§ Mr. Key
In calculating future road traffic patterns for road schemes, the starting point is a good understanding of the existing patterns of movement. This is usually obtained by roadside interviewing supported by manual and automatic traffic counts.
Working from this basis, the future pattern of movements is derived by application of growth factors. To ensure consistency with other appraisals, these are constrained at the district level to centrally produced estimates which are, in turn, constrained to agree with the national road traffic forecasts. Within these constraints, analysts are encouraged to take account of local variations in growth, as a result of new development, and so on. The key variables involved in these calculations include projections of the numbers of households, numbers of workers, and numbers of jobs.
Calculation of future traffic flows on roads—with and without schemes—is critically dependent upon the characteristics of the road network. The process of determining the routes that travellers take through the network depends upon the nature of the network, with congested urban networks requiring more complex methods than relatively uncongested inter-urban networks. Typically, relevant network characteristics can include 835W carriageway type and width, hilliness and bends, junction types and geometric characteristics, and the operation of traffic signals.
The methods available for calculating future road traffic patterns and projections are described in detail in the Department's "Traffic Appraisal Manual". A short summary document is also available. These are available through the House of Commons Library.