Driving down emissions for low carbon public transport

14 Oct 2020

 By Kathryn G. Logan, Astley Hastings, John D. Nelson

In the UK, road transport is the only sector where emissions have continued to grow, increasing by 6% to ~118 MtCO2e by 2017, the equivalent of 21% of the UK’s total GHG emissions (ONS, 2019). Car use remains one of the most popular methods of travel with 31.5 million licensed vehicles, contributing to 62% of the total transport GHG emissions in 2016 . If the UK is to meet its net zero emission reduction targets from transport, focus needs to be placed on shifting away from personal vehicles towards low carbon public transport.

As one of the world’s most popular methods of public transport, reducing the environmental impact of buses further will reduce a significant level of emissions, particularly during rush hour traffic. Similarly, rail is often perceived as a ‘green’ mode of transport as the ~2 MtCO2e produced in 2016, was the equivalent of only ~2% of total UK transport emissions. However, if both buses and trains are not upgraded to either electric or hydrogen, as well as having the available infrastructure, emissions could remain static.

Low emission travel potential

By transitioning towards public transport, a significant number of personal vehicles can be taken off the roads. For example, by assuming the maximum capacity of an average bus is 80 and a small personal vehicle is four, at least 20 personal vehicles could be taken off the road if buses were fully utilized. Similarly, for trains, assuming a capacity of 447 individuals on a regional train, 112 personal vehicles could be taken off the roads, significantly reducing road transport emissions and congestion. Furthermore, although the UK aims to ban the sale of petrol and diesel vehicles (and vans as well as hybrid vehicles) from 2035 onwards, with EVs likely to fill this gap in the transport network, there will remain a need to encourage the use of public transport.


Emission standard ratings requirements have been implemented for buses, with Euro VI the latest standard introduced for new diesel engines. Electric buses have been introduced in London, Manchester, and Birmingham. In London, the introduction of electric buses, low emission bus zones, and Euro VI diesel buses, are expected to reduce bus emissions output by 84%. In Aberdeen, single-decker hydrogen buses have reduced emissions by 150 tonnes of carbon dioxide in comparison to Euro VI buses, with plans to expand this network to include double-decker hydrogen buses later in 2020. Through London’s and Aberdeen’s separate initiatives, both areas highlight the potential of even relatively small-scale switches to clean energy options and have seen decreases in the level of operating emissions produced.


In 2018, the UK Government announced that to reduce rail emissions further, diesel-only trains are to be phased out by 2040. Rolling stock replacement is a longer-term project compared to that of CFVs as the service life of trains are currently ~19.2 years, therefore new diesel trains commissioned in 2040 will potentially be on the tracks until ~2060, past the UK’s emission reduction targets. Therefore ideally, no new diesel trains should be commissioned after 2020, however, this is reliant on infrastructure. This transition will be challenging in the UK as almost 60% of the rail network is not yet electrified. Additional new infrastructure projects such as the High Speed 2 are expected to make improvements through track and signalling infrastructure allowing for enhancements in efficiency and decreasing travel time. Hydrogen trains have not yet been introduced in the UK, however, several countries are testing and already utilising this technology with the UK planning to run tests from 2020.

Current COVID-19 climate

Although this transition to low carbon public transport will remain a challenge for years to come the current COVID-19 pandemic poses further challenges. At the beginning of the outbreak in March 2020, transport use fell dramatically over the course of the month due to the UK Government’s lockdown measures limiting unnecessary transport and actively encouraging individuals to work from home where possible. During the pandemic, there have been behavioural changes in transport use with an increase in active travel, however significant and long-lasting changes to travel behaviour remain unknown. During the lockdown period within the UK, active travel has been encouraged as this will reduce the pressure on public transport systems and the road network. As restrictions on the lockdown are lifted public transport use, which has seen reduced capacity and limited seating to ensure individuals are social distancing, is struggling to recover. Car use remains low but has begun to increase steadily over the course of October 2020. As the UK Government continues to encourage social distancing, it is important that significant behavioural incentives are implemented to ensure public transport recovers

Environmental impact of low emission transport

Through this transition towards low carbon public transport, it remains inevitable there will be an impact on the environment through additional energy generation, and the required infrastructure (generation source, power distribution, charging infrastructure, and from the vehicles themselves). However, even with this additional energy generation, the environmental impact of electric and hydrogen transport, particularly from low carbon public transport, remains significantly lower than that of conventionally fuelled alternatives.

Further downstream impacts of fossil fuel usage such as ocean acidification and global temperature rises causing sea ice melt are all factors that impact the environment and therefore have a value from a natural capital and ecosystem service viewpoint. This is not to say that all renewable generation methods do not have any quantifiable negative effect on the local environment, however, the impacts are generally negligible when compared to no implementation.

About the authors

Kathryn G. Logan is currently working as an energy policy researcher at the University College Dublin. She is in the process of completing her PhD (passed her viva examination in September 2020) in Environmental Science from the University of Aberdeen.

Astley Hastings is a Research Fellow in the School of Biological Sciences at the University of Aberdeen. He is a chartered engineer and environmental scientist, with a BSc in Mechanical Engineering and a PhD in Environmental Science.

John Nelson holds the Chair in Public Transport, Institute of Transport and Logistics Studies (ITLS), University of Sydney. He was previously the Sixth Century Chair of Transport Studies in the School of Engineering at the University of Aberdeen and Director of the Centre for Transport Research.