A £3 congestion charge covering only part of the city should be introduced in Cardiff, according to a new report.
The suggestion by the Centre for Cities think tank comes with Cardiff Council looking at options to bring in a charge on drivers in a bid to cut congestion and improve air quality.
It said the charge would be "better targeted" if it applied to a limited area around the city centre and the Bay, where congestion is highest, in a similar way to the Ultra Low Emission Zone (ULEZ) scheme in London.
This is different to the £2 fee for each car entering the council area which has been discussed previously.
The report's authors said it would be possible to raise the same amount of money with a higher charge in a smaller area.
The Welsh Government wants 45% of journeys to be made using public transport or active travel - such as walking or cycling - by 2040. Welsh cities, rather than rural areas, will make the largest contribution to making this ambition a reality, according to the report.
Cardiff has the greatest potential for increasing numbers using public transport due to the higher proportion of jobs based in the city centre than in Swansea, Newport and Wrexham, said the Centre for Cities.
Congestion is also higher in the capital than elsewhere, although it fell by 30% between 2019 and 2022.
However, the biggest challenge is the quality of public transport infrastructure, according to the think tank.
Amongst suggestions to improve public transport, the report said scrapping 20mph speed limits for buses could motivate more people to use them, as well as creating specific services to industrial estates and business parks.
The Welsh Government is looking at reforming bus services across the country.
Earlier this month, Deputy Climate Change Minister Lee Waters said: "It is becoming ever clearer that the privatised operating model for bus services is broken".
He added the system needs "reform".
The report said franchising bus services would increase coverage by creating routes that are not covered by private operators.
However, a subsidy would be needed to do this, according to the think tank, with the money needed to fund it raised partly by a congestion charge.
The Centre for Cities said money needed to reform the public transport system could also be raised by charging workplaces for parking spaces or giving the Welsh Government the power to increase fuel duty.
It also said funds could be raised through council tax and business rates.
Paul Swinney, director of policy and research at the Centre for Cities said: "Evidence from London and other cities worldwide shows congestion charging works where there are reliable uses and trains to provide an alternative.
"It also raises money that can be invested back into the public transport network.”
Councillor Dan De’Ath, Cardiff Council's cabinet member in charge of transport, said: “We welcome the report’s principal conclusion, that Cardiff offers the greatest potential to drive up public transport usage in Wales. This conclusion reinforces our call for further public transport infrastructure investment in the city and underlines why any type of road user payment must be on the basis of additionality, not replacing current funding.
"The report identifies a number of potential means to generate funding to invest in public transport, and the Cabinet will give full consideration to those, alongside the extensive consultation and equality impact assessment that will be carried out in developing a robust business case for any form of road user payment scheme."
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