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Published estimates

There are a number of published estimates. All seem to refer purely to the CO2 emissions from the fuel used.

Bus Travel:

g / km	g / mile	Source	Notes
60	100	NRF [1]
89	140	Defra [2]	(June 07) the most detailed source
89	140	DfT [3]	used in the Defra calculator, based on [2]
90	140	Hillman [4]	(London buses)
90	140	Lynas [5]	(London buses)
110	170	[6]
160	250	Hillman [4]	(outside London)
170	270	Lynas [5]	(outside London)

Figures taken from the original source are shown in plain text; Figures after conversion are in italics.


Coach travel:

g / km	g / mile	Source	Notes
28	40	Goodall [7]
50	80	[6]
80	130	Hillman [4]
80	130	Lynas [5]
89	140	[8]	referring to National Atmospheric Emissions Inventory, and unusually making no distinction from buses

Figures taken from the original source are shown in plain text; Figures after conversion are in italics.

Key source used

The source used as the basis for the Carbon Independent calculator is the Defra document [9]. This calculates the average CO2 emission from UK buses as 0.089 kg per passenger km.

Some key points are:

• the CO2 emissions relate purely to fuel use
• the average passengers per bus is 9.2 (from 5.2 billion vehicle km and 48 billion passenger km)
• the average CO2 emission for the buses is 822 g / km
• the total UK vehicle km by buses is 5.2 billion per year [giving a total CO2 emission of 4.3 million tonnes, i.e. 0.07 tonnes per person per year]

Basis for calculations

As with other modes of vehicle traffic, there should be some consideration of CO2 emissions from:

• energy used in oil exploration and refining
• construction of vehicles and their maintenance
• construction of roads and their maintenance
• running costs of the organisation e.g. heating of offices, and staff business travel. But this seems to be minimal in comparison with vehicle emissions since Merseytravel (source [10]) gives a total of 126 tonnes per year for the bus and train travel authority for Merseyside (population 1.4 million), i.e. less than 90 g per person per year, which is very much smaller than the UK average of 70 kg per person per year for fuel CO2 emissions from buses.

There is also the philosophical problem of how to allocate responsibility for CO2 emissions. Consider a passenger boarding a bus that is empty apart from the driver. The typical bus produces about 1.3kg CO2 per km travelled (National Express data quoted by Goodall [7]). Should this passenger be counted as being responsible for all of the 1.3kg CO2 per km travelled? It does not seem right that those individuals determined to support an infrequent bus service, or having no alternative, should be penalised by being awarded a higher carbon footprint than someone driving and definitely increasing carbon emissions, and also adding to road congestion and noise.

On the other hand, the bus was probably travelling the route whether the passenger got on it or not, so it could be argued that the CO2 emissions are the same whether or not the passenger gets on the bus, and so the emissions should count as zero.

To answer this dilemma, we need to consider what would happen over a period of time as more and more people get on the bus and use the bus service. It is true that initially the carbon emissions are unaltered, but as the bus becomes more and more full on a regular basis, eventually the bus company will put on a larger bus or increase the frequency of the service. So rather than looking at the extra emissions from one passenger, we need to find the average increase in CO2 emissions from many additional passengers (which we might term the average marginal cost). To make a simple analogy, if we are interested in the cost of the tea in a cup of tea made from a teabag, we don't take it to be the cost of a whole packet of teabags, and nor do we take it to be zero (if the packet of teabags is already open) - instead we take the cost of a whole packet and divide it by the number of teabags. It is just the same with buses (and coaches, trains, and planes etc).

So we should calculate CO2 costs per passenger assuming occupancy levels that are large enough for additional services to be contemplated. Occupancy levels in city centres are an indication of this, and in one survey of city centre bus occupancy (four cities in Scotland [11]), occupancy varied from 40% in the later morning to 80% in the morning peak. Clearly, many less popular and off-peak services run at occupancy levels much less than this, and this is why the average bus occupancy is only 9 passengers. On routes where occupancy levels are low, the driving force for the provision of bus services is not just the fares paid by the passengers but the decision by society that a certain minimum level of bus service must be provided, which is funded by a subsidy if necessary. It is the governments (local and/or national), acting on behalf of the citizens, who specify this minimum level of service, and who are consequently responsible for some of the carbon use - i.e. the part that is not accounted for by the passengers using the service - and this level of CO2 emissions should be added to that of other services provided by governments such as an ambulance service and so on, rather than being averaged over all bus passengers.

So we should partition the 4.3 million tonnes CO2 emissions from UK buses between a fixed total (the minimum acceptable bus service) and a rate per passenger km.

A reasonable approach would seem to be to take 1.8 million tonnes for the fixed total (0.03 tonnes per person) plus 2.5 million tonnes at a rate of 50g per passenger km, i.e. 80 g per passenger mile.

There then needs to be some adjustment for the fossil fuel used in oil exploration and refining (which is overall around 85% efficient - see car emissions page), and for bus manufacture and maintenance.

So the figure of 80 g / mile is adjusted to 100 g / mile.

Coach travel is taken to be similar.

References

[1]	National Energy Foundation http://www.nef.org.uk/greencompany/co2calculator.htm (viewed 26.1.08)

[2]	DEFRA (June 2007) Passenger transport emissions factors: Methodology paper http://www.defra.gov.uk/environment/business/envrp/pdf/passenger-transport.pdf

[3]	Carbon Emission Assumptions (Sep 2007) UK Department for Transport journey planner http://www.transportdirect.info/Web/Downloads/TransportDirectCO2Data.pdf

[4]	Hillman M, (2006) Personal carbon allowances British Medical Journal 332 1387-1388

[5]	Mark Lynas (2007) Carbon Counter (Collins)

[6]	http://www.carbonfootprint.com/results.php (viewed 13.8.06)

[7]	Chris Goodall (2007) (Earthscan)

[8]	Carbon Emission Assumptions (Sep 2007) UK Department for Transport journey planner http://www.transportdirect.info/Web/Downloads/TransportDirectCO2Data.pdf

[9]	DEFRA (June 2007) Passenger transport emissions factors: Methodology paper http://www.defra.gov.uk/environment/business/envrp/pdf/passenger-transport.pdf

[10]	[21] Merseytravel (2006) Environmental Sustainability Report 2005/2006 http://www.merseytravel.gov.uk/pdf/AnnualReport-2006.pdf

[11]	www.scottishexecutive.gov.uk/Publications/2003/12/18622/29900

First published: 2007
Last updated: 16 Sep 2023