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

Proposed Development

Indicate the year of opening. Indicate opening of phases and final opening, if applicable.
Include a copy of the planning application and reference previous applications on the same site (as appendices).
Provide a tabular breakdown of the Gross Floor Area (GFA) for land use/uses. Retail Floor Area should be listed if applicable and available.
Provide times/details of shift working arrangements.
Clearly indicate the total number of car parking spaces to be provided by numbering.
Provide a site layout plan (minimum scale 1:500) of the development showing proposed access arrangements, local junctions and existing site details. This may be based on a topographical survey or an OS base.

Modal Choice/Trip Attraction

For trip attraction the TRICS Datacard should not be used other than for a very preliminary estimate of use. In calculating trip rates considerable care should be taken to ensure that the correct parameter settings are chosen (e.g. for a shopping centre with a petrol filling station, ensure that shopping centres without filling stations are omitted). A list of de-selected sites should be included in the TIA, together with justification for the sites chosen.
To ensure a robust assessment, the 85th percentile generation figure should be employed in the analysis of the traffic impact. In certain circumstances, and with justification, a lesser figure may be acceptable, but this will require agreement with the local Roads Service Division.
One recurring difficulty in relation to traffic generation relates to whether local data or global data from databases should be used. Local data has its advantages in that it is more likely to reflect the particular, unique circumstances in Northern Ireland. On the other hand a larger database will have information from a considerable number of sites thus allowing a more credible generation figure to be calculated (e.g. 85th percentile). Weighing up the local data against global data can be difficult. The developer should present a reasoned argument within the TIA on the traffic generation figure to be employed. Reference to rates previously accepted will not normally be taken as fair justification for their use. The need for a robust analysis of the impact of the development proposal is considered vital.

Trip Distribution

Distribution and assignment should be separate sections as recommended in the IHT Guidelines. Distribution is an important stage in the assessment as the impacts on links and functions follow from it.
A distribution consistent with that used for a Retail Impact Assessment should be used.
To assist distribution calculations it may be useful to carry out origin and destination surveys at a local site similar to the proposed development. Use of existing local turning movements or volumes to estimate a distribution for a new development is not recommended in the Guidelines.
The application of universal and generalised discounting factors should be avoided. In this regard there has been considerable research, presented at TRICS conferences and available from other sources. Care should be taken to ensure that all research is presented in the proper context and is directly related to the development proposal.
It should be emphasised that travel distance isochrones (contours of equal travel time), appropriate to the size of the development, should be applied.
It should be emphasised that detailed consideration should also be given to the components used in a gravity model, e.g. the exponent for time should be a function of the development’s nature and catchment’s transportation characteristics.
Sensitivity tests will enable Roads Service to ensure that different case scenarios are tested and evaluated and that upper and lower bounds for highway performance are established. It may also be necessary to carry out sensitivity testing on different variables used in a gravity model.

Trip Assignment

It should be emphasised that assignment methods should be clearly separated from distribution. The method used should be stated, e.g. manual or network model.

Assessment Years

Retail information should be provided to determine if there is growth in development traffic.

Highway Impact

Consultants should identify the time period with the greatest overall traffic flows i.e. consider weekday am/pm peaks, Saturday peak and development peak as necessary. Calculations should be clearly shown and choices stated.
There should be a logical sequence of traffic flow diagrams and a percentage impact drawing for individual movements (with calculations shown). Fonts used in traffic flow diagrams should be sufficiently large to allow the numbers and text to be clearly read.
Validation of modelling is required.
At an early stage Consultants should make Roads Service aware of their views on congestion, or the lack of it, on links in the study area. Roads Service will consider their views and provide clear guidance on this issue.
1:500 drawings detailing the geometric parameters at existing/proposed junctions, should be submitted in support of junction testing. Existing and proposed details should be clearly shown to enable the extent of the works to be clearly seen and to show that proposals are achievable and constructable, within the land available. It is not sufficient to show only kerb lines; footways and future boundaries are needed.
Junction testing with direct data option using short input time periods may be required, where the TRL suite of computer programs is used.
Detailed conclusions should be drawn from all junction testing. RFCs, queues (average and maximum) and delay information (average and maximum) should all be used to this end.
Consultants should confirm that design standards applied to roads and junctions associated with developments meet Design Manual for Roads and Bridges (DMRB) Standards. Proposals for departures and relaxations from standards should be made clear and shall have been approved in accordance with Roads Service procedures.

Road Safety

The PSNI provides limited road accident details. When the scope of the TIA is being discussed, consultants will be made aware of proposals for minor works, traffic management, accident remedial, traffic calming, bus priority, cycle provision, etc. This information may be of relevance to safety considerations.

Internal Layout

Good access road penetration of sites is necessary to avoid queues from developments interfering with traffic on public roads.
The swept paths of specific HGVs should be shown on drawings.

Parking Layout

Parking layouts at 1:500 scale must be provided with the TIA. The spaces on the drawing must be counted and labelled.
Clear tabular breakdowns of the number of parking spaces being provided are required.
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