How Does Scheme Appraisal Perform in Real Life?
P Richards, C Shipley, Atkins, UK; V Kovacevic, Highways Agency, UK
Before and after study evidence to assess accuracy of traffic model forecasts and lessons for future appraisals.
The UK?s Highways Agency programme of investment in Trunk Roads and motorways is known as the Targeted Programme of Improvements (TPI), and 78 schemes are on this list since it was announced in 1998, and around a third of these have now been opened.
Since 2001, the Highways Agency and Atkins have been evaluating scheme information from this programme to see if the predicted benefits for each road scheme have actually occurred after opening. Atkins now has a sample of over 25 schemes of different types and it clear that a range of outcomes is being shown. This paper provides an overview of actual results against predictions, paying particular attention to the accuracy of the predictions of traffic volume movements after opening.
During the appraisal stage of schemes, it is usual that modelling techniques are used to show how many vehicles will use a new road, as well as predicting relief from roads around the area. These predictions are fundamental in the justification of these schemes, as they directly influence the predictions of economic benefit, as well as environmental sub objectives such as noise and air quality, and as such accurate modelling is clearly an important part of the justification of the scheme.
Now that a reasonable sample size is being obtained, conclusions can be drawn on a number of issues, including:
? Are the Modelling techniques resulting in accurate predictions?
? Are there any lessons from these evaluations that should be applied to future appraisals?
? Is the appraisal of road schemes being compromised by the appraisal methods?
This paper will present answers to all of these questions, and therefore is an important review of how the UK justifies its road building programme.
Association for European Transport