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A holistic analysis of passenger travel energy and greenhouse gas intensities

Abstract

Transportation is a major energy consumer and emitter of greenhouse gases (GHGs). Exploring the opportunities for energy savings and GHG emissions reductions requires understanding transportation energy or GHG intensity, which is defined as energy use or GHG emissions per unit activity, here passenger-kilometres travelled. This aggregate indicator quantifies the amount of energy required or GHGs emitted to provide a generic transportation service. We show that the range of observed energy and GHG intensities of major transportation modes is remarkably similar and that occupancy explains about 70–90% of the variation around the mean; only the remaining 10–30% is explained by differences in trip distances and other factors such as technology and operating conditions. Whereas average occupancy levels differ vastly, they translate into roughly similar levels of energy and GHG intensity for nearly all major transportation modes.

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Fig. 1: Energy intensity versus vehicle occupancy for light-duty vehicles, buses, railways and fixed-wing aircraft.
Fig. 2: Life-cycle GHG intensity versus vehicle occupancy for light-duty vehicles, buses, railways and fixed-wing aircraft based on data in Fig. 1.
Fig. 3: Energy intensity versus average trip distance for the transport modes shown in Figs. 1 and 2.

Data availability

All data were derived from public databases. The datasets used in the analysis have been deposited at https://doi.org/10.5281/zenodo.3727541.

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Acknowledgements

We thank D. Daniels and M. Schipper (both US Energy Information Administration) for providing the NHTS 2009 raw dataset with estimates of household vehicle energy use.

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A.W.S. led the data collection, data analysis and preparation of the manuscript. S.Y. contributed to data collection, data analysis and preparation of the manuscript.

Corresponding author

Correspondence to Andreas W. Schäfer.

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The authors declare no competing interests.

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Extended data

Extended Data Fig. 1 Energy intensity versus occupancy for railways in Canada, India, and the US.

Four electric commuter railways in the US are added to the figure. The energy intensities of electric railways are 0.49, 0.91, 0.55 and 0.45 MJ electricity/pkm for NY, PA, PA and IN, respectively. To ensure consistent comparisons, the final energy (electricity consumed) is converted to the secondary energy required to generate electricity in the corresponding US state, and multiplied by the final-to-secondary energy ratio for diesel fuel, which is 0.83 according to the latest GREET model. For example, the fossil-fuel equivalent EI of 0.55 MJ electricity/pkm is 0.55 /0.36 x 0.83 = 1.26 MJ/pkm, given the secondary-to-final energy efficiency of 36% for the electric power system in PA. The dotted lines represent a hypothetical 1:1 decline in energy intensity with vehicle occupancy, the limiting case of zero-weight passengers. All trajectories thus need to decline at a lower than 1:1 ratio.

Extended Data Fig. 2 Energy intensity versus occupancy for fixed-wing aircraft operating in US domestic travel.

All data relate to US domestic air travel in 2015. The dotted lines represent a hypothetical 1:1 decline in energy intensity with vehicle occupancy, the limiting case of zero-weight passengers. All trajectories thus need to decline at a lower than 1:1 ratio.

Extended Data Fig. 3 Energy intensity versus occupancy for intercity, regional, and urban buses in Australia, Canada, European cities, Sweden, Taiwan, and the US.

The dotted lines represent a hypothetical 1:1 decline in energy intensity with vehicle occupancy, the limiting case of zero-weight passengers. All trajectories thus need to decline at a lower than 1:1 ratio.

Extended Data Fig. 4 Energy intensity versus occupancy for light-duty vehicles.

The dotted lines represent a hypothetical 1:1 decline in energy intensity with vehicle occupancy, the limiting case of zero-weight passengers. All trajectories thus need to decline at a lower than 1:1 ratio.

Supplementary information

Supplementary Information

Supplementary description of data sources.

Reporting Summary

Supplementary Data

Data underlying Figs. 1–3 and econometric analysis.

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Schäfer, A.W., Yeh, S. A holistic analysis of passenger travel energy and greenhouse gas intensities. Nat Sustain 3, 459–462 (2020). https://doi.org/10.1038/s41893-020-0514-9

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