Applicants are strongly advised to seek clarification using the TfL pre-application advice service before starting to develop traffic and highway modelling so that a programme for review and audit can be agreed with the relevant parts of TfL. It should be noted that depending on the specifics of the proposal and modelling required this process can be lengthy - therefore engagement at the earliest opportunity is advised. TfL may also require the cost of model reviews to be covered, depending on the scale of the assessment.

Guidance on modelling

Model scoping

The following areas should be discussed and agreed with TfL:

  • Geographical and temporal scope of modelling, including time periods and assessment years
  • Approach to traffic growth and cumulative impact assessment
  • Committed highway schemes to be included
  • Software to be used eg stand alone junctions, network models (LINSIG, TRANSYT), micro simulation (VISSIM) etc
  • Survey specification and data collection (to be undertaken by the applicant)
  • Data relevant to survey days will be provided by TfL (signal timings, timing sheets etc)
  • TfL may have existing models that are suitable for use - these can often be provided to third parties. However, if any models are provided it will be the responsibility of the third party to ensure these are fit for purpose

Base Modelling

Following agreement of the model scope, the following steps should be undertaken:

  • Base models to be produced by the applicant
  • Base models will need to be calibrated and validated in line with TfL modelling guidance
  • Base models submitted to TfL for technical audit with any supporting information (scaled site layouts, survey results validation report etc)
  • Models must be submitted electronically, and allowing enough time for TfL to be able to respond (to be agreed on a case-by-case basis)
  • TfL will provide feedback on base models. Amendments to base models may be required and should be made and agreed before progressing to proposed model stage

Proposed Modelling

Following sign off of the base models by TfL, the proposed models can then be produced:

  • Base models amended to produce version controlled proposed models
  • Future year base models may also be required, taking into account committed development, planned highway schemes and traffic growth
  • Proposed models should then include agreed trip generation and distribution, any further growth assumptions and proposed changes to the highway layout
  • Proposed models submitted to TfL for technical audit with any supporting information (scaled site layouts, trip generation and distribution assumptions etc) allowing enough time to respond
  • TfL will provide feedback on proposed models. Amendments to proposed models may be required, and should be made and agreed before progressing to impact assessment stage

Impact Assessment

  • Impact of proposals on traffic journey time, reliability and predictability will be assessed by TfL. The criteria used for assessment will relate to the modelling software used
  • Mitigation of unacceptable impacts will be required and should be developed by the applicant. Depending on the nature of the mitigation, the impacts on the wider network may also need to be assessed at this stage
  • TfL reserves the right to ask for sensitivity testing of the model using alternative trip rates or distributions to ensure any conclusions drawn are robust
  • Testing of different highways options may be necessary at this stage to determine the effectiveness of different interventions

Mitigation of development impacts

Where mitigation is required to make acceptable development which would otherwise be unacceptable in planning terms, it is expected that this will be secured in full at no cost to TfL. There will normally be a requirement on a developer to enter into a Section 106 agreement, and where necessary a Section 278 agreement, to deliver highways improvements.

However it is acknowledged that in some circumstances, and in particular in areas where a large amount of growth is anticipated, it may not be realistic to expect a single development to deliver all the highways mitigation required. In these instances funding may be secured through limited pooling of contributions, Section 106 agreements or other mechanisms designed to secure money for transport infrastructure necessary to support development in the defined area. Importantly, it will be necessary to consider whether any restrictions on the occupation of developments prior to mitigation being delivered are necessary.

For more details see the Mitigation and Implementation section.

Implementation and TMA approvals

If a development is granted planning permission, any changes to the highway network for which TfL is responsible will require a scheme approval from TfL. This approval will consider the impacts of the proposals on all modes in line with TfL's network management duty. These webpages include guidance on the process for entering into a Section 278 agreement, but where alterations to traffic signals are proposed as part of Section 278 works, the scheme will need to be incorporated as part of TfL's existing work programme.

TfL will audit the design of new signals and any associated modelling, and produce a Traffic Signals Supplementary Report (TSSR), which will recommend whether or not a scheme should proceed. This process is covered in greater detail in both TfL's Traffic Modelling Guidelines and the Model Auditing Process Design Engineer Guide:

If the approach to modelling outlined above is adopted this will facilitate the TSSR process - it has the potential to save both time and money for the developer and for TfL. However, if the data used for the models at the time of TSSR submission is greater than 18 months old, TfL reserves the right to ask for data and assumptions to be checked or new surveys to be carried out as appropriate to ensure that any modelling reflects prevailing traffic conditions.

Growth Areas, highway assessment

In some cases development proposals will be of a much larger scale or will form a significant part of an Opportunity Area (or Growth Area) as identified in the London Plan. In these cases the developer will be expected to undergo more extensive pre-application discussions with TfL and will require use of TfL's strategic highway and public transport models as set out in the Strategic Impact Assessment section. In such cases the impact on both public transport and highways will need to be assessed and balanced including the appropriate weight that should be given when using different models.