Myth: Trams have more greenhouse emissions than cars

Fact: This was a well-publicised finding from a conference paper, which makes the claim without providing any solid evidence for it. The best evidence currently available supports the reverse conclusion. Tram travellers need feel no guilt about contributing excessively to climate change.

In early 2003, a paper entitled Comparison of Emissions from the Public Transport System and Private Cars, by RMIT engineers Ed Boyapati, Astrid Hartono and Joel Rowbottom, appeared in the 8th Cairo International Conference on Energy and Environment. It purported to show that Melbourne's trams emitted 1.23kg of carbon dioxide (CO₂) per passenger kilometre, whereas cars emitted just 0.25kg CO₂ per passenger kilometre. Trains were found to be just as bad as cars; only buses came out ahead.

The findings were reported by several media outlets, and were soon picked up by the ABC Science Unit, where inquiring minds stumble upon them to this day.

By the time the ABC picked up the story, Dr Boyapati and his coauthors had moderated their finding to 0.74kg CO₂ for trams compared with 0.25kg for cars. This is still an alarming figure, and the ABC's Greenhouse FAQ pulls no punches when it comes to the question of tram travel:

So, on what evidence have Dr Boyapati et al. reached such a provocative conclusion? The truth is we simply don't know, because their actual paper doesn't provide any.

What does the paper actually say?

The six-page paper contains two major sections. The first, Comparison of GHG Emissions from Various Modes of Transport states that the emission figures were obtained by surveying the loading conditions on trains, trams and buses at various times and calculating the emissions per passenger-kilometre using published figures for the vehicle's energy consumption. The final figures are then provided. No actual survey data is given, even in summary form, from which it would be possible to determine whether the survey sample is an accurate reflection of actual tram use patterns. Usually when such studies are reported, the researchers carefully outline the experimental design and provide a summary of the survey data. No such information is provided here.

The second major section, Flow On Effects of Trams on GHG Emissions, is devoted entirely to rehashing old 1950s arguments against trams, most of which we deal with elsewhere on this site. The authors assert (without any evidence) that trams add to pollution by delaying cars, that trams cost six times as much as buses, that trams take valuable road space away from cars, that tram wires pose a safety hazard, that tram stops in the middle of roads are ugly, and so on. The only claim for which any evidence is presented is that roads with trams have more casualty crashes than those without - a claim we deal with elsewhere.

It is unclear why a research paper in 2003 would devote so much space to airing all these 50-year-old arguments without offering any new perspective - unless what is being intended is simply a gratuitous tram-bashing exercise dressed up to look academically respectable.

According to the website for the 10th Cairo Conference, papers in these conferences are not subject to the full peer review that is the normal process for checking the correctness of research results. (Only a 250-word abstract is reviewed, essentially to ensure that the paper is relevant to the conference and says something interesting.) To the best of our knowledge, Dr Boyapati's findings have not appeared in any peer-reviewed journal or conference.

Is the paper's conclusion true?

Vehicle occupancy

Short as the paper is on detail, it does advance a plausible argument. It is certainly true that burning brown coal to make electricity to run tram motors is less energy-efficient than burning fuel in an internal combustion engine directly. If trams carry very few passengers on average, it may be that the greenhouse emissions per passenger are so high that even driving a car is benign by comparison.

This is far from the first study to show how easy it is to make public transport look bad in energy terms, simply by making sufficiently pessimistic assumptions about patronage. A background study for the 1991 Victorian Bicycle Strategy (produced by RTA, the predecessor of Vicroads) implied in its results that tram passengers generate slightly more greenhouse emissions than car occupants; it did so by assuming an average occupancy of just 10 passengers per tram. (It's important to note that in energy terms the tram passenger still came out ahead in this study, using about half the energy of a motorist; the problem is not with trams per se but with coal-fired electricity, which generates higher emissions per unit of useful energy.) The RMIT study presents a worse conclusion than this older study, so even though we don't know what occupancy figure they are using, it must be even lower than 10 per tram.

But even the figure of 10 per tram is almost certainly too low. In 1987 detailed surveys were conducted for the 'Metplan' public transport strategy, and revealed an average occupancy of 21 passengers per tram (500 million passenger-kilometres divided by 24 million vehicle-kilometres). Since 1987, occupancy on Melbourne's trams has shown a modest increase, especially outside peak hours and on weekends when patronage has traditionally been low. In 1992, using updated patronage figures, researcher Patrick Moriarty estimated average tram occupancy as 26 passengers. His estimates were published in the peer-reviewed journal Road and Transport Research in June 1992.

Using the updated figures from the 2006-07 State Budget of 149 million passengers (excluding one-off Commonwealth Games passengers) and 23 million vehicle-kilometres per year, together with the estimate of 4km average trip length from the Metplan discussion paper, we get an average occupancy of 26 passengers per tram - the same as Moriarty's 1992 estimate.

Vehicle energy consumption

Energy consumption estimates specific to Melbourne's trams are rare; among the only published estimates to be found in recent times are the 1991 RTA study and the 1992 paper by Patrick Moriarty mentioned above.

Moriarty's figure is almost certainly the most definitive, at least for the trams that were in use in the 1990s, not only because it appears in a peer-reviewed journal but also because it can be reproduced from credible official sources. Until it was privatised in the 1990s, the State Electricity Commission of Victoria published figures for electricity used by trains and trams. The figure for 1990 as reported by Moriarty was 49.5GWh. Based on the PTC's reported figure of 20.2 million tram-kilometres in 1990, we can reproduce Moriarty's figure for tram electricity consumption of 2.45kWh, or 8.8MJ, per vehicle kilometre.

The 1991 RTA study used an energy consumption figure for both trams and buses of 22MJ per vehicle kilometre. In both cases this figure is almost certainly an overestimate. 22MJ is more than double Moriarty's figure for trams, and for buses the National Greenhouse Gas Inventory states that Australian buses in 1991 used on average 11.8MJ per vehicle kilometre. While the 'light rail' figures in the Inventory are not reliable estimates for Melbourne trams (because they also include unrelated modes like the Sydney monorail), buses are a fairly standardised technology. This suggests the RTA figure for buses is a significant overestimate, and since the RTA bus and tram figures are identical (and derived in an identical manner), the same is likely to be the case for trams.

The chief barrier to obtaining accurate figures more recently has been privatisation, with its tendency to treat energy consumption and other figures as trade secrets. However, figures sometimes do emerge from third party sources. Tram manufacturer Siemens monitored the energy consumption of its Combino trams (one of the types now used in Melbourne) in actual operation in the cities of Basle and Potsdam, and obtained average consumption figures of 1.53 and 1.84 kWh per kilometre (5.5 and 6.6MJ per kilometre) respectively. Most trams manufactured in the last 20 years use regenerative braking to recover the kinetic energy dissipated when the tram stops. Even without any energy recovery, the respective figures were 9.4 and 9.5MJ per kilometre - similar to the Moriarty figure of 8.8MJ.

While not specific to the vehicles in use in Melbourne, James Strickland's vehicle efficiency page provides figures indicating that trams and buses use roughly the same energy per vehicle-kilometre, which is intuitively correct: while trams weigh more than buses, their steel-on-steel traction is more efficient than rubber-on-bitumen, and so the two differences tend to cancel (a fact Moriarty also points out).

A figure of 12MJ per vehicle kilometre is similar to that given by the National Greenhouse Gas Inventory for buses in 1991, the approximate vintage of the B-class trams which are the most numerous in Melbourne's fleet. It is also roughly twice the observed figure for a Combino tram, 25% higher than for a Combino tram with no energy recovery, and 36% higher than Moriarty's estimate from 1992. We therefore take this as a reasonable, but conservative, estimate of energy consumption for Melbourne trams today.

Putting it all together

The energy consumed per passenger-kilometre is equal to the energy per vehicle-kilometre, divided by the average occupancy.

Taking the occupancy figure from above of 26 passengers per tram, and the original very high estimate of 22MJ per vehicle-km from the RTA study, we obtain a figure of 0.85MJ per passenger-km. The equivalent figure for cars in peak hour is 3.7MJ per passenger-km. This means that even with the least favourable assumptions, energy consumption by tram passengers is a little over 20 percent that of car occupants. This makes tram passengers responsible for less greenhouse emissions than car occupants, even with the very poor greenhouse record of Victorian brown coal.

Using the more realistic (but still conservative) figure for trams of 12MJ per vehicle-km, and the same (relatively low) average occupancy, the equivalent figure is 0.46MJ per passenger-km. Thus on conservative assumptions, energy consumption by tram passengers is about 12 percent that of car occupants.

When converting energy use figures to greenhouse emissions, it is important to consider the energy source being used and also the 'embodied energy' involved in manufacturing the fuel. The way to make trams (and trains) look worse off compared with cars is to assume all electricity comes from Victorian brown coal and to ignore all the energy used in transporting crude oil and refining it into petrol. A more careful analysis needs to consider that much of Victoria's electricity comes from natural gas, hydroelectric and renewable sources, and that liquid fuels require a substantial amount of energy to extract and manufacture.

Taking into account the fuel mix in Victorian power generation and the energy expended in the petrol supply chain, we estimate that a typical tram passenger is responsible for about half the greenhouse emissions of a typical motorist. Of course, this figure will improve if more passengers are attracted to the system; in European cities with well-run tram systems, energy use by tram passengers is estimated at less than 5 per cent that of motorists, compared with our 12 per cent. And emissions per passenger in peak hour are a good deal less, even though there are more trams per hour running.

Our estimate here is in keeping with those obtained in credible studies of transport energy use. These come up with widely divergent results because estimating total passenger kilometres travelled by public transport is a notoriously difficult exercise, requiring large sample sizes to get reliable results, and made even more difficult since the introduction of automatic ticketing since there are no longer any staff with day-to-day knowledge of passenger volumes. We have chosen to rely on the Metplan figures because no study since then has surveyed Melbourne tram users so comprehensively. Nonetheless, it is worth noting that the Millenium Cities Database for Sustainable Transport, compiled by Peter Newman and Felix Laube of Murdoch University, gives energy figures for tram use in Australia and New Zealand of 0.36 megajoules per passenger kilometre. Further calculations based on other sources can be found on our energy use page.

(It's also nice to see that the ABC do their homework: the more recent Carbon Cops fact sheet on trams is a fairly accurate reflection of the available evidence.)

But even if our calculations or a well-designed empirical study were to find that the occupancy of trams is so low that the emissions per passenger are worse than in cars, this would just be another fact pointing to the need to increase public transport patronage in order to improve the efficiency of the system. We don't like the idea of running empty trams any more than the tram-hating road engineers do, but the solution is to encourage more use of trams by providing faster and more frequent services, not to get rid of them and remove all hope of reducing emissions from transport.

© 2007 Public Transport Users Association Inc. (PTUA), Victoria, Australia. ABN 83 801 487 611.
General copying and distribution on a non-commercial basis is permitted subject to proper acknowlegement.
Authorised by Tony Morton, 247 Flinders Lane, Melbourne, for the PTUA

Common Urban Myths About Transport

What does the paper actually say?

Is the paper's conclusion true?

Vehicle occupancy

Vehicle energy consumption

Putting it all together