It took an average of three minutes and 38 seconds to travel one kilometre (0.6 miles) in the centre of the city last year, mapping technology company TomTom said.
Reducing speed limits from 30mph to 20mph has been one of Sir Sadiq Khan’s main transport policies as mayor, as he aims to reduce road casualties.
More than half of the capital’s roads now have a 20mph limit, according to Transport for London’s website.
It says that a person is five times more likely to be killed if they are hit by a vehicle travelling at 30mph rather than 20mph.
Meanwhile, drivers lost an average of 141 hours stuck in rush hour traffic in 2025, the analysis found.
The worst day of congestion in London, on 10 September last year, coincided with a Tube strike.
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TomTom traffic expert Andy Marchant said: “Low average speeds in London are often interpreted as extreme congestion, but that misses the full picture.
“Average speeds are heavily shaped by static factors such as the widespread 20mph limits, street design that doesn’t match today’s needs, and consistently high traffic volumes, which means journeys can be slow even when roads are flowing.”
Belfast tops congestion list
Colombia’s Barranquilla was the only one of the 492 towns or cities in the study whose average road speed was slower than London’s last year.
London was also the UK’s eighth most congested town or city at 52%, based on the percentage of average additional time spent in traffic compared with free-flow conditions.
Belfast had the highest congestion level at 58%, followed by Edinburgh (58%) and Cambridge (54%).
Mr Marchant added that this year will be an “interesting time for urban mobility”, with autonomous vehicles set to be trialled across 20 London boroughs.
A spokesperson for the mayor of London said: “Comprehensive TfL data suggests that journey times on the TfL road network actually remain at similar levels to those in 2019, while analysis from around the UK suggests that lower speed limits have not increased congestion.”
She added: “Studies have shown 20mph limits and zones may improve traffic flow and cut traffic jams.”
