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Understanding Transport Assessments

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Last updated - March 2015



Pages in this series

1 - Transport Assessment Introduction

2 - Trip generation

3 - Routes taken by traffic

4 - Pass by, diverted and other trips

5 - Other aspects

6 - Transport in Environmental Statements

The material on these pages is draft, and may be updated from time to time. If you do happen upon it then I hope it's useful and interesting, but please don't regard it as being anywhere near complete.

Summary

This page is an introduction to what a Transport Assessment for a development proposal is, the other pages in this series, and how these pages may help people other than professional transport consultants understand them. I can't guarantee complete accuracy or uncontestibility. My aim is to be helpful.

Who should be interested in these notes on Transport Assessments?

The material presented here is aimed at helping Planning Officers and Councillors in Local Authorities and on Planning Committees to understand Transport Assessments (TAs) that they need to assess, as well as anyone with an interest in a development that they feel may produce extra traffic on the roads. I assume that transport consultants have access to more technical material so would not need this information.

TAs can be very complex documents, but the reason is not always due to any inherent complexity of the process, but may stem from a desire on the part of the writer for readers not to fully understand them. While a good TA should be simple to read and understand there is sometimes;

  • a lot of jargon that is often used in different and confusing ways,
  • the scope for excessive detail being presented in some parts of the assessment but a curious lack of detail and jumps in the logic in other parts, and
  • the scope for quite major errors of creep in due to the hurried way in which they are written with late demands for changes by clients, and with writers not fully understanding the area they are writing about or even the complexity of what they are writing.

It has been said that transport assessment has the complexity of rocket science but the accuracy of a circus cannon. Most transport professionals baulk at the thought of unpicking the details of how a TA has been done and whether it is correct or fair. The inherent inaccuracy of prediction coupled with the knowledge that most people (including Local Authority staff and Planning Committee members) will not feel able to challenge what is written, ensures that consultants have got used to getting away with showing that some major developments will have little traffic impact.

At the moment these pages concentrate on the traffic prediction parts of a TA, because that is where there is most need for technical explanation. The 'other issues' section looks briefly at treatment of walk, cycle, public transport and the use of 'Travel Plans', but in those areas logic, commonsense and local knowledge are often adequate to question anything suspect.

What is a Transport Assessment?

A "Transport Assessment" is a document that is required to accompany any planning application for a large development. A small application requires no such assessment and medium sized ones require a "Transport Statement". Government guidance used to give more information, but at the time of writing is somewhat sparse.

The aim of a transport assessment or statement is to make an assessment of what the transport implications will be, and to show how any problems ("negative impacts") can be put right ("mitigated"). An assessment should take all modes of transport into account, and a large TA will probably include a "Travel Plan" which will include measures that can be shown to reduce impacts - these are nearly always measures to reduce car use to the site.

How is a Transport Assessment produced?

There is an entire section of the transport profession devoted to writing TAs. But they are commissioned by developers who want to see their development built. Some ethical consultancies will only take on a project where they think that the transport impacts will not be seriously negative. But the pressures on the transport consultancy industry are such that others may not be so ethical, or some may want to believe that a proposed development can 'work', especially if they are commissioned by regular clients from whom they would like more work in the future.

Never be fooled into thinking that a Transport Assessement is a 100% objective analysis. Transport consultants don't have to sign a Hippocratic Oath, and all transport consultancy is done for 'the client', whoever that may be. Having said that most transport consultants I've worked with are honest people who feel uncomfortable when asked to do something they don't think is right.

In order to gain planning permission developers need to show that the traffic impacts will not be ‘severe’, so they won’t want to pay for a TA that doesn’t back up their case. ‘Severe’ impacts (which have never been defined but which I discuss more under 'other issues') usually mean where serious extra congestion will occur at key local junctions or on local roads. The assessment of congestion (or delays at junctions) is the very last stage of the analysis, after extra traffic has been forecast. So if congestion is predicted consultants will sometimes go back in the process to alter the earlier figures until the ‘correct’ result is produced. Hence, in a sense, the tail often wags the dog.

The elements of a Transport Assessment

Since proposed developments are all different the guidance (that exists - there is little official guidance any more) says that Transport Assessments can be done in a number of ways, but most assessments for large developments are focussed on access by vehicles (because it is vehicles that are likely to cause problems needing solutions), and follow the broad structure of:

  • A description of the site, location, and what is proposed
  • A description of the current access to the site by walk, cycle, public transport and car
  • An assessment of current transport use of the site (if any)
  • A prediction of how much traffic will be generated by the site, and which routes it will use to reach the site in the local area
  • Subtracting any existing traffic from the expected traffic to calculate the extra traffic
  • Measuring how much traffic is currently using the local roads. (And if there are empty buildings, the traffic associated with what would be there is sometimes calculated and added to what currently exists - "extant" land uses)
  • Running a computer model to assess how much extra congestion would be likely to occur if the development were built
  • Coming up with proposals, usually in discussion with the Local Transport Authority (Unitary or County Council, or area wide authority in London or Metropolitan areas) to address the problems predicted, with the result usually being to provide money for the Transport Authority to put in measures such as traffic lights, bus lanes etc.

The chart below attempts to explain the traffic prediction part (third to seventh bullet points) in graphical form. Hopefully by reading what is above and looking at the chart some sense can be made of it!

Getting the 'right' result

For the traffic prediction part of a TA, most consultants will initially conduct what they feel is a fair assessment of the traffic implications. For most smaller developments it will be found that there will be few problems caused, but if there is any doubt, or if congestion is shown to the likely, consultants have plenty of scope to change assumptions and parameters in their calculations.

But even for developments with few traffic impacts there is an incentive to downplay the potential traffic levels because it will mean there is less to pay Local Authorities for mitigation.

In many cases consultants are right to downplay extra traffic. For instance, traffic to some developments will be largely due to people who are passing by anyway – a petrol station is an obvious example! Many new developments will not generate entirely new journeys but change the day to day travel patterns that people are making already. This is looked at in greater detail in other sections.

Other pages in this review

These pages look at various aspects of how Transport Assessment results can be manipulated. Separate pages (do or will, when written) look at:

  • 2 - Trip generation rates - how much traffic is likely to use the site, which may be difficult to forecast
  • 3 - Routes expected to be taken by traffic to the site - this is important because different roads will have different levels of traffic and different likelihoods of becoming congested, or dangerous
  • 4 - Assessment of the amount of traffic that is passing by or making short diversions, and (for mixed use developments) traffic that visits two or more land uses on the same trip - these can be used to correctly adjust figures, or to imply much traffic 'disappears' from the calculations
  • 5 - Other aspects such as how much traffic was, or is currently using the site, what is said in Travel Plans etc
  • 6 - A brief guide to the treatment of transport issues in Environmental Impact Assessments, or Environmental Statements, where transport and traffic are treated somewhat differently

Back to Transport Index

Gordon Stokes, 2015