The Theory of Smart Growth -How to Test if a Partial Roll Back of the Green Belt is Good for Growth and Sustainability

There has been much discussion in recent weeks on whether a partial roll back of the Green Belt on sites without other designations would help economic growth and meet London and the SE’s housing needs.  For example Centre for Cities, Adam Smith Institute and the London Society.

Whatever you think of the idea it is good that the debate has moved on from a crude abolish the Green Belt or roll it back from the M25 approach which is where it stood three years ago.  Now at last think think tanks have discovered GIS and are looking at actual geographical differences.

Id like to suggest a sensible step forward in working out whether or not this makes economic and sustainability sense or would just contribute to unsustainable sprawl.  In doing so Id like to apply the broken city model used in an earlier post. In this model the urban economies of aggregation rise with city population, but the urban diseconomies of size rise with city size.   The diseconomies rise can be offset with funding of public transport and other infrastructure – but once there the scale of disecenomies exceeds the economies their is no longer an economic case to physically expand the city.

Eventually a city will grow to the point at which physical expansion no longer makes sense, though you can still grow the city’s population through increasing its density and investing in public transport and other infrastructure.  There will be increasing returns to scale with density and city size and the benefits of public transport investment, until transport carrying capacity is reached, at which point you need to start doubling or tripling up the public transport network and there is a large threshold infrastructure cost.  At some point it might make no sense to continue to densify or expand the original city – but rather to expand other cities or towns below their optimum size instead.

It helps explain why some cities such as Jakarta and Mexico City have dramatically slowed growth, they have reached the point where they have become ‘broken’, and why others such as Tokyo and Moscow have slowed down, because public transport investment has not kept up, whilst other that continue to invest, such as Beijing and Singapore continue to grow.

This insight is a combination of :

A) the New Economic Geography of Krugman and Others with its insights on city size.

B) The well known ‘Henry George Theorom’ in Regional Science on the optimal tax level on land value uplifts from public transport investment

C) The Insight from transport economics on the relationship between city density and public transport costs

D) Threshold Analysis theory

The beauty of this approach is it allows you to test various alternative policy options such as  densification, expansion and Garden Cities, it can be extended to cover loss of Ecosystem Services and the hedonic valuation of the ordinary countryside and newly created open spaces.

What the theory seems to predict is the best way to expand a city is through Smart Growth, densifying, expanding around transport nodes with spare capacity and expanding along similar nodes or in new settlements along such nodes.  If such nodes are beyond reasonable commuting distance the theory predicts you will get greater benefits from expanding employment in other housing markets and diverting housing growth and public transport investment to those areas.

If a Green Belt site near a public transport node has limited capacity and this is prohibitive to expand the theory suggest it is better to grow either within or beyond the Green Belt providing you can grow employment in those locations, otherwise the diseconomies of sprawl with outweigh the economies of aggregation.

4 thoughts on “The Theory of Smart Growth -How to Test if a Partial Roll Back of the Green Belt is Good for Growth and Sustainability

  1. CPRE briefing on Green Belts gives some advice:-
    ‘Influencing Local Plans so that as little Green Belt land as necessary is released for development. You can make the case that, as Green Belt boundaries should only be altered in ‘exceptional circumstances’, no more than one or two sites should be released for development. It can be helpful to evaluate the cumulative effects of any proposed changes on the overall openness and integrity of the Green Belt, as well as assessing the five purposes individually.’

    The question is what methodology should be used to assess the five purposes of the Green Belt. Here in Chester, the study was done by ARUP.

    ‘The purpose of the study is to provide an independent, technical assessment of 10 Green Belt Areas previously defined by the Council, before evaluating the resultant land parcels against the purposes of the Green Belt as defined by national policy.
    The outcome of the study will be used by the Council, alongside their wider evidence base and objectives for growth, to inform decisions on options for releasing land from the Green Belt through the local plan making process. ‘

    Ten parcels of Green Belt land around Chester were appraised:-

    The Arup report considered ten areas of land around Chester for the five purposes of the Green Belt.

    Link to Examination Document Library – The Green Belt Study is document LEB33:

    http://consult.cheshirewestandchester.gov.uk/portal/cwc_ldf/cwc_lp/submission/submission?tab=files

  2. Pingback: Questions to Ask Anyone with a Hardline Pro or Anti Green Belt Voew | Decisions, Decisions, Decisions

  3. Pingback: Most of the Solution to the Housing Crisis Lies Outside the Green Belt – A Response to Martin Wolf | Decisions, Decisions, Decisions

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