Summary

The Urban Dynamic Model (UDM) is a simulation of how transport interacts with population, employment and land-use over long periods of time, typically ten years or more.

It was developed to help understand how transport could contribute to economic regeneration by improving the ability of employers to recruit a workforce and their access to customers and suppliers, and by improving access to employment opportunities.

Advanced techniques for modelling transport have been available for many years. However it is the use of system dynamics which makes the UDM model unique in the market. System Dynamics provides the means to capture the wide dynamics of the real world in a way that just is not possible with more traditional models. For example, the vicious circle of congestion is familiar to us all but is almost impossible to capture in traditional models. It looks like this:

• improved transport reduces travel times and costs;

• this makes business easier, increasing the recruitment catchment of employers and their connections with important customers and suppliers;

• this generates increased employment and even increased population as people migrate in to take advantage of the rising employment;

• this generates more transport activity;

• and this increases congestion of the roads and crowding on public transport, putting up travel times and costs once again.

Once we consider the wider context of transport in society and the economy many more of these circles appear, some vicious, some benign. Whichever they are, System Dynamics provides the language and tools for describing and modelling them.

What was done

The UDM was developed incrementally over a number of years, beginning with a fairly simple model built with the Ithink modelling package in 2000. This was used in one of a series of large scale transport studies commissioned by the government at the time, known as the Multi-Modal Studies. Its role at that time was to test the regeneration claims being made for a proposed by-pass that would have passed through an area of outstanding natural beauty.

Models of this type must be able to represent the spatial characteristics of the study area, usually using a zone structure, and the road and public transport links connecting the zones together. This requires significant computing power, and after the first study the model was completely rebuilt in Vensim, which offers substantially improved power and analytic tools.

Other applications followed, and then in 2005 we were commissioned by the Department for transport to carry out a study into the impact of transport on business location decisions. A customised version of the UDM was used, and this study allowed us to carry out calibration of many of the parameters in the model that had been hard to estimate previously.

Current guidance from the Department for Transport emphasises the wider role of transport in supporting the economy, but also points to the need for reducing carbon dioxide emissions. The UDM fits the bill as a support tool in this context, and is perfectly suited to studying the balance between transport, the economy, population, and CO2.

Results

The UDM is generic, and has been applied widely in the UK to help local authorities develop integrated transport and land-use strategies. Conclusions vary from one application to another, but some of the findings we have reported include:

• measures to generate employment can appear to have a significant effect if examined very locally, but we often find that some employment is transferred from elsewhere, and the net effect can be less than might first appear;

• there is, not surprisingly, a tension between the desire to generate jobs and employment and to reduce CO2 emissions;

• changes in land use can have a very big impact that can often dominate the effects of transport;

• city centres remain the best locations for many types of employer because they provide the best access to a workforce and bring ‘agglomeration’ benefits that employers enjoy by being clustered together.

Value

Studies of major programmes of transport investment typically involve expenditure of tens or hundreds of millions of pounds. The measured benefits are usually expressed in money, and expressing both the social and economic impacts of investment and can also run to hundreds of millions of pounds.

Contact and further information

John Swanson:

Steer Davies Gleave
28-32 Upper Ground
London SE1 9PD
+44 (0) 20 7910 5542
email john.swanson (omitted ‘at’ to prevent spam) sdgworld.net