Rethinking Zero Carbon Urban Form in the Arc

The Oxford Cambridge Arc Project gives us an ideal opportunity to rethink appropriate urban forms of Garden Communities.

From previous government announcements on the Arc the predominant thinking seems to be an approach of having a ‘few big blobs’ of development along a rail corridor. Once you start to adapt evaluation methodologies to zero carbon reality it is clear that this is no longer appropriate – let me explain.

The two key drivers in evaluation methodologies are as follows:

  1. Regional Economic models which evaluate urban and industrial economies of aggregation;
  2. Cost Benefit Ratio evaluation of transport options.

The first is indisputable, once you accept that the urban area is a functional city region not just a single blob.

The second is problematic once you consider the fixed constraint that zero carbon planning provides.

Look at rail. New rail has a high upfront fixed cost (track, land etc.) and a moderate variable cost per passenger (trains, labour) and an almost zero marginal cost once you have the track and trains. Which is why it makes sense once you have the track and trains to subsidise the journey.

However CBA is based on the evaluation of the economic cost of time. The more trains stop the longer it takes to get to a destination. This means that the CBR of an express limited stop service will always be lower than a short stop commuter service. You add more passengers as you increase capacity but the benefit in terms of time per passenger goes down, so the CBR is always lower. This is clearly influential in the planning of East West Rail for example, where across the whole length of the project it will only have two new stations and net less stations than at present. Options such as four tracking will always have a lower CBR than dual tracking, diesal will always have a higher CBR than electrified services.

The problem with CBA often is calculating the counterfactual ‘do nothing’ scenario. Once you exclude positive carbon as a policy option this transforms the decision space. The cost of global warming is incalculable. Therefore CBR should only be applied to alternative zero or negative carbon options.

Applying this principle to new settlement/rail options you would have to exclude express service/big blob new settlements. The housing at the edge of the settlement would be car based and the time taken for interchange too high. Options need to be based around high frequency, high capacity short stop services around routes with high density development within walking distance of stations, like classic train and streetcar suburbs.

This is not to say you should have ridiculously short distances between stations, such as the current Marston Vale pattern sensibly proposed for rationalisation. Dutch planning teaches us that if you have good cycle infrastructure to stations you can extend distances between stations. On Marston Vale for Example you could have high density development around fewer stations and employment areas in between, which is certainly not what you find in current local plan proposals.

Once you understand this you gravitate towards a polycentric urban form in a string of pearls arrangement. You manage the higher frequency services through a combination of interleaved stopping services, passing loops and ideally 4 tracking. Also as recent reforms to the Green Book indicate CBR is not the only criteria, you also need o consider policy objectives such as the ability to move people in a zero carbon network. I often find a ‘headway obession’ (headway is time to turn around trians), in uk transport planning, which is haven’t found for example working with the Japanese, where ability to move the masses is always the priority.

This polycentric urban form was of course advocated by Fred Pooley in his pre development corporation plan for Milton Keynes. Set aside the silly monorails it is still a good pln, the best ever unbuilt plan in the UK. So new settlements in the Arc need to be Polycentric, between 50-150,000 in population based around networks in multiple stops. By contrast all we seem to have in the Arc is two big blobs at Tempsford and North of Cambourne.

Indeed once you start looking at CBR for rail and BRT in the ARC new options open up. It would even be worthwhile I think to split the central section South of Sandy, one route North and one Route South of the Sandy Hills both converging at Shenford South of Cambridge, the reason being that this opens up large development opportunities in the southern corridor, plans for which I have published before.

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