The carbon footprint of different travel options is a misleading guide to action

By environmental researcher Lisa Hopkinson

Some time ago an eco-conscious friend of mine told me he preferred to drive his family to the Peak District as it was more environmentally friendly than taking the bus. With all the bus cuts it is undoubtedly more convenient and cheaper to drive but more environmentally friendly? Really?

The information my friend was using as the basis for his decision was a well known infographic derived from Government statistics which compares the emissions per passenger kilometre from different modes of transport. This typically shows that a bus emits more than twice the carbon dioxide per passenger km than a car with four passengers.

For example, the latest 2022 Defra statistics show that an average petrol car emits 170g of carbon dioxide vs 96g for an average bus, and 35g for national rail per passenger km. 

If you assume there are four passengers in a car then the emissions per passenger km reduces to a quarter or around 43g which many of the infographics show. But for some reason they don’t do the same with a bus or train. In fact, in our heads for a family of four travelling by bus we tend to multiply the emissions by four as you would when estimating how much it would cost to take the bus or train. This produces a highly inaccurate representation of what is the most eco-friendly form of travel. Hey presto, driving is the new veganism.

While the Government statistics may be statistically robust, interpreted this way they can be a highly misleading guide to what mode of transport you should take if you want to limit your carbon emissions.

Firstly the infographics typically show an average of all vehicles and modes of driving, and the Government statisticians are very clear on this point stating in the caveats that “if a car is older than average, the journey may be more polluting. If the journey encounters less traffic than average, the journey may be less polluting.” This is also the case for buses, trams and trains.

Secondly these average figures are based on the existing very low occupancy figures for buses – around 13 passengers outside London and 19 in London. So just as when you have two people in a car the emissions are roughly halved, so doubling the bus occupancy will halve the emissions per passenger. As a typical single decker bus has capacity for up to 75 people there is plenty of room for additional passengers before the bus company needs to lay on extra buses.

If the bus was full or even half full then by my rough estimates emissions per passenger km would be lower than for driving with a car with four people in it. Even if the bus gets so full that the bus company needs to add an additional bus, the bus emissions per passenger km are still likely to be lower than driving as the occupancy rates will be much higher and the emissions per passenger km will be much lower.

Thirdly, and perhaps most significantly, it shows average emissions not additional emissions. When making a decision whether to drive or take the bus (or train) on the basis of carbon emissions, it should really be the marginal or additional carbon emissions than the average emissions that count. If a bus or train is travelling anyway and has plenty of seating room at off-peak times, then taking the bus or train will add next to no additional carbon emissions to the atmosphere (perhaps fractionally more fuel due to the additional weight of the bus or train) whereas taking a car will add the entire amount of emissions from that journey. In this case the additional emissions from taking the car are still the same but the additional emissions from taking a bus, tram or train are next to nothing.

Digging into the BEIS Transport and Environment statistics reveals a lot of other interesting information.

The latest BEIS statistics, for 2022, give a comparison of the emissions for different modes of transport for a single passenger for several example journeys. For a longer journey, say Glasgow to London (400 miles), the coach outperforms even an electric car, while the train emits less than a third of the carbon of a petrol or diesel car. 

For a shorter journey, say Sunderland to Newcastle (14 miles) taking the train emits a little less than an electric car, while the bus emits slightly less carbon than driving a petrol car per passenger km.

If we want to reduce people’s car use we need to give them accurate, honest and useful information to inform their decisions. This is not some academic exercise. Transport Scotland has done a lot of good quantitative and qualitative research to find out what interventions can reduce car travel and what people think about reducing traffic and how it will affect them.

In their route map to achieve a 20% reduction in car km by 2030, participants in a focus group discussed measures that could help them to reduce their car use. This included “the benefits of a single source of information to help people plan journeys by alternative modes and enable people to compare the environmental impact of different travel options.”

So what we need is an infographic that shows the additional emissions if someone chooses to make a particular journey by different modes. So choosing to drive a car, even an electric one, will typically add more emissions than taking the bus, tram, coach or train for a given journey. And definitely do not fly if you can avoid it!

About the author:
Lisa Hopkinson is an environmental researcher with more than 30 years’ experience in Hong Kong and the UK in the charitable, educational and private sectors. She has variously worked as a consultant, campaigner, political aide and researcher on numerous sustainable transport projects. She is a trustee of the grant-making charity, the Foundation for Integrated Transport (FIT). The views expressed here are her own.