Source: IEA (2019).
Transportation modes have a variety of environmental performance that can be measured in terms of CO2 emissions by unit traveled. For passengers, it involves grams of CO2 per passenger-kilometer, and for freight, it involves grams of CO2 per ton-km. Massification (economies of scale) is a key element in the environmental performance of transport modes and a key argument behind modal shift initiatives, such as towards public transit for passengers and towards rail and maritime shipping for freight.
It is important to underline that the performance of passenger and freight vehicles is not readily comparable, since they are using different units of reference; a passenger versus a ton. The economic value of the activities is also difficult to compare since a passenger driving a car may not be using it for the same purpose as someone taking a flight. A similar observation applies to freight. What is being carried by air cargo (high CO2 emissions per ton-km) is of much higher added-value than the average freight carried by maritime shipping (low CO2 emissions per ton-km).