Traffic illustration

Lone drivers use the most energy

​​The number of people in each vehicle is the single most important factor explaining the energy and greenhouse gas intensity of travel. This is shown in a new study by researchers from Chalmers and University College London, who also warn that self-driving vehicles could increase both energy consumption and emissions from passenger transport.
– On average, about 1.5 people travel in each car in industrialized countries. But that number could actually decrease to less than one person per car, when automated vehicles enter the market. This could lead to a tripling in light-duty vehicle energy intensity, says Sonia Yeh, at the department of Space, Earth and Environment. 
Occupancy is a central concept when it comes to calculating and assessing energy consumption and emissions for passenger transport. If you drive a car alone, the occupancy is 1 person kilometer per vehicle kilometer, or 1pkm/vkm. With two people in the car, the occupancy rate increases to 2 pkm/vkm. But there are also trips that have fewer than one person in the car. Sonia Yeh, professor in the division of Physical Resource Theory explains:

– In taxi travel, we don’t count the driver. For example, if a taxi driver drives 15 km to pick up a passenger,  and drive the customer 20 km to the destination, and drive another 5 km to finish the shift and go home. Since half the trips is empty, the trip average occupancy is 0.5, or 0.5 passenger kilometers for every vehicle kilometer. So the problem with taxi, some shared mobility, and automated vehicles, is that there are a lot of “empty miles” to pick up or drop off passengers or moving vehicles around. This could lead to even a tripling in light-duty vehicle energy intensity, an increase that would be difficult to compensate by fuel-saving technology.

An increased occupancy rate in the cars would reduce both emissions and energy consumption per passenger kilometer, but the occupancy has instead decreased for the last several decades due to the increase of two car household for example. Today, there are really no examples where that trend has been broken.

– Price based incentives, such as making single driver rides more expensive or shared rides cheaper, can be implemented. But previous studies show that people are generally not very sensitive to price, especially if they have to wait longer or if the trip takes longer. says Sonia.

– Public transportation in Sweden has very low GHG emissions in general. To reduce transport GHG emissions further, the most effective strategies are to reduce trip distance, decarbonize fuels and increase occupancy. The current situation with the corona pandemic makes the situation trickier, as people are avoiding public transportation or shared mobility to reduce transmissions. There remains the hope for electric vehicles powered by fossil-free electricity to reduce greenhouse gas emissions from passenger transport.

Sonia Yeh and her colleague at University College London, Andreas W Schaefer's, article “A holistic analysis of passenger travel energy and greenhouse gas intensities” was recently published in Nature Sustainability.

Published: Fri 24 Apr 2020.