individual Council areas are not National Statistics. DfT provides the estimates that it produces for individual local authority areas as being the best that it can produce from the limited amount of data available to it - rough indications of the likely volumes of traffic on roads in each Council area, for use with caution as no better estimates are available. Therefore:
- it is not possible for DfT to quantify the possible margins of error around the estimates for individual local authority areas;
- they are not classed as National Statistics;
- more detailed breakdowns of the estimates for individual Council areas are not published.
3.1.3 DfT's 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). It defines an urban road as 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 (using the Population Census boundaries for settlements); a rural road as 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. E.g. 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 estimate these adjustments to have a small impact on the overall traffic estimates.
3.1.4 DfT's urban / rural classification of roads differs from the built-up / non-built-up classification of roads, used for the DfT traffic estimates prior to 2003. The built-up / non-built-up classification was based on speed limits, with 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, urban / rural traffic figures are not comparable to built-up / non-built-up traffic figures: the two could differ noticeably for some local authority areas. It will not be possible to quantify this, because each set of DfT's estimates were produced using only one of the two classifications, so there is no table which cross-tabulates the traffic estimates by both urban / rural and built-up / non-built-up. Also 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.
3.1.5 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 2002 edition described how this affected the traffic estimates produced by DfT's previous methodology, and caused discontinuities in the series of figures for traffic volumes on major roads. DfT's traffic estimates are no longer 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. The new estimation method which DfT introduced in 2003 also removed some other discontinuities from the figures (again, details of these were given in previous editions).
3.2 Traffic flows at selected sites
3.2.1 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.
3.3 Traffic on specific trunk road routes: average time lost
3.3.1. Estimates of the time lost by traffic on particular routes are produced by Transport Scotland's Trunk Road Network Management (formerly the Scottish Executive Trunk Roads Network Management Division). The figures are estimates of the additional time taken compared with the time that would have been taken had the vehicles been travelling in Free Flow Speed conditions. The reasons for the delays may vary from month to month and from route to route, and include traffic congestion, roadworks, increases in traffic for particular events and seasonal factors. Routes with high time lost throughout the year are most likely to be affected by congestion.
3.3.2 The Free Flow Speed for a stretch of road generally represents the speed that is seen outwith periods of high traffic flow and other known events on the road network (e.g. traffic management for roadworks etc). The early hours of the morning are generally excluded, as they often have a higher than usual percentage of heavy goods vehicles, which usually travel at speeds lower than the overall free flow speed. The Free Flow Speed for each stretch of a particular route is derived from information about the actual speeds of vehicles travelling on that road. The additional travel time at a particular time on a particular day is then calculated from the average speed of vehicles using that stretch of road from its Free Flow Speed. E.g. on a kilometre stretch of road, the average speed of vehicles (in a particular 15 minute period) was 60 kilometres per hour, and that the Free Flow Speed for that stretch of road was 100 kph. The additional travel time per vehicle in that period would be calculated thus:
- average time taken to travel 1 km at 60 kph = 1 minute
- time taken to travel 1 km at Free Flow Speed of 100 kph = 0.6 minutes
- so, additional travel time per vehicle = 0.4 minutes
If 300 vehicles went through in that period, the total additional time would be 300 x 0.4 = 120 minutes. (NB: vehicles with average speeds above Free Flow Speed are treated as if they were travelling at Free Flow Speed, so their reduced travel time does not offset any of the additional travel time incurred at other times.)
3.3.3 Such figures can be aggregated to produce a number of additional travel time values, such as the average time lost per vehicle-kilometre for a route for a month. This represents the average delay encountered by a vehicle travelling one kilometre on that route. As it is an overall average for the month as a whole, it could conceal considerable day-to-day and/or hour-to-hour variation - for example, a stretch of road which has only one or two periods with very long delays due to congestion (perhaps when there is a lot of traffic to events such as football matches), and traffic travelling (on average) at or above Free Flow Speeds at all other times, will have a low overall average time lost.
3.3.4 The average time lost per vehicle-kilometre is only one of a number of possible measures of the delays that are due to traffic congestion and other factors. Reports (see section 5.4) provide information on a range of such measures, and give more detailed information about (e.g.) the levels of traffic, speed and congestion/delay on each of the routes which Transport Scotland is monitoring.
3.4 Estimated consumption of petrol and diesel
3.4.1 The estimates for the consumption of petrol and diesel of road traffic relate to the areas in which the vehicles travelled rather than where the fuel was purchased or the locations of the registered keepers of the vehicles. These figures should be treated with caution and will be removed from future editions.
3.5.1 The atmospheric pollutants listed in Table 5.12 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 these pollutants. The Air Quality Strategy for England, Scotland, Wales and Northern Ireland contains air quality objectives for nine pollutants (benzene, carbon monoxide, lead, nitrogen dioxide, ozone, particulates (PM10), sulphur dioxide, 1,3-butadiene and polycyclic aromatic hydrocarbons (PAHs)). 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 ones to which transport is understood to contribute significantly).
AIR QUALITY OBJECTIVES FOR SCOTLAND
Date to be achieved by
running annual mean
31 Dec 2010
running 8hr mean
31 Dec 2003
31 Dec 2004
31 Dec 2008
hourly mean not to be
exceeded more than 18
times a year
31 Dec 2005
31 Dec 2005
24-hour mean not to be
exceeded more than 35
times a year
24-hour mean not to be
exceeded more than 7 times
31 Dec 2004
31 Dec 2004
31 Dec 2010
31 Dec 2010
daily maximum (measured as
an 8 hour running mean) not
to be exceeded more than 10
times a year
31 Dec 2005
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 Transport Scotland Trunk Road Network Management (formerly Scottish Executive Trunk Roads Network Management Division) (TRNM).
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 each of the approximately 2,100 (in 2006) 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 3.1). 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:
- The road link traffic counts are taken manually, for 12 hours in one day, on a rotating basis (on average about once every four years), at each of the approximately 2,100 (in 2006) 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). Traditionally, roughly one sixth of all the road links on the major road network were counted each year in Scotland, but the proportion counted each year has risen, and was about 22% in 2005 (compared with around 30% 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. Up to and including 2002, about 300 or so counts were taken each year. However, following a study of possible ways of improving the road traffic estimates for Scotland, the then Scottish Executive (SE) increased the number of counts (in 2006, there were about 480 or so per year). These 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.
- 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 around 150 core sites on major roads (including motorways) in Great Britain, of which about 25 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 2,100 (in 2006) link counts:
- 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.
- 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, it is split (for the purposes of the calculations) at the boundary into two separate links. Similar calculations are performed for each new link: the length within the relevant local authority (which DfT obtains from the GIS) is multiplied by the average traffic flow calculated for the original link (regardless of the Council area in which the traffic count was taken - because the original link was a section of road between major intersections, the traffic flow should not vary much along its length).
4.1.7. DfT compared its 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 TRNM, the Highways Agency in England and the Welsh Assembly Government in Wales. In general, there was a much closer correlation between the two sets of data than for the estimates which DfT had made in 2002 and earlier years. 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.
4.1.9 DfT's estimates of the total volume of traffic on major roads in each local authority area are based on 12 hours in one day manual counts at an average of under 10 (up to 2002: under 15 for 2003 onwards) sites on major roads per Council per year - so they are clearly not based on much data. And, because the manual traffic counts are taken on a rotating census basis, there may be several years between successive counts at a particular site: in which time, there could be large changes in the volume of traffic there. The estimates therefore provide only a broad indication of the likely volume of traffic on major roads in each Council area. DfT notes that there could be some large percentage errors in its traffic estimates for the major roads in some local authority areas. Therefore, DfT's estimates for individual Council areas are not classed as National Statistics.
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 TRNM.
4.3.2. The method used differs from that used for the major roads, because far fewer data are available for minor roads: up to and including 2002, only 200 or so 12 hours on one day manual traffic counts per year were 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. There was little change in the composition of the panel of sites until 2003, when, following a study of possible ways of improving the traffic estimates for Scotland, SE increased the number of minor road traffic counts in Scotland to about 320 or so per year.
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 5 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 number of manual counts per year at minor road sites across Scotland represent an average per local authority area per year of only 6-7 (up to 2002) and only 10 (2003 onwards) - 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 panel survey. The estimates for 2000 to 2003, inclusive, were produced by applying year to year change factors which were derived from the data collected by the GB-wide core automatic traffic classifiers. The methodology was changed for the production of the estimates for 2004, when the overall percentage changes in minor road traffic volumes between 2003 and 2004 were calculated using information, from the panel survey, about the percentage changes in traffic flow levels at each of the sites for which comparable results were available from the manual counts taken in the two years. In all cases, the estimates also took account of information about changes in the length of the minor road network.
4.3.10. Given the assumptions that DfT has to make, and the fact that its estimates of the total volume of traffic on minor roads in each local authority area are based on 12 hours in one day manual counts at an average of 6-7 (up to 2002: about 10 for 2003 onwards) sites on minor roads per Council per year, 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. That 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 5.4, with no breakdown by type of minor road within local authority. DfT notes that there could be some large percentage errors in its traffic estimates for the minor roads in some local authority areas. Therefore, DfT's estimates for individual Council areas are not classed as National Statistics.
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 Average time lost by traffic on specific trunk road routes
4.4.1 Transport Scotland's Trunk Roads Network Management Directorate (TRNM) produces the estimates of the average time lost by traffic on specific trunk road routes. The routes for which the estimates are produced are those sections of the trunk road network which presently experience congestion, or which are thought likely to experience congestion over the coming years, and which are therefore covered by TRNM's congestion monitoring work.
4.4.2 Contractors working for TRNM produce the estimates from two sources of data about the speeds of traffic on those sections of the trunk road network: automatic traffic counters and so-called floating vehicle surveys.
4.4.3 The automatic traffic counters use sensors which are buried under the surface of the road. They run continuously, and record the numbers of vehicles passing each site, and the speeds at which they travel. The counters collect large amounts of data, which are then aggregated and stored as overall figures for 15-minute periods. Data are available from automatic traffic counters at over 300 locations on the monitored routes, with information collected about the speed of traffic in both directions at each location.
4.4.4 The speed data for each section of road covered by a particular monitoring site are validated and calibrated using what are called floating vehicle surveys. In these, vehicles drive the routes at speeds which are representative of the traffic flow in which they are travelling (by balancing the numbers of vehicles that they overtake and which overtake them) and record their speeds and times taken along the route. A particular stretch of road is surveyed several times, on different days and at different times of the day, in order to obtain a representative range of results. The surveys also provide some information which is unavailable from the automatic traffic counters, such as the time which is taken by traffic queuing at junctions.
4.4.5 The contractors produce the estimates by combining the information from the two sources, using a specially-developed methodology and considerable computer processing of the data. A more detailed description of the method of producing these estimates appears in the reports described in Section 5.4.
4.5 Scottish Household Survey
4.5.1 Information about the Scottish Household Survey is given in Chapter 12.
4.6 Estimated consumption of petrol and diesel
4.6.1 The figures for the petrol and diesel consumption of road traffic are estimated by AEA Energy & Environment, which was commissioned to do this by the Department of Energy and Climate Change. AEA produce the estimates using a range of data, including: (a) information from equipment, located alongside many A roads, which monitors the levels of various substances emitted by vehicles; (b) average fuel consumption factors (expressed in terms of grams of fuel per kilometre driven) for different classes of vehicles; (c) the Department for Transport's information about the traffic flows on each link of the major road network; and (d) the DfT's estimates of the total volume of road traffic on minor roads. AEA estimate the consumption of petrol and diesel separately for each type of vehicle for each Council area, producing more detailed estimates than appear in Table 5.11.
4.7.2 Figures on fuel deliveries should be used with caution since they may not reflect actual fuel consumption in Scotland. This stems from the underlying data being based on company-level reports that may not distinguish properly between Scotland and the rest of the United Kingdom. The main reason for this is that the refiners (who provide the data) have lost market share to hypermarkets (who do not provide data). Information about imports made by non-refiners is apportioned on the basis of the refiner's figures for the country of delivery. However, these details may not be accurate if the fuel is delivered to a different country/region from that of the invoice address or if there are intermediary suppliers.
4.7 Pollutants and air quality objectives
4.7.1 The information on pollutants is taken from the Scottish Government online publication Scottish Environment Statistics Online. Some of the data are additionally published in the then 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.
4.8 Emissions of greenhouse gases by Transport allocated to Scotland
4.8.1 These figures are based on data used in Greenhouse Gas Inventories for England, Scotland, Wales and Northern Ireland: 1990-2008, compiled by AEA (Environment) for the Department for Environment, Food and Rural Affairs (DEFRA), the Scottish Government, the National Assembly for Wales and the Northern Ireland Department of Environment. In this inventory:
the figures are classified on the basis of the source of the emissions - so, for example, the Transport figures do not include a share of the emissions from the power stations that produce the electricity used by electric trains.
The figures given in the tables take account of removals of carbon dioxide as a result of Land Use, Land Use Change and Forestry (LULUCF).
4.8.2 The way in which emissions are allocated to the different countries within the UK are described in the Greenhouse Gas Inventories report. In summary, the bases of the different estimates are:
- road transport - the estimated volume of traffic on the roads within each country. The estimates for carbon dioxide are constrained so that the total for the four countries agrees with the internationally-reported overall total for the UK as a whole (which was calculated from the total volume of fuel sold within the UK);
- railways - emissions from railway locomotives in Great Britain are disaggregated based on diesel oil consumption data for passenger services and National Atmospheric Emissions Inventory (NAEI) estimates for freight services. The data used in the 2006 inventory was reported for each railway company, whose area of operation can in most cases be allocated to one of the four constituent countries;
- civil aviation - estimates of emissions from domestic aviation are calculated based on aircraft movement data from the UK's major airports. The total number of domestic flights from each of the devolved administration areas has been calculated, and based on this, a fraction of the total UK emission has been allocated to each constituent country. This approach is also used to allocate emissions from aircraft support vehicles;
- national navigation - the disaggregation of emissions from navigation and coastal shipping has been derived in a similar way to the approach used for aviation, based on port movements in each constituent country;
4.8.2 Road Transport carbon dioxide (CO2) emissions are estimated using vehicle kilometre data constrained so the sum of the UK areas equate to the total for the UK inventory (where that total is derived from fuel sales data of petrol and DERV within the UK as specified in the reporting guidelines of the Intergovernmental Panel on Climate Change). A criticism of this method is that the presentation of results does not always provide a CO2 emission trend that is directly consistent with the vehicle kilometre trend data, as the fluctuations in UK fuel data have a more significant impact on the resultant emission trends. As an alternative, road transport CO2 emissions from the constituent countries of the UK may be estimated solely by vehicle kilometre data unconstrained to the UK total derived from fuel consumption data.
4.8.3 The difference in results between the constrained and unconstrained methods at DA level largely reflects the difference in the results at UK level between bottom-up calculated fuel consumption using vehicle km data and fuel consumption factors and the fuel sales data in DUKES. The reason for a disparity has previously been attributed to cross-border fuel sales ("fuel tourism") although model uncertainty was always emphasised as an additional, and probably a major explanation for the differences.
4.8.4 Any change in the methodologies or the factors used to calculate fuel consumption will affect the magnitude of the difference between calculated fuel consumption at national level and sales figures from DUKES and so, in turn, it will affect the disparity between the DA CO2 emissions from the constrained and unconstrained approaches. The disparity has varied slightly between 1990 and 2008. For 1990, CO2 emission estimates for Scotland constrained to match UK fuel sales, were 0.3% higher than unconstrained emissions. For 2008, constrained estimates were 0.7% higher than unconstrained estimates, while for 2003 unconstrained estimates were 1.7% higher than constrained estimates.
4.9 Carbon dioxide emissions per passenger-kilometre
4.9.1 The figures are taken from the 2010 Guidelines to Defra/DECC's Conversion Factors to Company Reporting, 2010, Defra/DECC.
Figures are consistent with the factors used in the compilation of the UK's National Atmospheric Emissions Inventory (NAEI) and in the Greenhouse Gas Emissions Inventory compiled for Scotland and other constituent countries in the UK by AEA Technology Energy and Environment.
Figures are estimated using data for GB/UK as a whole and so do not relate specifically to Scotland. There are no estimates of emissions per passenger-kilometre for Scotland alone. The basis of the estimates is as follows:
- Road Transport - The factors used are estimated values for the average petrol and diesel car fleet in 2009 travelling on average trips in the UK. This has been divided by an average car occupancy rate of 1.6 passengers to calculate average emissions per passenger kilometre.
- Rail - the national rail estimate refers to an average emission factor for diesel and electric trains in 2007. The light rail and tram factors are based on an average of the annual electricity consumption and passenger kilometre data provided by network operators in 2008, and a CO2 emission factor for electricity generation on the national grid from the UK Greenhouse Gas Inventory.
- Air - the emission factor is an aggregate representation of typical CO2 emissions from illustrative types of aircraft for the three types of air services - domestic, short haul and long haul. The long haul estimate is based on a flight length from the Guidebook of 6482 km, short haul 1108km and domestic 463km. A 9% uplift factor has been applied (from IPCC Aviation) to take into account non-direct routes (i.e. non straight line) and delays/circling.
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. DfT also has a Geographical Information System (GIS) website which provides statistics of major road traffic flows for Great Britain. The website enables users to access Annual Average Daily Flows (numbers of vehicles), and traffic (thousand vehicle kms) for each major road link in Great Britain. Information can be found at www.dtf.gov.uk/matrix, alternatively contact Sophie Davies at DfT Statistics Roads 2 branch (020 7944 6599)
5.2 For enquiries about DfT's methods of estimating road traffic, contact Penny Allen of the Department for Transport (0207 944 8057).
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 Transport Scotland's Trunk Roads Network Management (0141 300 8282).
5.4 Time lost by traffic on trunk roads - see Congestion on Scottish trunk roads
http://www.transportscotland.gov.uk/strategy-and-research/publications-and-consultations/j10343c-01 or Stuart Hay of the Transport Scotland Trunk Road Network Management (Tel: 0141 300 8282).
5.5 Scottish Household Survey congestion figures - Andrew Knight of the Scottish Government Transport Statistics branch (tel: 0131 244 7256).
5.6 Scottish oil deliveries (including petrol and diesel) - see Chapter 3 and Table 3.9 of the annual DECC publication Digest of UK Energy Statistics, available on DECC's website (or tel: 020 7215 2718 email@example.com).
5.7 Petrol and diesel consumption by road traffic - see Energy Trends in June 2009 or Laura Williams of The Department of Energy and Climate Change (Tel: 0300 068 5045).
5.8 Pollutants - see Scottish Environment Statistics Online www.scotland.gov.uk/stats/envonline/menu0.asp or Sandy McPhee of The Scottish Government, Environment Statistics branch (0131 244 0445).
5.9 Carbon dioxide and other greenhouse gases emissions allocated to Scotland - Sandy McPhee of The Scottish Government, Environment Statistics (0131 244 0445).
5.10 Carbon dioxide emissions per passenger-kilometre is available from http://archive.defra.gov.uk/environment/business/reporting/conversion-factors