“Other Things Being Equal” A Spell That Binds Us To The Past

In the first of an occasional series of musings, BCyC member Sam Saunders explains why the Department for Transport could be misleading us into thinking that an exponential growth in car use is inevitable.

“Other Things Being Equal” is a phrase I first heard from economists in the Latin form “ceteris paribus”. It’s a note of warning that (for example) when the price of umbrellas goes up, sales of umbrellas go down, other things being equal. It explicitly does not rule out the possibility that extra rainfall could make sales can go up, despite the price increase.

The need to remember the “other things being equal”rule can be seen in some of the muddled reactions to the recently published Road Transport Forecasts 2013: Results from the Department for Transport’s National Transport Model (DfT 2013).

The headline figure from the DfT’s 35 page document has been that motor vehicle traffic on the strategic road network will increase by 45.6% by 2040. The non-strategic roads (the ones paid for by local authorities like Bristol) will have to cope with an increase of 41.5%. These numbers have been subjected to some pretty strong criticism in cycling circles.

The forecasters working on the National Transport Model (NTM) did explain that their projections are based on economic and demographic variables that are known to have influenced the growth in motor vehicle use over recent decades. It must be said, though, that they also explain that their predictions assume a number of other things staying the same. Changes in these “sources of uncertainty” would affect the predictions. Figure 9 gives the picture:

(copied from DfT paper p.15)

Six things in that scenario could change and each of them has the potential to make the prediction unreliable. So what the NTM report actually does say is not contradicted at all by the following statement:

“If Government policy changes, the trend in traffic growth could be reversed.”


There might be good reasons why Government would not want traffic volumes to actually decline. A lot of livelihoods are tied up in selling more motor vehicles and a lot of tax revenue might, on balance, be lost. Nevertheless, anyone looking at streets in any city would have to agree that there is a strong practical case for not letting congestion, danger, pollution, noise, or the loss health-giving  activity levels get any worse than they already are.


Queens Road mid-morning in Bristol

As a cycling organisation, the Bristol Cycling Campaign is not anti car, van, or lorry. What our strategy document suggests is that we have already got too much of a good thing and its sheer volume is making life difficult. A sustainable future is not being planned for at national level and the city is struggling to work against the national tide. The prospect of a 40% increase in traffic on top of an already unmanageable load (let alone the NTM’s high-side estimate of 60%) is bleak. Central and local government need to put more political energy into changing the default traffic policy. The sorts of new policy we need are already set out in the Bristol Cycling Campaign’s strategy document:

“Excessive and inappropriate motor vehicle use must be made less convenient, and fairly priced, e.g. through congestion charging and parking management schemes. Integration with public transport must be made as easy as possible. Development control policies must provide for high levels of cycling, and rigorously applied. A danger reduction strategy to make our roads free from fear and harm must be followed. Transport planning models must ensure cycling is properly valued.” 


In a city like Bristol it is no longer possible to plan for more walking and cycling without also encouraging a reduction in the use of motor vehicles. If policy does change then the predictions of the National Transport Model no longer hold. That’s why the idea of “other things being equal” is so important. If the arguments of sustainable transport groups, from the Bristol Cycling Campaign up to national and international movements are accepted then everything else can change.

Look at this graph showing relative changes in the mileage covered by different types of road user in Bristol since the year 2000.

Index of changes in miles travelled by different transport modes in Bristol 2000-2012 (data source DfT)

Bristol, to its great credit, has already pursued an active policy of encouraging cycling. The amount of cycling has doubled over 12 years. Unlike the Department for Transport, Bristol has been building cycle infrastructure as money allowed and has looked for funds to support more. Cycle mileage could increase by another 200% in the next decade and space on the roads for essential motor vehicles could increase.

My point is that the National Transport Model, in its measured and non-confrontational way, has said very clearly:

If you don’t like the look of the future of traffic, you could change government policy.

You can’t really blame the statisticians for pointing at what is most likely to happen if you don’t make policy changes. They are hired hands honestly doing what they were asked to do. Models have to be based on past experience but sometimes we have to break away from the past. Other things remaining equal is not a good idea when the past seems to have driven us into a bit of a pickle.

For more musings please take a look at Sam Saunders’ blog samsaundersbristol which, amongst other things, includes a fascinating series on Considerate Cycling.