Rides on O'ahu's public bus system could someday be free under a new proposal by City Council Chairwoman Barbara Marshall.

"I think we have to look at any and all alternatives that may relieve traffic congestion," she said. "I think it's going to take something dramatic to get people out of their cars, and this is dramatic."

A bill she sponsored would not immediately suspend fares, but would create a process that allows the council to do so later if that is possible while balancing the city's next annual budget.

The city would give up more than $40 million in annual fare revenue if public transportation was made free, Marshall estimated. But doing so might boost bus ridership tremendously, relieving traffic pressure on roads and highways.

"I don't think anyone can deny that this would be a tremendous public service," she said. "I don't know if we can afford it. But we need to have a public conversation about it."

Ridership on the Big Island's smaller bus system has soared since fares were suspended there two years ago, she said. The adult fare for a single ride on O'ahu's system is $2.

Marshall said her proposal was not meant to undermine or replace the fixed-guideway mass-transit system Mayor Mufi Hannemann hopes to build.

But the cost of free buses pales in comparison to the $5 billion transit project, which will take years to build.

"I'm not saying we should do this instead of transit, but this may be an immediate solution to our growing traffic problem," Marshall said. "If we could increase ridership by 20 to 30 percent, that would be huge."

About 70 million passengers use the bus system each year. Increased ridership could require more buses and drivers, which would have to be taken into consideration if the plan moves forward, she said.

Hannemann said he had not seen details of the plan but would be concerned about the potential loss in city revenue.

Reach Johnny Brannon at jbrannon@honoluluadvertiser.com.