National administrations in England, Scotland and Wales are responsible for setting speed limits on motorways and trunk roads. Local authorities have the power to impose or vary speed limits on principal roads and on all other local roads. In order to monitor the compliance of drivers with these speed limits the Department collects speed data from traffic counting sites around Great Britain. The Department monitors traffic speeds and traffic levels at about 190 sites throughout Great Britain using automatic traffic counters. These are generally situated away from junctions, hills or sharp bends, at locations where traffic is likely to be free flowing and not near speed cameras. Thus, in principle, they provide information on the speeds at which drivers choose to travel when their behaviour is not constrained by congestion or other road conditions. These statistics do not indicate average traffic speeds across the road network. Any sites affected by long term road works are excluded. Of course incidents or congested conditions may arise at any site, and when this happens it will reduce speeds below those in free flowing conditions. This is not believed to have a significant effect on the figures published in this bulletin. In this bulletin the term ‘built-up’ is used to describe roads to which a 30 mph or 40 mph limit applies; the term ‘non-built-up’ describes other roads. The total number of vehicle records which were processed to produce 2005 statistics was about 860 million. In the earlier years of this survey only a sample of records were analysed but from 2002 onwards, all vehicle records recorded at each site were included (except for a few very slow-moving vehicles indicating that conditions at the site were congested). A description of the survey including estimates of the accuracy of the measurement of speed is given at Annex A. (Author/publisher)
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