Bristol is investigating the possibility of building an underground system to ease the city’s growing congestion problems.

The local council has said it could cost about £4bn to build three lines, including one linking the airport with the city centre.

Initial feasibility studies have been launched and the directly elected Bristol mayor, Marvin Rees, has travelled to China to try to drum up investment.

As well as a line taking passengers to the airport, which is to the south of the city, a second route could head north and a third east.

Few details have been released but Mhairi Threlfall, the Labour cabinet member for transport, said it was possible that lines could run overground, underground – or a combination of both. She said that when feasibility studies were complete the risks and opportunities would be made public.

Threlfall said: “Bristol has limited road space. We do need to be creative and inventive about how we tackle issues around congestion.”

Some opposition councillors have dismissed the idea as “pie in the sky”.

The council’s Conservative group leader, Mark Weston, said: “In any large project a lot of money is spent on paperwork, studies, reports. My fear is we could spend tens of millions on reports and in the end find that the project is unfeasible.”

Speaking at a full council meeting on Monday, he called for more information to be released about the work that has been done so far including a geological study and detail on how the scheme would be paid for. “I want the information that underpins the mayor’s optimism,” he said.

Fellow Tory John Goulandris said big infrastructure projects were beloved of politicians keen to leave a tangible legacy.

He said: “A Bristol underground which served the whole city and went out the airport would be wonderful. Equally welcome would be an expansion of our libraries, more parks, more toilets, a new hospital, more police on our streets. The stumbling block is money. It’s good to dream sometimes, but making dreams a reality is tough.”

The Green councillor Charlie Bolton said such a scheme could draw resources away from other important projects and suggested that spending “a couple of hundred million” on a comprehensive city cycle network would be more effective.

There was a mixed reaction on the streets of Bristol. Dave Green, whose commute by car into Bristol takes an hour each way, welcomed the idea. “It’s terrible getting into Bristol now. I’d love to sit on a tube train instead of in my car.”

Jan Spence, who cycles into town, said the scheme would never happen. “Nice idea – but I won’t see it in my lifetime, she said.