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Guide to Transport Assessment in Scotland

APPENDIX D - TRANSPORT ASSESSMENT AND TRAFFIC IMPACT ASSESSMENT COMPARED

1. Traffic Impact Assessments (TIAs) were designed to ensure that the traffic impacts associated with a new development would be accommodated, by providing the extra capacity to the road system (e.g. additional lane width or junction capacity enhancement) to ensure that congestion would not arise in the immediate area as a result of the proposal.

2. The guidelines for Traffic Impact Assessment (published by the Institution of Highways & Transportation in 1994, http://www.iht.org/) provided a step-by-step approach to carrying out a TIA. Based on a standard set of tools involving traffic counts, junction turning counts, forecasting of traffic likely to be generated by the development, and analysis of the effectiveness of different options to increase capacity, it provided a relatively straightforward but technical method.

3. Comparing TIAs with the wider Transport Assessment approach it is apparent that there are many differences between the two. These can be summarised as:

• Predict and provide focus - The TIA guidance was developed within the "predict and provide" paradigm, while the Transport Assessment process is designed to ensure that development is in keeping with sustainability principles. The TIA guidelines did not attempt to encourage measures designed to change travel behaviour. A Transport Assessment, however, is intended to help change travel behaviour and reduce car dependency.
• Based on past experience - The recommended method in TIAs gives great weight to the use of "comparable" developments to determine likely trip rates, using databases of recent developments (e.g. TRICS). This use of "comparables" assumes that the conditions pertaining to past developments continue to be acceptable. As a result provision for traffic access and parking is likely to be replicated in future developments. This should no longer be the case, with policy guidance now emphasising the need for very different travel and transport outcomes.
• Traffic or transport focus - TIA guidance relates in the main to assessing the traffic impact of development proposals and accommodating it, rather than to influencing the travel or mode split outcomes. It therefore refers to vehicle trips, whereas the emphasis in a Transport Assessment should be on person trips.
• Modes considered - In TIAs, modes of travel other than the car are not emphasised, and are seen as relevant mainly in terms of provision of infrastructure. The Transport Assessment process will require more weight to be given to encouraging, as well as making possible the use of, alternatives to the car.

4. Table D.1 below summarises the main differences between TIAs and Transport Assessments. While Transport Assessments must cover all modes of transport from a person trip perspective with the emphasis on walking, cycling and public transport, TIAs generally concentrate on car trips and ignore all but traffic impacts. Transport Assessments however, involve the consideration of a wider range of criteria, based on the new Scottish Transport Appraisal Guidance (STAG) (Scottish Executive, 2001) approach which centres on the five objectives arising from the transport White Paper of: environment, economy, safety, integration and accessibility.

5. Most importantly it is in dealing with the negative transport impacts of development that TIAs and Transport Assessments differ. Whereas a TIA will aim to achieve a no net detriment to the integrity of the road network through enhancements to road infrastructure, e.g. an increase in road space, including junction capacity, a Transport Assessment will start from the basis of planning for desirable mode shares and will propose a package of measures designed to give access to the site while reducing the role of car access as much as possible. Such measures may include the preparation of a Travel Plan, fiscal incentives to discourage car travel to the site and infrastructure improvements to make walking and cycling more attractive or public transport service improvements.

TABLE D.1: TIAs AND TRANSPORT ASSESSMENT COMPARED

6. Some aspects of the TIA approach will be relevant to Transport Assessments nevertheless, since many new developments will result in increased traffic in the immediate locality which still needs to be addressed. Issues relating to parking provision, road safety, possibly junction and link capacity will therefore need to be addressed within a Transport Assessment. The main difference is that the options for 'mitigation' are wider (involving restraint, provision for other modes, or acceptance of reduced capacity etc.) rather than simply increasing highway capacity to meet forecast demand.

7. In summary, the TIA approach forms part of a much broader consideration within a Transport Assessment. Transport Assessments are concerned with access to developments by all modes, with sustainable modes taking as great a share as is feasible. But there will always be a requirement for vehicular access, and although reduced from a car-dominant approach, TIA techniques will still be of value in the assessment of the residual traffic effects of developments after all measures to encourage sustainable transport have been taken.

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