This methodology represents greenhouse gas emissions associated with the road transport in the US. The data and calculation methodology is based on those provided in the Greenhouse Gas Protocol worksheet GHG emissions from transport or mobile sources (version 2.2), published in June 2011.
The emissions methodology is based upon emissions factors which describe the rate at which greenhouse gas emissions are produced during road-based travel.
As with other leading greenhouse gas data and calculation methodology sources (e.g. IPCC, EPA), this methodology assumes that CO2 emissions are directly related to the quantity of carbon which is oxidised during combustion and therefore directly proportional to the quantity of fuel consumed, whereas emissions of CH4 and N2O are more closely related to other factors, including distance travelled.
Therefore, in the best case scenario - where both fuel consumption and distance travelled can be specified - CO2 emissions are calculated on the basis of fuel consumption and CH4 and N2O emissions on the basis of distance travelled. In those cases where only one of fuel consumption and distance travelled can be specified, however, the Greenhouse Gas Protocol methodology converts the original (EPA) emissions factors into the required basis (distance, fuel volume) using 'typical' fuel efficiency data for the vehicles types represented. As such, calculations of CO2, CH4 and N2O can be made on the basis of either fuel consumed, distance travelled or - ideally - both.
Since the CO2 produced during the burning of biofuels in not a net contributor to atmospheric warming (see here), fossil- and biofuel-associated CO2 are explicitly defined.
All emissions factors represent emissions associated with entire vehicles (e.g. car, motorcycle), which can be contrasted with similar transport-related emissions factors which describe emissions on the basis of individual passengers. Emissions for individual passengers can nevertheless be derived by 'sharing' vehicular emissions across all vehicle occupants, if the occupancy of the vehicle is known.
Emissions - expressed in terms of mass (e.g. kg) - are calculated by multiplying these rates (mass emitted per distance or volume [fuel]; e.g. kg CO2 per mile or gallon) by a distance (e.g km) travelled or fuel (e.g. gallon) consumed.
The rate at which road transportation produces greenhouse gas emissions varies with the type of vehicle, depending on factors such as the type of fuel used (e.g. gasoline/petrol, diesel), and emissions control standards. Therefore, emissions factors for a broad range of generalised US road transport scenarios are provided.
A total of 75 specific types of road transport are represented and are differentiated by vehicle type (i.e. bus, car, light and heavy goods vehicles, motorcycles), fuel type (gasoline/petrol, diesel, ethanol, LPG, LNG) and emissions standard. For each vehicle represented, specific emissions factors are used to calculate greenhouse gas emissions according to the activity data provided. In total, the following emissions factors are provided for each vehicle type:
- Fossil CO2 emissions per unit distance travelled
- Biogenic CO2 emissions per unit distance travelled
- CH4 emissions per unit distance travelled
- N2O emissions per unit distance travelled
- CH4 emissions per unit fuel consumed
- N2O emissions per unit fuel consumed
Activity data required
According to this methodology, greenhouse gas emissions can be calculated on the basis of fuel consumed, distance travelled or - for most accurate results - both. Therefore either or both of these values must be specified in order to make an emissions calculation.
If multiple passengers are under consideration, this number can be specified, in which case vehicle emissions are shared amongst all passengers.
Calculation and result
The returned quantities for this methodology represent CO2, biogenic CO2, CH4, N2O and CO2e emissions associated with the activity data (distance/fuel) specified. In most cases CO2 emissions are entirely represented by fossil CO2 as only biofuels (in this case, ethanol) or biofuel blends contain biogenic carbon. CO2e emissions represent all three gases (fossil CO2 only), converted using these global warming potential)
Other Greenhouse Gas Protocol transport methodologies are available representing general transport fuel consumption, fuels consumed in specific transport contexts, freight transport, passenger/public transport, road transportation in the UK (with heavy goods) and other regions.