Annual road traffic estimates
Annual road traffic estimates are mainly based on around ten thousand manual counts, which are combined with Automatic Traffic Counters (ATC) data and road lengths to produce overall estimates.
Manual counts take place on major roads (motorways and ‘A’ roads) and minor roads (‘B’,’C’ and unclassified) where trained enumerators count vehicles of each of eleven types (pedal cycles, two-wheeled motor vehicles, cars and taxis, buses and coaches, light vans, and six separate categories of goods vehicle) for the 12 hours from 7am to 7pm.
Traffic data are also collected continuously from a national network of around 180 ATCs. In addition to counting traffic, the ATCs also record some of the physical properties of passing vehicles (which are used to classify traffic by type) and vehicle speeds. Data on the weights of passing vehicles have also been gathered from a small number of these sites.
The total activity of traffic on the road network in Great Britain is measured in vehicle miles/kilometres. Different procedures are used to calculate traffic estimates for major and minor roads:
Major road traffic: The Department produces estimates of annual average daily flow (AADF) for each link of the major road network. They are produced using 12-hour manual data counts from a large number of sites and traffic profiles derived from automatic counters at about 190 sites.
Traffic estimates are calculated for each link of the network by multiplying the AADF by the corresponding length of road factored up by the numbers of days in the year. Therefore, a major road link of length 2 miles with an AADF of 50,000 has a traffic figure of 100,000 vehicle miles (2*50,000). This equates to 36.5 million vehicle miles a year. Because every major road link is counted, in principle, total traffic on major roads can be obtained by summing the traffic figures for every link.
Minor road traffic: In the base year, for each minor road class in each local authority, an AADF is estimated based on a sample of traffic counts, including those projected forward from counts done in earlier years. These AADFs are then multiplied by the total road length for the relevant minor road category to give an estimate of traffic for that road category. Traffic for the latest year is obtained by calculating changes in traffic flows from a sample of minor road manual counts.
Frequency: Calendar Year