Passenger emissions metrics

Is it better to travel by car, bus, plane or train? A common approach would be to look at the average "carbon emissions per passenger kilometer" for each mode of transportation. But if we want a fair comparison, we have to make sure they are all calculated the same way:

Two ways to calculate:

• A. Assume the vehicle is packed with some arbitrary number of people. Divide the vehicle's fuel economy by that number.
• B. Look at the total yearly emissions of the bus companyairlinetrain system, and divide it by the total yearly passenger kilometers traveled by all its customers.

'A' tends to be optimistic. 'B' tends to be realistic or pessimistic. (...)( Note that 'B' can't necessarily be applied to personal car travel, although it can still apply to rideshare car travel. )

Why the difference, you ask? Because, for example, a bus full of people can be quite fuel-efficient (good by 'A'), but that doesn't help much if the bus company also has to send a lot of buses that don't often get filled (bad by 'B').

Why this matters

For informed consumer choices

I've been trying to get an honest evaluation of which modes of transport are really greener. But if I just compile a table of "carbon emissions per passenger kilometer", there's no guarantee that it's a proper comparison. What if some of the data points were obtained by methodology 'A', and some by methodology 'B'? What if the cited sources(...)( especially ones that are easy to find, such as news articles )don't specify (nor give enough clues to determine) which methodology was used? Or(...)( for proper academic sources )what if the methodology is hidden behind a paywall?

For systemic changes

It's possible that bustrainair travel companies could save a huge amount of carbon emissions by not servicing low-traffic areas or hours.

This of course would be terrible customer service. It also might even go against government policy in some countries. But if the status-quo carbon emissions are bad enough, maybe the companies should just do it anyway (first lobbying to change whatever policies get in the way). Even if all the customers lost(...)( most of which are rich or middle-class, by the way; with some exceptions )would travel by car instead, maybe the overall carbon emissions would still be less. We'll never know unless we gather the numbers that matter.

See also

• Air travel
• Public transit