The DfT and their car traffic forecasts

This graph, from the Department for Transport’s 2013 Road Transport Forecasts (which summarises the results from their National Transport Model) has been doing the rounds on social media this week.

Screen Shot 2014-11-28 at 10.38.59

It shows that the amount of distance we are travelling by car, per capita, in Britain has fallen consistently since the early-2000s; and yet their model predicts that this decline will reverse, and car miles per person will increase by 15% by 2040.

What is just as remarkable, however, is the Department for Transport’s own analysis of this graph –

Figure 16 below shows that, according to our forecast, miles per person will increase by 15% percent by 2040 (9% above pre-recession levels) despite an increase in GDP per capita of 66% and fuel cost decreasing by 24%. [my emphasis]

The key word here being ‘despite’.

The DfT believe that increases in GDP per capita, and falling fuel costs, should really push car miles per person even higher than the projected 15% increase. Coupled with a projected 20% increase in English population by 2040, the DfT are forecasting that overall road traffic will be 46% higher in 2040 than 2010.

They acknowledge that the effect of their ‘key drivers’ on road traffic levels (GDP per capita, population, and fuel prices) is becoming less elastic, as the market becomes saturated –

As explained in section 2, the elasticity of miles per person to key drivers is falling over time, and will keep falling into the future as the market moves further towards saturation.

However, they still think that this 15% rise in car miles per person will happen, principally because of falling costs per mile, meaning people will be incentivised to travel further.

This increase in miles per person [15% on 2010], however smaller than it would have been in the past, reflects the fact that people will be able to travel longer distances with their cars, as the cost per mile will decline sharply compared to ability to pay.

Whether people will actually want to do this – to spend more time stuck in cars – appears to simply be assumed.

The other interesting detail from this report is… London. This document essentially acknowledges that the National Transport Model has failed to predict that the amount of car traffic in London would fall as much as it has –

… analysis of our forecast from 2003-2010 shows that although the NTM predicts a fall in London car traffic of 1.5%, this was not as great as the actual 7.8% fall in traffic count statistics.

What’s the explanation?

We believe that the reason for this short-term model error and long-run discrepancy with other forecasts is due to:

Car Ownership – the number of cars per person in London has been relatively flat over the last decade. While we have different car ownership saturation levels for different area types, including London, these may need to be re-estimated.

Public Transport – London has seen high levels of investment in public transport, capacity and quality improvement on buses and rail based public transport. London will continue to see high levels of investment in public transport with increase in capacity into the future, e.g. Cross Rail. We will need to revisit our modelling on the impact this may have on car travel.

Road capacity, car parking space cost and availability – There is evidence to suggest that In recent years London road capacity has been significantly reduced due to bus lanes, congestion charge and other road works. There is also a significant constraint and cost to parking in London which would reduce the demand to travel by car. We will need to revisit our modelling on the impact this may have on car travel.

On each of these three factors, the DfT are admitting that their model needs to be ‘revisited’ – their model simply hasn’t correctly taken into account the effect of public transport, and reallocation of road space, on the amount of car traffic that might be on the roads.

It’s also worth noting this ‘London’ example appears to show that levels of car ownership – which the DfT tie closely with GDP per capita – might be much more strongly affected by these other two factors assessed here, public transport and use of road space. Again, a challenge for the DfT modellers.

It seems that the DfT are admitting that their model doesn’t accurately take into account factors beyond income, population and fuel costs, their ‘key drivers’ – which is hugely significant if, as is likely to be the case, urban areas (in particular) in England continue to reallocate road space to other modes of transport, and prioritise these other modes, ahead of car travel.

Certainly, planning for future growth in car travel using a model that the DfT itself admits isn’t properly reflecting other factors on car demand looks increasingly silly.

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22 Responses to The DfT and their car traffic forecasts

  1. paulc says:

    something seriously borked with their models as it consistently ignores the existing trend…

    Personally, I think they are doing this to try and justify large budget requests for their department…

  2. Dan B says:

    I find it positive that TfL are saying that a reduction of capacity has reduced demand for driving. #space4cycling

  3. KristianCyc says:

    Trouble is they use these predictions to allocate funding to projects. They do not account for any reallocation of road space, so predict that car usage will rise with the economy, then choose not to fund any applications for projects that reallocate road space and instead fund projects that increase space allocation to driving.

    All in all it’s a heads-i-win, tails-you-lose situation for the automotive/oil industries, who can always guarantee their preferred mode of transport is subsidised, prioritised and catered for.

  4. ORiordan says:

    Yeah, they aren’t going to come up with an answer that says driving is declining so you can cut the roads budget.

    It would be interesting to know if the models include the *time* spent driving. The Dft seem to say that if GDP goes up and oil is cheap, then people will drive more and more but that translates to more and more time spent stuck in a car somewhere. Generally speaking, I think there is a limit to the time most people want to spend in a car and beyond that point, they will choose to spend their time doing something else rather than keep on driving more and more.

    Another factor in London is the increasing age that people are getting driving licenses, so the proportion of people in their 20s with driving licenses is much lower than it was 10 years ago.

    As these cohorts get older, the number of people driving can be expected to decline further.

  5. ORiordan says:

    … sorry… I just skim read the post and see you have already mentioned the point regarding time spent in cars!

  6. Danny Yee says:

    I think a graph showing their *past* predictions better illustrates just how bad their modelling is

    • T.Foxglove says:

      “a graph showing their *past* predictions”

      I’ll give them 1989 GB High Forecast, as it appears that actual traffic in 1990 out performed their predicted high but since then it is just laughable.

      Almost as if they are following the teachings of George Bush: “Fool me once shame on you; I can’t be fooled again”

    • fonant says:

      These two graphs need to be combined, both to show historical traffic levels since 1970 and to show how predictions of increases in the past have been completely wrong every time.

      • Jitensha Oni says:

        Your wish is my command:

        Figure A is a quick and dirty mangling in an image app. They do seem to be improving after 25 years – they may get it right in another 25; but what it looks like is that they are somehow making the >2025 model approximately fit the 1990-2005 data, once the “2010-2025 anomaly” clears itself. There must be something flagging the current situation as an anomaly. I think we should be told.

        On the other hand all it would take to change the model output to something like Figure B, would be a change of sign from Total = (Initial + New) to Total = (Initial – New).

        Where + or – may represent pm’s aspiration

  7. Danny Yee says:

    The 1989 high forecast was roughtly right for one year, but that’s hardly a test of a 35 year forecast!

  8. fonant says:

    I think in many towns and cities the car usage is already at saturation. So unless they knock down houses and build huge urban motorways I can’t see urban car usage going up much at all.

    Private motoring is terribly inefficient for most journeys, given that most journeys are short (less than five miles) and in urban areas. This doesn’t matter when there is spare road capacity, but these days the roads are pretty full at peak times.

    In the meantime people are re-discovering the efficiencies (both for individuals and for society) of cycling and public transport usage. London motor traffic is falling because the city has spurned this inefficient and polluting mode in favour of underground trains, buses, cycling, and walking.

    One big question this “forecast” ignores is “do we really want yet more traffic on our roads?”.

  9. pm says:

    I suspect its less a forecast and more an aspiration.

    • pm says:

      …they are going to allocate funds with a view to try and bring this about, and if they don’t manage it, they’ll keep on trying.

  10. rdrf says:

    It’s silly to say that : “Certainly, planning for future growth in car travel using a model that the DfT itself admits isn’t properly reflecting other factors on car demand looks increasingly silly.”

    Because it is not a question of “silly”. Forecasting by the DfT and its antecedents has ALWAYS been about accommodating and colluding with increasing motor vehicle use. Take a look at John Adams work such as “Transport Planning: Vision and Practice” from decades ago, along with his critiques of Cost Benefit Analysis – another numbers trick set up to support more motorisation.

    Because of this some green academics like John Whitelegg have advocated “backcasting” – wherein you work out what you want in he future and then work out what to do to achieve it.

    Either way, the thing is to examine the ideological underpinnings. For example – why should motoring become cheaper ? Even without other costs going up, shouldn’t motoring have become more expensive? Shouldn’t traffic law be enforced (which means that a larger number of those not prepared to/able to drive properly wouldn’t be out there in the first place)? And how about reducing car parking at origins and destinations? Or reallocating road space to more benign modes (which, incidentally, I haven’t noticed happening very much in London).

  11. cyclestrian says:

    DfT should relabel the left axis as hours per year spent in a car, or even % of waking hours spent in a car.

  12. Can I just note that the NTM is simply a bit of arithmetic based on what has happened in recent years. What I recall from reading the estimates a while ago is that they do contain an exp[licit statement to the effect that increased car use is a best-guess at what might happen in the next few years if no body or no circumstances do anything about it.

    Blame lies with Ministers and their “advisers”. It is their job to imagine the future and make it better. If they wanted to they could try to understand and use the NTM forecasts to justify radical (or gradual) measures to shift away from our fatal dependency on motor vehicles.

    Poking fun at statisticians is good fun, but it does lead to more honest critics having their own data rubbished in unkind ways.

  13. I have just discovered a gem! This guy is suggesting we need to build more roads because… car travel *isn’t* and won’t increase!

  14. ‘Rebalancing Britain: from HS2 Towards a National Transport Strategy’ -sounds interesting. It popped into my inbox the other day. I know it’s mainly about making better use of the rail network, but, I thought I’d have a look at their vision for cycling since it’s a ‘National Transport Strategy’. Yes, you’re right, not a single mention. Shame really, as one of the key problems with rail travel is getting to and from the station at either end -just where you might think bikes could make travelling by rail become more convenient.

    Click to access Rebalancing%20Britain.pdf

  15. Pingback: Cycling in middle England – going nowhere fast | As Easy As Riding A Bike

  16. Pingback: Comments: DfT’s car-use forecasts – June 2016 and As Easy As Riding A Bike blog November 2014

  17. Pingback: More on: ‘DfT and their car traffic forecasts’ (2014) with rdrf comment

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