资源环境科技发展态势分析平台

A long-standing belief is that buses and railroads use less energy for personal transport than passenger cars and airplanes. Several patterns emerge from a new analysis of data by Andreas Schafer and Sonia Yeh. Here, we define Energy Intensity as Energy Use (E) divided by Passenger-Miles Traveled (PKT), which is a good expression of what personal transport is all about.

First, regardless of the mode of transport, the energy intensity drops dramatically with greater occupancy of the vehicle. A lightly occupied urban bus uses about 10 times more energy than intercity bus traveling the same distance, largely because the latter is usually full.

Second, under customary conditions of occupancy (e.g., 2 passengers per car and 100 passengers on an airplane), the energy intensities of passenger cars, buses, railroads and commercial aircraft are all about the same. A fully occupied wide-body jet has about the same energy intensity as a passenger automobile for each passenger carried one kilometer. It is noteworthy that private business jets stand out in their high energy intensity—they have no economies of scale in occupancy or size.

Third, different technologies make a minor contribution to these comparisons. An electric car has only slightly lower energy intensity than a traditional gasoline-powered internal combustion engine for each passenger carried one kilometer. The electric car has large energy costs imbedded in its batteries and derives its power from the grid. Only a solar-powered car may dramatically affect these relationships.

Fourth, travel distance can affect energy intensity, especially for aircraft. Aircraft use lots of energy during take-off and landings, so up to a point, longer trips have lower energy intensity. Above a travel distance of about 2000 km (about 1200 miles), the added weight of fuel carried on the airplane adds to energy intensity. Similarly, intercity buses achieve lower energy intensity because they avoid the frequent stops and starts of city buses.

The convenience of a personal automobile—being able to go almost anywhere at any time—reinforces the likelihood that you will travel alone, raising your energy intensity. Avoid the urge. Personal choices and environmental policies that increase vehicle occupancy will have the greatest reduction on our environmental impact.

Reference

Schafer. A.W. and S. Yeh. 2020. A holistic analysis of passenger travel energy and greenhouse gas intensities. Nature Sustainability 3: 459-462.