Chapter 18 Forecasting future conditions

The objective of the transportation planning process, as it relates to highway schemes and improvements, is to understand how proposed changes and interventions in the highway network will operate once implemented and what the ongoing economic and environmental impacts of the changes could be in future years. Forecasting the usage of highway and transport networks in the future in terms of traffic flows, public transport patronage and so on, together with performance levels such as journey times and crowding, is a critical component in any transport appraisal.

The concept of ‘predict and provide’, where essentially unrestrained demand for car travel was used to justify major highway schemes, has largely been abandoned in favour of a variable demand approach. This means that the impact of any transport policy or intervention, including new highway schemes, may only be assessed satisfactorily with a robust forecasting methodology which takes into account the link between transport capacity and the demand for travel. The outputs of the forecasting process form the basis of the subsequent operational, economic and environmental appraisal of the proposed highway scheme.