The correct approach while drafting a Transport Assessment is to ensure that improvements to sustainable modes of transport are always considered first, with measures to increase the capacity of roads or junctions only considered at the end of the process to mitigate the residual traffic impact of the development.
Transport Assessments identify the impact of the development in ‘person trips’, which are then broken down by transport mode. This normally involves the use of the nationally accepted TRICS database that enables the calculation of person/vehicle trips rates for different forms of development. Alternatively the use of bespoke survey data from other development sites are used, which can give a more accurate estimate of trip generation if a suitably representative site can be found in close proximity to the development site being considered.
Once development trips have been estimated, the distribution of trips can be predicted though various techniques, including gravity models or using existing travel pattern information or other empirical data. Information is also gathered on any existing person/vehicle trips from the existing use of the site and these are discounted from the network, to produce a net increase or decrease in trips within agreed assessment periods, which are usually considered during weekday AM/PM and weekend peak hour periods.
Once the net trip predictions are known, the local transport network is examined to establish whether capacity exists that can adequately accommodate development trips. When assessing the traffic impact on the highway network, this usually requires assessment of the immediate highway links and junction connections, with the scope of assessment limited to the highway network where the potential for material impact is anticipated. The traffic assessment will potentially include junction modelling of traffic movements in existing, opening and future design year scenarios, which are normally at least 5 years hence. Surveys of existing traffic flows on the local network are required to establish the base situation, with traffic growth factors applied to predict future design year conditions, derived through the National Trip End Model (NTEM) forecasts or other empirical data.
Where the traffic assessment identifies that the level of additional traffic movements cannot be accommodated by existing infrastructure, then off site highway improvements need to be considered to ensure that the impact of the development is neutral, i.e. that it is no worse than existing circumstances. Similarly pedestrian/cycle routes and public transport facilities are examined and if improvements are necessary to accommodate development trips, then improvements need to be considered to these facilities to encourage access by sustainable modes.
The operational safety of a development is a key consideration and a detailed appraisal of the recorded personal injury accident records for the local highway network is usually required, determining casualty, linked factors and common problems that may need to be addressed as part of the development.
The assessment concludes with a summary of its findings and recommendations outlining the developments compliance with National Planning Policy Framework (NPPF 2012) and Local Plans.
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