London's Ultra Low Emission Zone — Roadside NO2 Down Sharply in Central London, but Independent Analysis Finds No Added Effect From the 2023 Expansion

London's Ultra Low Emission Zone began in central London in April 2019 and was expanded in stages, most recently London-wide in August 2023. Vehicles not meeting emission standards pay a daily charge.
City Hall's modelled estimates put roadside NO2 in 2024 at 27% below what it would have been without the ULEZ and its expansions: 54% lower in central London, 29% in inner London and 24% in outer London. The first-year effect of the London-wide expansion in outer London was up to 4.8% lower NO2 than expected. These are the authority's own estimates, advised by an expert group chaired by Imperial College's Gary Fuller.
An independent University of Birmingham study (Tong, Shi, Bartington, npj Clean Air) uses machine learning on hourly data from 124 monitoring sites. It finds NO2 fell 19.6% and NOx 28.8% at central roadside sites within three months of the 2019 launch, but detects no significant impact on NO2 or NOx from the 2023 expansion, attributing this partly to an anticipation effect and a compliant fleet (only 7.4% of vehicles non-compliant by 2023). Pollution remains above WHO guidelines.
The practice is best read as a policy with strong early evidence in the centre and diminishing, contested returns from later expansion, which makes it a useful cautionary reference for cities planning phased zones.

Read the full analysis: https://www.birmingham.ac.uk/news/2025/londons-ulez-cut-air-pollution-high-vehicle-compliance-left-little-room-for-post-expansion-gains