US road transport by Greenhouse Gas Protocol

Summary

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 methodology

Emissions model

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.

Model data

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)

Similar methodologies

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.