The emissions methodology is based upon emissions factors which describe the rate at which various forms of passenger transport emit greenhouse gases in relation to distance travelled. These emissions factors represent emissions associated with individual passengers, which can be contrasted with similar transport-related emissions factors which describe emissions on the basis of entire vehicles or per-unit-of-freight.
Emissions - conventionally expressed in terms of mass (e.g. kg) - are calculated by simply multiplying these rates (mass emitted per distance per passenger; e.g. kg CO2 per km per passenger) by a distance travelled (e.g km). The emissions associated with multiple passengers can be obtained by simply multiplying by the appropriate number of passengers.
The rate at which passenger transportation produces greenhouse gas emissions varies with the mode of transport, depending on factors such as the fuel efficiency (i.e. the distance acheived per unit of fuel consumed) of the particular type of vehicle (e.g. bus, plane, train) as well as the passenger loading. Fuel efficiency may be related factors such as the type of fuel used (e.g. petrol, diesel, biofuel, electricity) and the physical dynamics of the transportation context (i.e. road, rail, water, air). The loading of passengers indicates the extent to which the emissions of the entire vehicle can be 'shared' between individuals. Therefore, emissions factors for a broad range of generalised passenger transport scenarios are provided.
A total of 22 specific types of passenger transport scenarios are represented and are differentiated by transport type (i.e. bus, rail, ferry, plane, taxi) and subtype (e.g. 'underground' versus 'local' rail, 'domestic' versus 'short-haul' flights). In those cases where individual passengers occupy dissimilar fractions of the vehicle (e.g. 'economy' versus 'first class' air travel; foot passengers versus car-based passengers travelling by ferry), additional differentiation into particular passenger classes is made in order to resolve the disporportionate attribution of emissions on this basis.
Each passenger transport type is represented by six distinct emissions factors which differentiate greenhouse gas emissions into the following types:
- direct CO2 emissions: carbon dioxide emissions produced during travel, i.e. fuel combustion
- direct CH4 emissions: methane emissions produced during travel, i.e. fuel combustion
- direct N2O emissions: nitrous oxide emissions produced during travel, i.e. fuel combustion
- total direct emissions: all direct emissions, i.e. CO2 + CH4 + N2O
- indirect emissions: emissions associated with stages in the fuel production chain such as raw material extraction and fuel delivery
- total or 'life cycle': the total of direct and indirect emissions
Using this methodology
Choosing a specific activity type
To use this methodology, the particular type of passenger transport scenario which best represents the users emissions scenario must be specified by making selections from each of the type, subtype and passengerClass drill down choices.
Activity data required
According to this methodology, greenhouse gas emissions are directly proportionate to distance travelled which therefore must be specified - using the distance profile item value - in order to make an emissions calculation.
In addition, the number of passengers can be specified by setting the appropriate value using the passengers profile item value (default = 1).
Users can choose to specify their activity data in a comprehensive range of appropriate units.
Calculation and result
The returned emissions quantities for this methodology are inclusive of CO2, CH4 and N2O. The following discrete amounts are returned:
- CO2: CO2 emissions
- methaneCO2e: CH4 (methane) emissions
- nitrousOxideCO2e: N2O (nitrous oxide) emissions
- totalDirectCO2e: total direct emissions
- indirectCO2e: Indirect emissions
- lifeCycleCO2e: Full life cycle emissions
This methodology represents per passenger emissions and therefore the returned emissions should be considered - by default - to represent those attributable to an single passenger (the default value for number of passengers is 1). If multiple passnegers are specified - by setting the passengers profile item value to a positive value other than 1 - the returned emissions quantity represents the emissions attributable to the group of passengers.
Great circle distance 'uplift' factors
Following IPCC advice, the DEFRA methodology stipulates that flight distances which are defined as direct 'great circle' distances between two locations should be inflated by 9% in order to account for the typical indirect routing of actual flights and deviations related to congestion. The calculations for air travel made herein assume that the distance specifed is known to be the actual distance flown - rather than the distance estimated on the basis of the Great Circle - and therefore this adjustment factor is not applied. In order to make calculations based on Great Circle estimates between two locations (airports or specifed by latitiude/longitude) see the Great Circle route methodology.