Studies into the Impact of Transport Projects

[DynamicSpirit]

I'm trying to find out more about the impact of transport schemes.

The particular context is that I live in SE London, where certain local authorities and TfL are trying to get several new road crossings built over the Thames. I'm personally very concerned about these, to the point where I'm likely to get involved in local groups campaigning against them. But I'd also like to make sure I have understood the issues - to the point where I can offer constructive, well-informed critiques of the schemes and of possible alternatives.

Hence the question: Can anyone who knows about this kind of area point me to the right places to look, or to particular articles or books to read? I want stuff that is seriously researched, and data that is unbiased. (I'm not particularly interested in opinions by individuals or pressure groups that are not backed up by reasonable evidence or research).

I'm interested in anything that might be relevant to building proposed Thames crossings, including:

• Details of traffic in the area - where it goes to/from, including usage of the Dartford and Blackwall tunnels.
• Case studies and research into the impact of similar schemes - or new roads or railways in general - elsewhere.
• Stuff about the causes and effect of induced traffic.
• Research into the impact (social and economic) of schemes like this on communities.
• Anything else likely to be relevant.
• Methodologies for projecting traffic levels (I'd love to know how TfL have done their forecasts for how they think the proposed road crossings would affect traffic levels)

Any suggestions or (informed) advice appreciated!

 

[radamfi]

To appraise a new scheme, a traffic model is usually built. For highway schemes in the UK, the SATURN package from Atkins is normally used. That involves building the network of nodes and links representing the highway network and a trip matrix representing the current or future demand. Then the trip matrix is "assigned" to the network, and the modelling software then shows how much traffic is on each link and details of queueing, traffic speed etc. A "base" model is created first, to try and replicate existing conditions, and then the matrix is "growthed" to represent future years. TfL may have its own methodology for growthing matrices but there is always the TEMPRO program which anyone can download which shows expected growth rates for all districts in the country:

https://www.gov.uk/government/collections/tempro

Trip matrix development is highly complex but essentially involves creating a "prior matrix" which is derived from surveys, such as roadside interview surveys and household interview services. No matter how detailed your survey is, it will still have holes in it, so a "matrix estimation" process is normally used where existing traffic counts are mixed with the "prior matrix" to create a matrix that when used in assignment creates a validated base model.

However, often an existing model is used as a starting point for matrix development and TfL have built HAM traffic models covering various regions of London, so for river crossings they could take extracts from the HAM models.

In the old days, you used to have a "fixed trip" matrix, which assumed that traffic demand stayed the same after building a scheme, but there was the SACTRA recommendations in the 90s which stated that in certain circumstances induced traffic would occur. So today's models can have a "variable demand" component to modify the growthed future year matrix. This may or may not take account of public transport services.

There will be a "do minimum" case, where a model is built assuming no scheme is built (but taking into account other committed schemes) and a "do something", a model representing a scenario including the scheme.

Appraisal of traffic schemes is usually conducted following WebTAG guidance which can be downloaded here:

https://www.gov.uk/guidance/transport-analysis-guidance-webtag

and the specific advice for variable demand modelling is here:

https://www.gov.uk/government/publications/webtag-tag-unit-m2-variable-demand-modelling

When a traffic scheme goes through to public inquiry, the objectors often hire a transport modelling expert who will attempt to find flaws in the modelling process and departures from the WebTAG guidance.

There are numerous traffic modelling books but I've never read any of them! As well as WebTAG above, the SATURN manual could be useful, particularly the early chapters which give an overview of the modelling process which is similar regardless of package. Don't worry about understanding the equations. Most practitioners don't understand them anyway!

http://www.saturnsoftware.co.uk/saturnmanual/chapters.html

 

[Busaholic]

In a nutshell, the problem is nobody has any real idea of how many people/cars would use any new crossing until it's built, and that would be partly dependant on the level of charges. Look at the overwhelming success of the East London Line once incorporated into London Overground, completely unpredicted by anyone.
On a personal level, as one who grew up in the then Borough of Woolwich I happen to feel very strongly that any replacement of the Free Ferry should be on the same basis i.e. financed through taxation, not at point of use, but I expect that argument will hold no sway.

 

[radamfi]

In a nutshell, the problem is nobody has any real idea of how many people/cars would use any new crossing until it's built, and that would be partly dependant on the level of charges.

Stated preference surveys can be carried out but demand forecasting is always a tricky business. For example, the DfT's national road traffic forecasts were embarrassingly wrong because they assumed that road traffic would continue increase as long as real GDP increases. They didn't bank on road traffic flat-lining or falling from around 2002 onwards, especially in London where it peaked in the early 90s.

 

[telstarbox]

TfL's Traffic Modelling Guidelines may be of interest too - on their website.

 

[edwin_m]

To appraise a new scheme, a traffic model is usually built...

There are also multi-modal models which include all the public transport links as well as roads. Many transport authorities have a contract with a consultant to build and maintain a model of their region, which can then be used relatively (relatively!) easily to test a new proposed transport link. So transport proposals in those areas are usually tested via the multi-modal model, unless they are relatively simple such as incremental enhancements to train services where the rail-only MOIRA is usually sufficient.

 

[radamfi]

There are also multi-modal models which include all the public transport links as well as roads.

There are some software suites such as Cube and Omnitrans which are genuinely multi-modal, however it is often preferred to model the highway and public transport in separate packages as multi-modal suites lack features that are available in dedicated highway and public transport packages. In those cases, a demand model would be employed in order to split the trips between road and public transport.