Road operators can use different criteria to determine the cost of their service:
The weight transported,
The volume transported, or
The surface area occupied.
Our methodology
At Greenly, we use emission factors in kgCO2e/tonne.km. In the realm of carbon accounting, this "mass" approach (vs. a "volume" or "surface" approach) is the one most commonly used.
Indeed, most road freight-related emission factors published in trusted databases are emission factors in kgCO2e/tonne.km (e.g. the UK's GHG Conversion Factors for Company Reporting database or the French Environment Agency Base Carbone database). To be consistent when computing GHG emissions, these emissions factors and a "mass" approach are systematically used at Greenly.
Example
If a 7.5 t lorry transports 1,500 kg of goods over a distance of 100 km for one company (no other company uses the lorry), the corresponding GHG emissions are computed as follows:
GHG Emissions = 0.378 kgCO2e/t.km (EF of the lorry) * 1.5 t * 100 km = 56.7 kgCO2e
Heavy packages are therefore penalised compared to lighter packages (the emission factor is multiplied by the weight of the good). However, it is frequently said that the fuel consumption of a lorry hardly depends on the weight transported.
This is indeed a shortcoming of the methodology. However, according to the UK Government, there is a difference in fuel consumption between an empty lorry and a fully loaded lorry (see "UK Government GHG Conversion Factors for Company Reporting" available on the UK government website).
In this database, the emission factors of 100% loaded lorries are between 17% and 65% higher than those of 0% loaded lorries. The weight of the goods is therefore not negligible.