User guide – Road traffic

Notes and definitions

Department for Transport traffic estimates

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 5.1 describes the method which the Department for Transport (DfT) used to produce the estimates for 1993 onwards. The method used prior to this is explained in the Road Traffic chapter of earlier versions of this publication. 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 5.6 describes the methods used.

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.

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.

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.

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).

Traffic flows at selected sites

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.

Traffic on specific trunk road routes: average time lost

Table 5.8 in previous editions of STS provided estimates of the time lost by traffic on particular routes. Due to a number of reasons including major changes to the network which would have required a substantial rework to the methodology, this table is no longer being updated.

Estimated consumption of petrol and diesel

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.

Sources

Method of estimating major road traffic volumes for 1993 onwards

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).

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.

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.

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.

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.

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).

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.

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.

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.

Method used to estimate traffic on minor roads for 1993 onwards

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.

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.

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 fewer 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.

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.

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.

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).

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.

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.

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.

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.

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.

Average time lost by traffic on specific trunk road routes

Scottish Household Survey

The Scottish Household Survey is a large household level survey run in Scotland. Data is collected on a range of topics including transport and travel. The survey also includes a Travel Diary component. This data is used to analyse travel patterns and choices.

Estimated consumption of petrol and diesel

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 5.10.

Further information

Within Scottish Transport Statistics, further information can be found in:

Chapter 1 – Road vehicles,
Chapter 4 – Road network
Chapter 6 – Road casualties
Chapter 11 – Personal Travel chapter (including travel to work)
Chapter 13 – Environment and Emissions

Other Transport Scotland Publications:

Transport and Travel in Scotland – includes more detailed analysis of SHS data, in particular:

Table 8 – Effects of traffic congestion
Table 5 – concessionary pass possession
Table 21 – Park and ride
Table 28 – Frequency of bus and train use
Tables 29 and 30 – Views on local buses and trains
Tables 31 and 32 – Concessionary pass use

Scottish Household Survey Travel Diary, published as part of Transport and Travel in Scotland – includes detailed tables using the Travel Diary dataset, in particular:

Table 2 – journeys by mode of transport
Table 2a – journey distance by mode of transport
Table 4a – mode of transport by journey distance
Table 5a – distance summary statistics by mode of transport

SHS Local Authority Results, published as part of Transport and Travel in Scotland – provide breakdowns of SHS data by Local Authority, Regional Transport Partnership and Urban Rural Classification. In particular:

Table 6 – Congestion delays
Table 16 – Proportion of journeys by mode of transport
Table 18 – Travel day
Table 19 and 20 – Distance travelled

The Department for Transport produces a number of related publications:

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