US energy use per passenger mile (Btu) Source: DoE Transportation Energy Data Book, 2012

There was one of those intense internet fracas last month that generate lots of heat and light yet pass by completely unseen by the non-connected world.

It started when Eric Morris argued that, contrary to the conventional wisdom, public transport’s emissions aren’t markedly lower than those from cars (see Save the earth, drive your car? and Can mass transit save the environment?).

He says that’s because trains and buses are heavy and use a lot of energy, but have low passenger loads most of the time.

There were a number of critical responses like this one from Angie Schmitt and this one by Michael Lewyn. The author of Human Transit, Jarrett Walker, also weighed in with a couple of fighting articles, The same “empty buses” fallacy, over and over and The “transit isn’t green because it runs empty” line.

Jarrett Walker’s counter-argument is essentially that the mission of transit agencies isn’t to maximise ridership. Equity objectives oblige them to provide services in areas that can’t be serviced efficiently. Such areas have “predictably low ridership” which increases emissions per passenger kilometre.

By and large transit systems perform considerably better on emissions than cars, but the gap isn’t as big as is often imagined. The US Department of Energy estimates transit rail consumes 37% less energy per passenger mile than cars, but buses consume more (see exhibit).

In 2008, Victoria’s former Commissioner for Sustainability said modes that rely on coal-fired electricity, such as Victoria’s trams and trains,

have GHG full fuel cycle intensity levels on an average per person-kilometre basis that are comparable to motor vehicles.

This report prepared for the 2008 Victorian Climate Change Summit shows the GHG intensity of cars in Victoria is only 13% higher per passenger kilometre than either trains or trams. Emissions from buses are 27% higher than those from cars.

The explanation for the relatively modest difference between cars and public transport is partly about factors like how the electricity is generated (e.g. Victoria uses brown coal), but is mainly the result of low load factors.

Those celebrated photographs comparing how many cars, buses and bicycles are needed to transport a fixed number of people give a misleading impression because they assume the bus is full (and the car is at average occupancy).

That’s largely true of trains and trams in the peaks, but not during the middle of the day, on weekends and at night.

A train that starts out full from the CBD in the evening will also progressively shed passengers as it approaches its destination. Another factor is peak hour public transport vehicles usually return empty or with just a few counter-flow passengers after they’ve reached the terminus.

Buses rank worse because they tend to be relegated to low volume feeder services and marginal routes. When buses are used for trunk services as they are with Bus Rapid Transit, they perform much closer to rail.

If we only consider services with high load factors – say peak hour CBD services – then trains and trams are much, much more sustainable than cars, measured in terms of passenger kilometres.

The performance of public transport also improves when lifecycle emissions are taken into account. Trains and buses don’t sit in parking spaces most of the time – they’re used for the greater part of each day.

Further, public transport enables us to live and work in denser locations. All other things being equal, density means we travel fewer kilometres and live in smaller dwellings that consume less energy e.g. to heat and cool.

The relative performance of trains and trams will also be better in cities where the electricity supply is cleaner e.g. a high proportion of hydro power.

On the other hand, if likewise we only consider the sorts of car trips that tend to have high occupancy levels – like non-work trips – cars will perform better too.

Public transport is necessarily more than crowded peak hour services – it has to be viewed as a system. As a society we expect governments to provide frequent services all day and late into the night when patronage falls off.

We demand a decent standard of service in locations like outer suburbs where public transport struggles for ridership because it’s very hard to compete with the flexibility and privacy offered by cars.

Nevertheless, I think we have to acknowledge that public transport isn’t as green as is customarily assumed. The fact that low patronage services address important social objectives doesn’t change that equation. It shouldn’t lead us to ignore the facts.

I think this offers an important “so what?” insight. Environmental sustainability is very important but it isn’t the sole purpose of public transport.

As a society, we invest billions of dollars of public funds in transit in order to provide mobility for those who have no other choice. We also do it to sustain dense agglomerations of activity. We’d continue to do so even in the absence of climate change.

It’s of course highly benficial that mass transit is more sustainable than driving. However we shouldn’t for a moment shy away from providing public transport services where they’re warranted because some types of services have (relatively) high emissions.

The real fallacy is the idea that we have to judge the sustainability of every activity equally, irrespective of the cost of mitigation or the value it delivers. It’s erroneous to think only in terms of costs and ignore benefits.

What we should do is acknowledge some activities are so socially beneficial that their relatively modest – or even poor – environmental performance is acceptable. I’d propose many “marginal” public transport services in outer suburbs should be in that category.

At the same time we should focus on improving the environmental performance of activities where the pay-off is high relative to the financial and political cost. It’s not wise to fight a war with equal intensity on all fronts.

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Peter Fray
Peter Fray
Editor-in-chief of Crikey