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Transport Assessments: development of guidelines

Development of best practice guidelines for the production of Transport Assessments in England and Wales. Difficulties in reconciling differing views on what should go into the guidelines delayed this project. Meanwhile, Steer Davies Gleave were asked to produce Transport Assessment guidelines for Scotland. These contained the same guidance (although with less detail than had been prepared for the England and Wales project) and were published quickly. In practical terms, this greatly reduced the need for the English guidelines, which were never published as a result of this project.

The problem with "residual demand"

The aim of transport assessments should be to keep the generation of motorised traffic from new developments to a low level. Ideally, there should be an aspirational level within an acceptable maximum. The common practice of TAs, however, is to

  1. Calculate (often using dubious assumptions or comparitors) the amount of traffic that would be generated if no attempts were made to reduce it;
  2. Set out some measures to reduce traffic generation (pay for a bus for one year, reduce parking below full provision etc.);
  3. Calculate the reduced traffic generation resulting from these measures;
  4. Calculate the "residual demand", and provide roads and parking to accommodate it.

The problem with this approach is that it is based on a starting point of full car dependence, and the residual demand usually will end up being much higher than what should be regarded as an acceptable maximum.

The starting point should not be car dependence, but  the acceptable maximum, based on strong aspirations for the use of non-car modes, or benchmarked from other locations. The bare minimum aspiration should be traffic generation no greater than that from existing similar or neighbouring developments.
Date(s)1999 - 2001
Client(s)Department of the Environment, Transport and the Regions (DETR)
Team(s)Tim Pharoah with Steer Davies Gleave and Llewelyn Davies


Transport assessment, guidelines, TAs, transport assessments, best practice

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