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Transport Assessment
Appendix D - Supplementary Guidance Relating to Traffic Impact Analysis (Page 1 of 2)

The Following Brief Notes Relating to various aspects of Traffic Impact Analysis (TIA) have been prepared to assist Developers and Consultants in understanding Roads Service’s Technical Requirements.


The purpose of this Guidance Note is:
  • To supplement the IHT Guidelines.
  • To provide information that will assist the completion of TIA.
  • To indicate where Roads Service will require those preparing TIA to adopt variations from the IHT Guidelines to reflect local conditions.
  • To provide consultants and developers with information that will assist them in understanding how Roads Service deals with TIA.

Structure of TIA

Consultants should structure their TIA using the headings in Chapter 3 of the IHT Guidelines. In addition the TIA should contain the following:
  • An introduction which should include the Planning Application number to which the TIA relates, the past planning history of the site and a reference to any meetings with Roads Service about the application.
  • A section giving the conclusions of the TIA. This should refer to any measures required to mitigate against the effects of the development. In particular the consultant should list what works/measures the developer is providing. Conclusions should include any assumptions used and have clear cross-references to the main body of the TIA.
  • Appendices containing drawings, tables, computer analysis, output, etc., all of which should be in a logical order and cross-referenced to the main body of the TIA.

Submission of TIA

TIA when submitted must be through Planning Service and will be an element of the Transport Assessment process. Developers or their consultants should, in addition, submit three copies directly to Roads Service. Additional copies may be required.
In addition to hard copies of documentation submitted, Roads Service encourages consultants to submit a copy of the document in electronic format (Microsoft Word format is preferred) to assist evaluation.
Computer modelling should be submitted in electronic format in a print file format. With programs such as LINSIG, sketches of lane labelling, etc. must be included.

Contact Information

The initial contact with Roads Service regarding a new Planning Application should always be with the relevant Development Control Section. The Development Control Officer dealing with the application may then direct the developer or his agent to another officer who will deal with the aspects relating to the TIA for the application. Contacts can be discussed at the same time as the scope of the TIA.
Development Control Officers deal with the internal layouts, number of parking spaces, sightlines and DC planning history for the site/area. As such it can be seen that the Development Control Officer is an important first contact and can discuss fundamental planning issues before further work is instigated. TIA should, of course include proposals, explanations and layouts, number of parking spaces etc.
Development Control Officers can explain the stages and staff involved in assessing TIA in simple terms. The Planning Service role can also be explained.

Amended TIA

Amendments may be required to TIA. As TIA is a supporting document to a planning application the public must have access to any amendments. In order to facilitate this, consultants should, where necessary, submit new sections and a revised conclusion or a new complete stand alone document containing all the alterations amendments, additions and background papers. These should include clear references to an original document. Roads Service will confirm when it is necessary to submit information through Planning Service.

Existing Conditions

Traffic Information

Roads Service is prepared to accept count information less than three years old, providing a location has not experienced significant development or infrastructure change.
Traffic surveys should not generally be carried out in July, August and the last two weeks of December or during other holiday periods. Surveys should usually be carried out between 07:45 and 09:15, and between 16:45 and 18:15. Data should be recorded every 15 minutes and notes made at relevant periods of queues, interaction of junctions, usage of pedestrian facilities, weather, site specific data (eg doctor’s surgery at end of crossing) etc.
A summary of the base survey information should be provided and must include dates, times and methods used. Weekday surveys should normally be carried out on Tuesdays, Wednesdays and Thursdays. A Friday survey may also be necessary if the development is for retailing.
Roads Service may have “proposed works” in the area that could, for example, affect traffic survey information. Contact with the appropriate Section Office is advised.

Descriptions and Drawings

It is vital to have a good description and drawings of the existing traffic situation on all links and junctions within the area of influence. Information on pedestrians (including the ambulant disabled), public transport, cycling and commercial traffic should be included as well as identification of reserve capacities, interaction of junctions, queues (average and maximum), delays, critical links/movements and junctions. Base survey information, including geometry, should be included in the TIA. Times of peak hours should be clearly calculated and stated.
Fonts used in traffic flow diagrams should be sufficiently large to allow the numbers and text to be clearly read.
It is vital that drawings included for unmodified junctions in the area of influence have sufficient detail to allow decisions to be taken. 1/1250 scale is sugggested as the absolute minimum.

Accident Information

Accident statistics for the area (drawing conclusions) are required. See Road Safety section.
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