Tuesday, January 7, 2020


Transport is one of the main sources of CO2 emissions and it is sometimes assumed that more buses and trains are the solution. Public transport has a collective value in its own right, but public transport per se is not always the silver bullet for huge reduction in CO2 emissions. Emissions per passenger-kilometre is an imperfect metric, but we have to accept that trains and buses do not always score well. They can, with too few passengers, have rather high emissions per passenger-kilometre, even compared to private cars. Policies to reduce total transport emissions can only be developed, therefore, if full account is taken of the multiple factors in play. Public transport will have a major role but our approach needs to be holistic, and integrated with other objectives as well as climate priorities.

It’s also the case that the economics answer to most problems – reflect the issue properly in prices and all will be well – does not always fit well with transport. Fuel is already highly taxed but much travel demand is essential and price-inelastic; higher fuel prices may have limited, or insufficient, impact on the demand for private vehicle use. This implies the need to look at wider policy options. These include the unglamorous world of other regulatory and planning interventions, including congestion pricing, traffic management and town and land use planning.

How does public transport score on emissions?

It is difficult to find statistics that are reliable, up-to-date, and truly representative, but it is not hard to find reasonable if approximate indications. The comparative (average) figure for the private car[1] is 171g per person-kilometre, falling to 85g for a driver and passenger, and 44g for a driver and three passengers. For buses the numbers likewise depend on fuel used, size and age of bus, type of journey and so on. My indicative estimates[2] for buses are DEFRA based and from the website https://carbonindependent.org/ . The figure for bus performance is an average CO2 emission of 822 g / km. The DEFRA statistics allowed Carbon Independent.org to estimate the average loading of UK buses at 9.2 passengers.

In very approximate terms this means that the bus must have at least nine passengers in order to get down to 90g per person-kilometre and be able to “compete” on emissions with the private car with one passenger. Comparable results have been quoted by the US Department of Energy. Seat occupancy rates in cities may generally meet this challenge, but in many suburban or rural areas this is much less likely to be the case. That is certainly suggested by the estimates cited in Carbon Independent.org, with much higher g/km figures cited for the average of bus journeys outside London. So for some journeys the bus will be generating lower emissions than the private car, but on lightly loaded routes, often those serving more remote areas, significantly more. Prima facie this is a disappointing finding for anyone expecting a simple solution to transport emissions.

A contrasting benchmark is the London Underground, with emissions estimated at a mere 9g per person-kilometre (and reduced further as the power sector moves towards complete decarbonisation); this offers an excellent example of public transport providing very clear emissions savings. A key factor is clearly utilisation or load factor, which in turn tends to reflect population density.

Of course the public transport network is about more than just passenger-kilometre comparisons. It is often an essential for other social and economic reasons – such as inclusivity, enabling economic activity, and reducing congestion. And for the personal choice of the environmentally aware, it will be preferable to choose public transport when it is available and meets the need, since the incremental emissions will be close to zero.

What mix of policies do we need?

The answer in the long run has to include low or zero carbon fuel sources. Electric vehicles (or equivalent alternatives such as hydrogen) have the potential, ultimately, to reduce emissions to close to zero. But while technology may be the primary means to achieving the ultimate goal of zero emissions, there should also be a big premium on the large, valuable and immediate gains to be made simply by reducing emissions from the stock of vehicles in use now and in the near and medium term. If more public transport per se is not a solution for the mitigation transport emissions then it’s worth examining other options. This blog has consistently emphasised the high value of immediate[3] emissions reduction as both postponing climate milestones and providing option value for the future. Immediate and urgent actions have a higher value, tonne for tonne, than future solutions.

Two factors that are a major contributor to energy consumption are speed and congestion, for obvious reasons. Speed matters because the energy requirement tends to rise as the square of velocity, as anyone who has used a trip computer to make this comparison on the motorway will testify. Congestion matters because of the loss of energy implied in constant stopping, starting, and idling.

The remedies are relatively straightforward to describe, at least in principle, although more complex to implement. Reducing and/or more strictly enforcing motorway speed limits could have a significant and immediate impact, with an additional benefit in terms of safety. Congestion pricing, applied successfully in London and elsewhere, could be used more widely in the UK and other major cities. Both these measures continue to have their merits even in an all-electric low carbon world, and are complementary to effective planning and good public transport systems.

The search for better and low carbon transport policies will continue, but it needs to be a mix of low or zero carbon sources of energy, clever urban planning, and economic incentives to reduce congestion, as well as well-designed public transport systems.

[1] BEIS figures as quoted by the BBC and shown in my previous posting of 2nd January 2020 on this subject.
[2] The author quotes estimates from different sources for out of London and in London buses, while aggregate figures are drawn primarily from a 2007 DEFRA study. Given the changes in bus fleets, and inclusion of some gas powered or hybrid vehicles, these are arguably dated estimates. However more recent 2015 figures released for the actual performance of new Routemaster buses on London routes show significantly higher fuel consumption and hence emissions than the indicative numbers I have used.

1 comment:

Ian Campbell said...

Thanks John for an interesting article.

I've got a few comments:

1. Re your "Fuel is already highly taxed", I'm afraid that I have to disagree. To comply with the Paris agreement and the residual CO2 budget of 420Gt, we need to phase out fossil fuels at a rate of 20% per year (https://www.carbonindependent.org/93.html). I feel that we have continued to use fossil fuels many years after we should have stopped because they are so cheap, and we need now to tax them out of existence.

2. Re "much travel demand is essential", I have to disagree again, and say that most travel is unnecessary - we need to reduce the need to travel. Travel to hospitals is all that I can think of as being essential.

3. Yes, the incremental (or "marginal") emissions are the important figures - so if a bus has very few passengers, but is provided as a minimum public service, the emissions are effectively zero.

4. If buses are traveling with very few passengers, such that the emissions per passengers are similar to private cars, the operators should be thinking of replacing some of the buses by minibuses, or even by taxis on demand - I came across this in the Yorkshire Dales a few years back.