Heathrow expansion could add 22 million car trips a year, Mayor warns

A third runway at Heathrow Airport could add 22 million car trips to London’s roads each year, the Mayor of London has warned. Sadiq Khan and Transport for London submitted evidence to the House of Commons Transport Committee stating that the expansion would create 60,000 additional highway journeys daily, increasing congestion and air pollution in west London.

The Department for Transport published the Heathrow Expansion National Policy Statement in June 2026. The document sets out the framework for deciding whether to approve the third runway. The Mayor’s submission argues the government’s case relies on outdated evidence from 2015 and fails to account for changes in business travel, airport capacity elsewhere in the UK, and net-zero carbon targets.

Read Next in News London5 min read

Silvertown Tunnel cycle shuttle costs £5,400 daily for 51 riders

TfL runs the service until 2028 under Department for Transport rules, despite low usage and calls to scrap it

Air quality gains at risk

London’s air quality has improved since the Ultra Low Emission Zone was introduced. In the Heathrow area, average nitrogen dioxide concentrations have fallen by 34%, and fine particulate matter (PM2.5) by 28%. The Mayor’s submission warns these gains could be lost if expansion goes ahead, as increased flights and road traffic would raise emissions.

The submission states the government’s environmental tests do not provide enough assurance that an expanded Heathrow can avoid serious environmental harm. It accuses ministers of relying on improvements from unrelated policies, such as ULEZ, to justify more flights without addressing the airport’s direct impact.

Related Borough Coverage5 min read

TfL escalators past lifespan force repairs and station closures

More than a quarter of London Underground escalators now exceed their expected working life, Transport for London data shows

Passenger numbers and transport strain

Heathrow’s annual passenger numbers are projected to rise by 79% to 150 million if the third runway is built. The Mayor’s submission says this would place intolerable strain on the local road network, increasing pollution and carbon emissions. It also questions whether public transport can absorb the extra demand.

The government has suggested Heathrow can expand without increasing road traffic, but the Mayor’s submission says this would require a significant shift to public transport. Upgrades to the Piccadilly Line and Elizabeth Line would help, but additional direct rail services to destinations such as Waterloo, Basingstoke, and Reading would be needed to meet demand.

What happens next

The government aims to secure planning approval for the third runway by 2029, with operations expected to begin by 2035. There is no indication ministers will reverse the decision. A Department for Transport spokesperson previously told the London Datastore that airport expansion would support jobs and connectivity while meeting decarbonisation goals.