Taxis and private hire vehicles found to be major contributors to urban air pollution, reveals Scottish study

A recent study by The Real Urban Emissions (TRUE) Initiative has unveiled a concerning trend: high-use vehicles such as taxis are disproportionately contributing to air pollution in Scotland’s major cities.

The findings are based on a comprehensive analysis of 1.4 million real-world vehicle emission measurements taken over three years in Glasgow, Edinburgh, Dundee, and Aberdeen.

The study's data highlights that diesel taxis and private hire vehicles emit significantly more nitrogen oxide (NOx) compared to regular passenger cars. This is largely attributed to the faster degradation of their emission control systems due to higher usage rates. Taxis registered between 2009 and 2016 were found to emit 84% more NOx than passenger cars of the same age group. Notably, the introduction of newer vehicles into the taxi fleets in cities like Dundee and Edinburgh has not reversed this trend; instead, average emissions have increased.

Another alarming discovery is the state of older diesel taxis fitted with diesel particulate filters (DPF). The report indicates that some of these vehicles are emitting particulate matter at levels suggesting their DPFs are either malfunctioning or have been removed altogether.

In contrast, the report offers a glimmer of hope in the form of newer diesel and petrol passenger vehicles, which show marked improvements in NOx emissions. The latest Euro 6d models, for instance, are producing 95% less NOx compared to older Euro 3 models. Despite this, there are still concerns, as some of the newest vehicles continue to exceed on-road emission limits.

The study also draws attention to older Euro V trucks, which are shown to be particularly prone to increased emissions over time. These vehicles, often with defective or tampered emission control systems, represent a significant risk and point to the need for stricter regulations.

The report concludes with a series of policy recommendations aimed at tackling these issues. Key proposals include adding NOx emissions testing to the national vehicle testing programme, introducing stricter age or mileage limits for taxis and private hire vehicles, and implementing measures to accelerate the transition to zero-emission vehicles.

The findings present a clear challenge for policymakers in Scotland as they seek to reduce urban air pollution and its associated health risks. With high-use vehicles playing such a significant role in emissions, targeted action will be essential in mitigating their impact on city environments.

"Analysing the results of this study over several years allowed us to build a better picture of real-world emissions in these four cities," says Kaylin Lee, researcher at the ICCT. "It allowed us to offer insights and provide recommendations that reflected the year-to-year changes in the fleets versus a single window of opportunity."

Derek McCreadie, Transport Scotland Air Quality Senior Advisor, said: "Remote sensing provides insight into real-world emissions of diesel and petrol vehicles in the urban environment, detecting emissions that diverge from the type-approval limit and typical levels. Now that Low Emission Zones have been introduced in four major cities, these technologies improve our understanding of real-world vehicle emissions and provide an evidence base for the delivery of policies to improve air quality and benefit the health of people in Scotland."

Sheila Watson, Deputy Director of the FIA Foundation, a core founder and partner of the TRUE Initiative, added: “It is shocking that the highest use vehicles in Scottish cities are the most polluting. Even when Dundee and Edinburgh taxi fleets are being upgraded to newer standards, their emissions are still getting worse. It is clear that there needs to be a change in how we move in urban areas. We have got to break the urban reliance on cars and shift to public transport, walking, and cycling for our environment and our health.”