About The STHC Carbon Footprint Calculations
From a sustainable travel perspective, calculating the carbon footprints of the journeys to school is the next logical in the process once we have worked out the distances travelled. However though the theory is straightforward, and officially maintained carbon footprint calculation factors are readily available, there's more to it than just "doing the math"! Also with the collection of pupil usual mode of travel data from schools becoming anything but straightforward over the last few years, we have had to ensure that there's a lot more to the STHC analysis output than just carbon footprint calculations.
Scroll down to find out more about the background behind our carbon footprint factors, the factors we use in our calculations, how they need to be normalised for different modes, how they have evolved over time since the STHC began and a discussion about their use.
Background
Once the length of a journey to school has been calculated, the next logical step from a sustainable travel analysis perspective is to calculate the carbon footprint. The maths is straightforward enough:-
Carbon Footprint For Mode (kg CO2) = Distance Travelled (km) x Carbon Footprint Factor For Mode (kg CO2 emitted per km)
However before we get our calculator out, we need to consider the limitations of our analysis:-
- Our distance analysis is "straight line, as the crow flies", using the pupils home postcode centroid as the starting point rather than their actual address. In other words we don't know the length of the actual journey on the ground to a great degree of accuracy.
- For pupils coming by car, we don't know what the parents do after they have dropped the pupil(s) at the school. Do they go straight back home again? Do they go on to other destinations like work or the shops, in which case the whole journey shouldn't be attributed to the 'school run'.
- Also we don't know what sort of cars they are. Big engined cars or small engined cars? Petrol cars or diesel cars or hybrid / electric cars?
- "Half way house" modes like "park and stride", that will obviously reduce the carbon footprint of the car journey and we know occur in some schools, can't be recorded in the school information management system (SIMS).
- For pupils travelling by public bus or school bus, we don't know what proportion of their journey is actually taken up with walking to / from the bus stop at either end.
In other words not everything is known, or known as accurately as we would ideally like, so we shouldn't get too hung up on the absolute accuracy of our carbon footprint calculations. As with other indicators in the STHC, the most important thing about it is that it enables relative comparison between modes and schools over time.
With that in mind we can see that, just calculating the total CO2 emissions for a school, in of itself does not tell the whole story. More pupils means more total travel distance, means more CO2 emissions, right?
In order to be able to meaningfully compare schools of different sizes we need to calculate a "per pupil average" carbon footprint for all pupils at the school, not just those that travel by CO2 emitting modes. That way we can make a "like for like" comparison.
As well as this basic STHC Sustainable Travel Indicator, there are other questions we want our carbon footprint analysis to answer:-
- For the pupils travelling by car in any given school, what are the relative proportions of pupil numbers, distance travelled and carbon footprint compared to the overall total for the school? For example in one STHC school there are 88 pupils travelling by car (10.4% of all pupils), collectively travelling 332 km (15.7% of the total 'school run' distance for the school), emitting 68 kg of CO2 (37.6% of the total carbon footprint of the total 'school run' for the school).
- All forms of motorised travel emit CO2. Yes some less than others on an individual, per passenger basis, but how does this play out at the 'whole school' level? For example does a high proportion of pupils travelling by dedicated school bus from a long distance away to one school (eg. from the edge of a big town to a rural school), produce a similar overall per pupil carbon footprint as a school with fewer pupils coming by car from not as far away and the rest walking?
Of course all of the above is only relevant, and possible, if we have usual mode of travel data for the pupils that we have calculated the journey distances for. As this is increasingly impossible to guarantee for any individual school or pupil, even in authorities that still recognise its strategic value and want to still collect it, we need to ensure that the STHC analysis output still provides useful intelligence information even in its absence.
Carbon Footprint Factors Used
So what factors do we use in our calculations? We want to use "definitive" figures so the results are as "definitive" as possible and directly comparable.
We have always used the official UK Government “Greenhouse Gas Conversion Factors For Company Reporting”, researched and published by the UK Department for the Environment, Food and Rural Affairs (DEFRA), since 2002. We "standardised" on the factors published in 2007, which was when the STHC expanded out to become a nationally available service. The factors are as follows:-
| Mode of Travel | CO2 Emission Factor Per km | Reference |
|---|---|---|
 Car or Van(CAR) |
0.2042 kg CO2/vehicular km converted to 0.2042 kg CO2/passenger km |
Table 6 – Combined average for petrol & diesel cars |
 Car Share(CRS) |
0.2042 kg CO2/vehicular km converted to 0.1021 kg CO2/passenger km |
Table 6 – Combined average for petrol & diesel cars |
 Public Service Bus(PSB)  Dedicated School Bus(DSB)  Bus Type Not Known(BNK) |
0.0891 kg CO2/passenger km | Table 8 – Bus |
 Train(TRN) |
0.0602 kg CO2/passenger km | Table 8 – National rail |
 Metro/Tram/Light Rail(MTL) |
0.0650 kg CO2/passenger km | Table 8 – Light rail & tram |
 London Underground(LUL) | 0.0526 kg CO2/passenger km | Table 8 – London Underground |
Source: “Greenhouse Gas Conversion Factors For Company Reporting”, DEFRA (2007). (URL: http://webarchive.nationalarchives.gov.uk/20130402151656/http://archive.defra.gov.uk/environment/business/reporting/pdf/conversion-factors.pdf))
Normalising Carbon Footprint Factors For Different Modes
One issue to be wary of when using factors to calculate carbon footprints is that they are expressed in different ways depending on the mode. In terms of the STHC analysis, they come in 2 forms:-
- Vehicle types like Cars / Vans (CAR) - these are expressed as kg CO2 per vehicular kilometre. They will also be further broken down by fuel type eg. petrol, diesel, LPG etc., or at least an "average" for that vehicle type.
- Passenger carrying modes such as Public Service Bus (PSB) or Train (TRN) - these are expressed as kg CO2 per passenger kilometre.
In order for us to do "like for like" carbon footprint calculations, we need to convert the factor for Cars / Vans (CAR) into a "per passenger kilometre" one. In order to do that, we need to make assumptions about the number of passenegers in the vehicle:-
- Mode recorded as Car / Van (CAR) - We have to assume only one pupil passenger in the vehicle, and therefore the kg CO2 per vehicular kilometre factor is equivelant to kg CO2 per passenger kilometre.
- Mode recorded as Car Share (CRS) - We know there are at least 2 passengers but there may be more, we just don't know. Therefore erring on the side of caution, we have assumed only 2 passengers and therefore each one assumes 50% of the carbon footprint of the journey. Thus the kg CO2 per passenger kilometre factor is equivelant to 50% of the kg CO2 per vehicular kilometre.
It's the best that can be done given the limitations of the travel data we have, and at least it begins to refine the analysis to reflect the reality on the ground.
Evolution Of Carbon Footprint Factors Over Time
As you can see from the official page of UK Government “Greenhouse Gas Conversion Factors For Company Reporting”:-
- Figures were first published in 2002 and have been republished every year since.
- However the page linked to was only published for the first time in 2013 as part of the new, single, all-government website GOV.UK. Prior to that factors were published on the DEFRA website in not such a user-friendly, "all years in one place" kind of way.
- the science behind determining and measuring all the individual factors involved in greenhouse gas emissions is constantly being developed and improved over time.
- DEFRA have republished their list of factors every year in response to new research, however not every factor is revised every year. and some have been replaced all together with others as new insight is gained.
- Factors related to travel are only one contribution to greenhouse gas emissions, so most of the publications concern the likes of power generation etc.
The STHC first began as a generally available service to all local authorities in 2006/07, so the factors published in 2007 were the ones first used in the STHC output. However:-
- we needed to be able to continue to do "like-for-like" comparison between analysis years.
- we didn't have the technical capability to be able to produce different carbon footprint calculations using different years factors to enable users to "compare and contrast".
Thus we "standardised" on the 2007 figure in our STHC carbon footprint calculations to enable like for like comparison. With the STHC Data Portal it is now technically possible to use more than one factor in the output so users can compare and contrast. The ability to do so is on our portal development road map.
Discussion
CO2 emissions are very much a longer term sustainable school travel indicator because, although the concept is fairly easy to understand - the further you drive the more CO2 is produced - tackling the issue is not so straight forward.
All forms of motorised travel emit CO2. Yes some less than others theoretically on a per passenger basis, but it all depends on how many pupils are travelling by that mode and how long the journey is as to whether or not it's "sustainabile". We can only work that out through a combination of spatial analysis and pupil usual mode of travel data.
Reducing total CO2 emissions will require sustained action by the school community over a longer period of time, supported by continued STHC analysis resources.
Carbon footprint calculation is not an exact science, but if we assume that any shortcomings are not biased in favour of any particular mode, it can give stakeholders a crucial insight into the underlying "sustainability" of travel to their schools, which distance analysis alone cannot provide. But only if pupil usual mode of travel data is collected and made available for analysis of course ....