Slowing down trains on the Underground could cut journey times for Londoners, a study has claimed.
The introduction of faster Tube services encourages more people to use the network but in many cases that causes commuters in the suburbs to drive to stations, according to researchers.
This puts more vehicles on the road at already busy times, intensifying congestion in localised areas and increasing overall travel times.
The findings come in a study - Multiplex networks in metropolitan areas: generic features and local effects - led by French geographer Marc Barthelemy. It analyses the effects different transport networks have on each other.
Coaxing more people into using the Tube would lead to reduced road congestion in central London. But researchers found the negative effect on traffic in the outskirts outweighed those gains.
“Changing one network can have an effect on the congestion of the others," Mr Barthelemy told the New Scientist.
“If you want to give more access to more people, then increasing the speed of the subway is not the best solution."
The study also looked at New York, where congestion is so bad that speeding up trains does help.