Kate Mackenzie Trains, planes, and empty buses

A few reports around yesterday suggested that planes might be less environmentally harmful than trains. If you thought that sounded odd, so did Infrastructurist, who were somewhat annoyed with the simplified headlines given to some reports.

The paper itself, by Mikhail V Chester1 and Arpad Horvath at UCL Berkeley, says there is more to it than the environmental cost of transport systems than the journeys themselves – a point summed up in its title: “Environmental assessment of passenger transportation should include infrastructure and supply chains”.

One part of the paper compares the greenhouse gases emitted simply in operating a mode of transport to the total cost per passenger kilometre travelled (PKT).

To estimate this second figure, the paper goes quite far into the background costs of various types of transport.  For example it considers the electricity production, steel production, and transport of materials that go into making cars and buses, and the material extraction and production used in building roads. The study also looks at the cost of connecting to transport modes, such as driving to the train station.

Which has a slightly deleterious effect on the virtuousness of rail:

Rail modes have the smallest fraction of operational to
total energy due to their low electricity requirements per
PKT relative to their large supporting infrastructures [20].
The construction and operation of rail mode infrastructure
results in total energy requirements about twice that of

And by contrast, boosts air:

Aircraft have the largest operational to total life-cycle
energy ratios due to their large fuel requirements per PKT
and relatively small infrastructure.

However the authors also look at the question of occupancy. They write that comparisons of the environmental impact of different modes of transport often assume average occupancy; which does not acknowledge that occupancy levels can drastically affect relative performance:

For example, an SUV (which is one of the worst energy performers) with 2
passengers (giving 3.5 MJ/PKT) is equivalent to a bus with
8 passengers. Similarly, CA HRT with 120 passengers (27%
occupancy giving 1.8 MJ/PKT) is equivalent to a midsize
aircraft with 105 passengers (75% occupancy). Similarly,
commuter rail (with one of the highest average per-PKT NOX emission rates) at 34% occupancy (147 passengers) is equivalent to a bus with 13 passengers or a sedan with one

The paper doesn’t dramatically undermine any arguments for rail – in fact the emissions of empty buses are probably the most striking point – but it seems to suggest all of these factors should be taken into account, particularly planning locations for train stations, bus routes and so on.

Infrastructurist, (which does write about trains quite a lot), makes another point:

What’s totally missing in their “complete” estimates for these various transportation modes are the virtuous effects of rail: creating denser communities where people tend to walk more, own fewer cars, live in smaller abodes, and spend less time stuck in traffic jams. And planes create no such positive effects–which isn’t an anti-aviation argument, it’s just a fact.

And a commenter on the blog points out, substituting inputs from renewable sources is potentially easier with rail systems that rely solely on electricity, than for vehicles that rely on liquid hydrocarbons.