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 130 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. 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. Any sites affected by long term road works are excluded. These speeds will not be the same as average speeds over whole journeys or sections of the road network. 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. The sample of locations is small and the results are subject to sampling error. A description of the survey including measurement errors is given at Annex A. The sampling errors of average speeds are given at Annex C. (A)
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