By Mark Kinver
Science and environment reporter, BBC News
Trains have been considered to be one of the greenest forms of transport
Policymakers must consider more than just "tailpipe" emissions when assessing the impacts of different modes of transport, say researchers.
Many analyses overlook greenhouse gases emitted in constructing and maintaining travel infrastructures, they added.
The team found that, based on passenger kilometres travelled, off-peak urban bus services were more carbon-intensive than flights by commercial aircraft.
The findings have been published in the journal Environmental Research Letters.
The researchers from the University of California, Berkeley, said the importance of tackling emissions from transport meant that decisions should not be based on partial data.
"Governmental policy has historically relied on energy and emission analysis of automobiles, buses, trains and aircraft at their tailpipe," they wrote.
"[This ignores] vehicle production and maintenance, infrastructure provision and fuel production requirements to support these modes.
"To date, a comprehensive life-cycle assessment (LCA) of passenger transportation in the US has not been completed."
The team selected a range of transport to be assessed, including a saloon car, an urban bus service and a mid-sized aircraft.
They then identified ways in which energy was being used and/or gases were being emitted.
As well as assessing "operational components", such as the fuel consumption of running an engine, the team also considered the impacts of "non-operational components", such as road construction, street lighting and maintenance.
The data was then presented in terms of grams of carbon dioxide equivalent per passenger kilometre travelled (g CO2e/PKT) to allow a comparison between the different modes' carbon intensities.
Co-author Mikhail Chester said he hoped the team's framework would encourage more people to consider the indirect impacts of transport systems as well as fuel consumption.
"In the past, we have had this 'direct consequence' approach to transport emissions," he told BBC News.
"A lot of transport components are interrelated. For example, you cannot drive a car without having roads and the rest of the infrastructure."
Within the study, the researchers assessed emissions from an urban bus service, comparing emissions during peak and off-peak periods.
Although the actual emissions for the vehicles did not change during the course of the day, there was a very stark difference between the two periods in terms of emissions per passenger.
"Very often, public transport services do not operate at the average occupancy level," Dr Chester observed.
"So I took the bus and showed off-peak verses peak emissions.
"I think this does highlight a lot of issues, one of which is that we should start thinking about policies that promote increased occupancy, as well as looking as ways to improve energy efficiency."
The study also showed that aircraft produced lower emissions than private cars, when considered on a passenger kilometres travelled basis.
Dr Chester suggested local authorities and policymakers should invest more effort into increasing occupancy levels - not just on public transport, but across all modes of travel.
"If the ultimate goal is to provide passenger miles as a service, then it would be a good strategy to encourage people to take the forms of transport that currently have empty seats," he said.
"We have modes of transport where we have already invested in the system and infrastructure, so it makes sense to look at ways to encourage more people to use it.
"It would be nice if we saw the thinking on the environmental impacts of transport move beyond the tailpipe and start to include lifecycle components," he added.
"I think it is critical that we look at both the direct and indirect impacts of our transport choices."