Traffic in Great Britain - quarterly bulletin

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Notes

1998

A number of changes to the contents and format of this quarterly bulletin were introduced in the issue covering the 1st Quarter of 1998
- (SB(98)3).eg:

  • Tables 1,3 and 4 (as of Quarter 1 1998) contain index numbers rather than actual values. This is to enable trends to be seen more clearly.
  • All quarterly figures in these bulletins are seasonally adjusted unless stated. This is aimed at allowing comparisons with all recent quarters, rather than just the same quarter in the previous year.
  • The quarterly figures for two-wheeled motor vehicles and buses & coaches are not shown separately, but together in the 'other vehicles' column. This is because of the unreliability of the figures.
  • The road classes shown in these bulletins have been redefined to highlight the low growth of traffic on built-up roads.

1999

  • DETR clarified the definition of rigid 2-axle goods vehicles (R2s) in 1999. Analysis of the 1999 data showed that counts in earlier years had included some light vans as heavy goods vehicles. As a result of this, 1999 estimates of R2 traffic are about 30 per cent lower than what might previously have been expected. Correspondingly, light goods van traffic estimates are about 15 per cent higher. The changes in the estimates vary by type of road and by area.
  • This clarification has brought the traffic estimates more into line with other DETR surveys of goods vehicles traffic. Data from earlier counts affected by the misclassification have been adjusted. For recent years, this has been done by reference to the differences found for 1999. For earlier years, it has been done using past data from the Continuing Survey of Road Goods Transport.

2001

  • Figures for 1999 and 2000 have been produced on a new basis and are not directly comparable with earlier figures. In Table 2, the 1999 values are shown on both the original and the new basis and quarterly figures are only shown for the period Q1 1999 onwards. For the other tables in this bulletin, the old values in index number form have been retained for the quarters prior to 2000. The latest index numbers have been derived by applying the latest year-on-year increases to the 1999 index values. The reasons for the changes are described below.
  • In 2000/2001, steps were taken to improve the quality of DTLR's major road network database used in producing traffic estimates. This was done using a geographical information system (GIS), together with Ordnance Survey (OS) data. One result of this work was that it identified inconsistencies in the road length data supplied by local authorities. Preliminary checks showed that this was mainly due to the variety of methods used by them to calculate road lengths. However, local authorities have useful local knowledge of their roads, particularly recent changes to their classification. Therefore, once the network data had been compiled, it was sent to each local authority for final checking. In some instances, this led to discussions with Government Regional Offices and the Highways Agency in order to establish the correct, up-to-date classification of roads. The end result of this extensive work was a much-improved road network database for the whole of Great Britain. It was decided to use this GIS-based information, rather than data supplied by local authorities, for grossing-up average traffic flows, in order to produce traffic estimates.
  • The road network data was further refined by information collected by DTLR traffic count contractors on the location of relevant speed limit signs. This enabled DTLR to be more confident about the built-up and non built-up lengths of each section of road. The net result of both these improvements has been a reduction in the estimates of principal road traffic, particularly on built-up roads.
  • Furthermore, DTLR began a review of the expansion factors used to convert the occasional, 12-hour manual count data to estimates of annual average daily flows. Highways Agency data strongly suggested that the expansion factors for some motorways, particularly the busiest ones in the South East and Midlands regions, were too low. Temporary adjustments to the factors have been made and these adjustments have led to a general upward revision to 1999 and 2000 motorway traffic estimates. Further work will need to be done to refine these estimates over the coming year.
  • Minor road lengths are now based on OS data, not local authority data, but this has not affected the quarterly minor road traffic figures to any extent.
  • The net result of these improvements has been little change to the estimates of total motor vehicle traffic for Great Britain for 1999 and 2000, but some changes to the composition of the overall figure. In general, the new motorway traffic estimates are now higher than before, whilst those for other major roads are lower than before.
  • The new figures for 1999 and 2000 are already considerably more accurate than those produced for earlier years. In the time available, it has not been possible to consider the changes that ought to be made to estimates prior to 1999. However, adjustments to earlier years will be carefully considered over the coming year, once the review of expansion factors is complete and the 1999 and 2000 values are finalised. It is intended to publish a full set of figures on the new basis in next years Annual Report.

2002

  • Figures for 1999 onwards have been produced on a new basis and are not directly comparable with earlier figures. In Table 2, the 1999 values are shown on both the original and the new basis and quarterly figures are only shown for the period Q1 1999 onwards. For the other tables in this bulletin, the old values in index number form have been retained for the quarters prior to 2001. The latest index numbers have been derived by applying the latest year-on-year increases to the 1999 index values. The reasons for the changes are described below.
  • In 2001/2002, steps were taken to improve the quality of DTLR's major road network database used in producing traffic estimates. This was done using a geographical information system (GIS), together with Ordnance Survey (OS) data. One result of this work was that it identified inconsistencies in the methods used by local authorities to produce road length data. Consultations with local authorities, the Highways Agency and Government Offices of the Regions helped to establish the correct, up-to-date classification of roads. The end result of this work was a much-improved road network database for the whole of Great Britain, and this GIS-based information has been used for grossing-up average traffic flows to produce traffic estimates.
  • The classification of built-up and non built-up lengths of each link are now based on information on the actual location of relevant speed signs, as provided by DTLR traffic count contractors in early 2002. Previously, traffic estimate calculations depended on local authority estimates of built-up and non built-up road lengths.
  • The net result of both these improvements has been a reduction in the estimates of principal road traffic, particularly on built-up roads.
  • Furthermore, DTLR began a review of the expansion factors used to convert the occasional, 12-hour manual count data to estimates of annual average daily flows. Highways Agency data strongly suggested that the expansion factors for some motorways, particularly the busiest ones in the South East and Midlands regions, were too low. Temporary adjustments to the factors have been made and these adjustments have led to a general upward revision to motorway traffic estimates since 1999. Further work is being done to refine these estimates.
  • Minor road lengths are now based on OS data, not local authority data, but this has not affected the quarterly minor road traffic figures.
  • The net result of these improvements has been little change to the estimates of total motor vehicle traffic for Great Britain for 1999 onwards, but some changes to the composition of the overall figure. In general, the new motorway traffic estimates are now higher than before, whilst those for other major roads are lower than before.
  • The new figures for 1999 onwards are already considerably more accurate than those produced for earlier years. In the time available, it has not been possible to consider the changes that ought to be made to estimates prior to 1999. However, adjustments to earlier years will be carefully considered over the coming year, once the review of expansion factors is complete and the 1999 and 2001 values are finalised. It is intended to publish a full set of figures on the new basis in the Annual Report to be published in July 2002.