By looking at the environmental impact of passenger transport – whether it be trains, planes or automobiles – beyond the exhaust fumes spewing from its collective tail pipe, researchers in the United States have discovered a significant spike in energy use and greenhouse gas emissions. By taking into account transport support systems – which includes sourcing raw materials, manufacturing, as well as the construction and maintenance of infrastructure – researchers at the University of California hope to provide a more detailed view for transport planners and policy makers. And produce a better outcome for the environment.

The research team, led by Mikhail Chester, examined airplanes, trains, cars and buses, including the support system of each, to demonstrate how much energy each required as well as how much greenhouse gas and air pollutants each produced over its lifetime. What they discovered were significant increases in energy use and green house gas production: aircraft rose by 30 percent, automobiles 40 percent and rail more than doubled, skyrocketing 155 percent.

As well as considering the use of fossil fuel such as gasoline, diesel and jet fuel, the research factored in electricity generation used in the extraction of raw materials, manufacture of the vehicles, construction of roads, railways and runways, and operation of support systems, such as railway stations, street lighting and airport ground support equipment operation. The environmental performance for each component in the transport's life cycle was calculated and then standardized per passenger-kilometer-traveled (PKT).

Using the PKT performance indicator instead of vehicle-kilometers-traveled (VKT) gave the researchers a clearer picture of the environmental impact because it calculated the number of people being transported. The researchers also analyzed the performance of the various transit modes for both low and high occupancy, rather than assuming average occupancy rates.

Even though automobiles, trains and aircraft have about the same total carbon monoxide (CO) emission factors, this approach showed CO emissions from automobiles are about 110 and 40 times larger per PKT than rail and aircraft respectively, due to their vastly different occupancy rates.

The team’s findings also demonstrated that there are many conditions under which different modes can perform equally. For example, an SUV (which is one of the worst energy performers) with two passengers uses the same energy as a bus with eight passengers. Similarly, a commuter train a little over a third full, a bus with 13 passengers and a sedan with a single passenger all produce the same levels of nitrogen oxide.

Interestingly, the team’s research showed that air and rail travel consume similar levels of energy, despite flying being made to seem far more detrimental to the environment than taking trains. In areas such as the US' east coast, which relies more heavily on fossil fuels for electricity to power rail travel, trains turn out to be even bigger greenhouse gas emitters than planes. Exacerbating the problem are the large CO emissions produced in making cement to build train stations.

While most of the energy consumption and greenhouse gas emission for road and air are caused by the automobiles and airplanes themselves, the results for rail are largely due to indirect factors such as building and looking after stations.

The study points to technological advancements in improving fuel economy and switching to lower fossil carbon fuels as the most effective ways to improve the environmental performance of road and air transport. For rail, it is necessary to look at reducing the amount of concrete used in station construction by switching to lower energy input and greenhouse gas-intensity materials.

The university's researchers believe their study will arm policy and decision-makers with the information they need to produce better outcomes for the environment. For example, policy-makers have targeted exhaust pipe emissions in recent years to remove sulphur, which causes acid rain, from petrol and diesel fuels. However, the study shows that the bulk of sulphur emissions stem from electricity generation used in making the vehicle.

In fact, all the modes of transport consume substantial amounts of electricity at some stage in their life, even though it may not be when they are actually moving us about. It could be while they are being manufactured, it could be making the fuel they run on – regardless, cleaner electricity generation will have an immediate positive effect on the environment. It is especially true of rail and is likely to become a much bigger factor for automobiles if electric vehicles take off.

However, the researchers believe a concerted effort is needed to tackle all indirect sources of emissions and energy consumption, which means closer scrutiny of every part of a transport system.

The study can be found in the journal Environmental Research Letters.