The motive for this study is a question raised by Folksam on how well the criteria currently used, which have a focus on CO2 emissions, reflect the total environmental impact of exhaust emissions. One of the questions is whether diesel cars, being more fuel efficient, are preferable to gasoline cars given the differences in for example particle and NO2 emissions. Since the latter emissions are known to have health impacts, if they are to be accounted for there is also the question of how to compare these health impacts of the cars with their safety aspects.
Regarding the specific question raised in this study about indicators for sustainable cars, we find that the indicators currently used, CO2 emissions, do not reflect the full environmental impact.
Different types of vehicle technologies result in different combinations of emissions. With the large variety of car models, and with important differences between type approval and "real driving" emissions, we conclude that apart from CO2 emissions, vehicle technology should be accounted for in the classification of cars.
Concerning the difference between gasoline and diesel vehicles, important aspects to consider are:
- differences in emissions of particulates where particle size or number and composition may be important to consider in addition to, or maybe even rather than, mass
- the difference in the ratio between NOx and NO2, as it affects local NO2 and ozone concentrations.
It was also provided an example of how external cost calculations can be used to assess the performance of a car model in relation to differences in risk for severe outcomes of an accident. In principle, the same type of reasoning as for other health impacts applies.
However, due to lack of data we have not had the possibility to explore the trade-off between smaller cars, with lower emissions but higher risk for severe outcomes of accidents, and larger cars. This is an issue left for future studies.