How the ULEZ Expansion Reshaped Air Quality and Traffic in Outer London: Two Years On

When the Ultra Low Emission Zone (ULEZ) expanded to cover every London borough on 29 August 2023, it was the most ambitious road-pricing intervention in the city’s history. The Mayor of London, Sadiq Khan, argued that extending the £12.50 daily charge from the inner boundary to the Greater London border was necessary to reduce toxic air pollution and meet legal NO2 limits. Two years on, the data offers a clearer picture of what has changed—and what remains contested.

The expansion was fiercely opposed by some outer London councils and residents, who said it would penalise lower-income households and small businesses reliant on older vehicles. Proponents pointed to the health costs of air pollution, estimated by the Greater London Authority at up to £3.7 billion per year. Now, with two full years of monitoring data, we can separate the measurable outcomes from the still-unresolved questions.

Air Quality Gains: What the Monitors Show

The most direct metric is nitrogen dioxide (NO2) concentration, which is closely linked to diesel exhaust. The Mayor of London’s Air Quality Annual Report 2024, published by the GLA, recorded a 20% reduction in roadside NO2 concentrations in outer London in the first year of the expanded zone, compared with the pre-expansion baseline. The London Datastore’s air quality monitoring network confirms that this reduction has been sustained into 2025, with average annual NO2 levels now below the legal limit of 40 µg/m³ at all but a handful of roadside monitoring sites.

The improvements are not uniform. The table below compares changes in roadside NO2 at selected monitoring stations across outer London, using data from the London Air Quality Network (LAQN) and the GLA’s annual reports.

Sources: LAQN monitoring data and GLA Air Quality Annual Report 2024. Pre-expansion baseline is the 2022 annual average.

The biggest drops are at sites that previously had the highest NO2 levels, often near major roads. The data suggests that the ULEZ is working as intended to reduce the most harmful concentrations, though ambient background levels (away from busy roads) have fallen less steeply.

Compliance Rates: More Clean Vehicles, Faster Than Expected

Transport for London’s monthly compliance reports show that the proportion of vehicles driven in the expanded zone that meet the ULEZ standards (Euro 6 for diesel, Euro 4 for petrol) has risen steadily. In August 2023, on the day of expansion, compliance was estimated at 85% across the whole of London. By August 2025, that figure had reached 97%, according to TfL’s latest data. The number of non-compliant vehicles detected per day fell from around 35,000 at launch to fewer than 10,000 in the first half of 2025.

This sharp increase in compliance is partly due to the scrappage scheme, which offered grants of up to £2,000 for cars and up to £7,000 for wheelchair-accessible vehicles. The GLA reports that the scheme funded the replacement of over 50,000 vehicles in its first two years. However, uptake has been lower among small businesses and self-employed drivers, many of whom operate vans and minibuses that are expensive to replace.

Traffic: Has the Expansion Reduced Congestion?

One of the opponents’ predictions was that the ULEZ would push traffic onto non-compliant routes or simply shift journeys to other times. The evidence so far suggests a small but measurable reduction in overall vehicle miles in outer London. TfL’s traffic data for 2024 shows a 3% drop in total vehicle kilometres in the outer London area compared with 2022, after accounting for general traffic trends. The effect is most pronounced in the early morning and late evening, when some drivers appear to have switched to public transport or cycling.

However, traffic levels on the orbital roads that form the boundary of the expansion (the Greater London border) have not increased significantly. TfL’s analysis of automatic traffic counters at the boundary shows no consistent pattern of displacement. The reduction in inner London traffic, where the ULEZ had already been in place since 2019, has continued, with a further 2% drop in 2024.

Economic Effects: The Hardest Evidence to Gather

The most contentious question is whether the ULEZ expansion has harmed businesses in outer London. The London Assembly Environment Committee’s report “ULEZ: One Year On” (2024) found that businesses reported higher operating costs due to vehicle upgrades, but the overall economic impact on high streets and local employment was not statistically significant in the first year. The report noted that the sample was small and that many businesses were already planning to upgrade their fleets anyway.

The GLA’s own economic analysis, published in 2025, estimates that the net effect on outer London employment is neutral, with any job losses in sectors like van-based trades offset by the positive health benefits (reduced sick days and healthcare costs). But these models rely on assumptions that are hard to verify. The real-world data on business closures and job losses linked directly to the ULEZ is thin, and no independent audit has been commissioned.

What Remains Unclear

Several important questions cannot be answered with current data. First, the long-term health effects of the NO2 reduction will take years to appear in hospital admissions and mortality statistics. Second, the impact on particulate matter (PM2.5), which is also harmful but less directly linked to vehicle exhaust, is still being studied. The ULEZ primarily targets NO2, and PM2.5 reductions are modest (around 10% in outer London, per the GLA’s 2024 report).

Third, the equity effects remain disputed. The scrappage scheme has mostly helped car owners, but households without a car do not benefit from the charge, yet they breathe cleaner air. The balance of who pays and who gains is not fully captured by the aggregate data.

Practical Checks

If you live in an outer London borough and want to verify the impact of the ULEZ in your area, you can check the London Air Quality Network’s interactive map for local monitoring stations. For traffic data, TfL publishes monthly compliance and traffic counts on its data portal. The GLA’s air quality annual reports are also available online. The next major update will come with the 2025 annual report, due in late 2026, which will include the full two-year health modelling.

Two years after the expansion, the evidence supports the case that the ULEZ has reduced the most harmful roadside pollution and pushed vehicle compliance to near-universal levels. The economic and equity trade-offs remain less clear-cut, and will require more detailed, independent study to resolve.