Two of the UK's 10 most congested roads in 2016 were in Belfast, data shows.
But Belfast did not feature in the 10 most congested cities in the UK - despite being ranked third in the UK by data company Inrix in 2015.
The southbound Lisburn Road from College Gardens to Wallace Park was the most congested road outside London, according to an Inrix study.
Drivers spent on average 49 hours in gridlock on the road last year, said the report.
That made it the sixth most congested road in the UK.
Also making the top 10 congestion list was the A24 from the Ormeau Road to Ann Street in the city centre.
It came in ninth in the congestion table. While it was busiest in the morning, the Lisburn Road was busiest in the afternoon.
The study, published on Monday, analysed 87 cities and large urban areas across the UK, as part of a wider survey of 1,064 cities in 38 countries worldwide.
The UK ranked as the fourth most congested developed country in the world and the third most congested in Europe, with drivers spending an average of 32 hours a year in congestion during peak hours.
Inrix estimates that the direct and indirect costs of congestion for all UK drivers totalled £30.8bn in 2016, an average of £968 per driver.
Ann McGregor, chief executive of NI Chamber of Commerce and Industry, said infrastructure was one of the biggest issues consistently raised by its members.
"Businesses rely on having infrastructure and transport routes in place whereby goods and products can be moved efficiently and without extra costs from one location to another," she said.
"Therefore it is vital that key projects across the country, for example the York Street Interchange, A5, A6 and the Belfast Transport Hub, all proceed which will ease traffic congestion and in the process help businesses with their operational costs, supply chain efficiency and overall productivity."