City supervisors, acting in their role as commissioners of the San Francisco County Transportation Authority, agreed Tuesday to a comprehensive study of whether to restrict cars on the downtown stretch of Market Street from the Embarcadero to Van Ness Avenue.

"There's a growing momentum to restrict cars, but it will probably be done in baby steps," said Supervisor Chris Daly, the chief proponent at City Hall of a car-free Market Street.

The heavily used 2-mile stretch is shared by bikes, buses, streetcars, cars, trucks and pedestrians. It cuts through the Financial District, the downtown shopping district and the seedy strip to the west of Powell Street.

"Having been a great street, Market Street has the scale, the social and historical significance, the architectural profile and the infrastructure, and hence the potential, to be great once again," Tilly Chang, deputy director of planning for the Country Transportation Authority, wrote in the agency's formal request to proceed with the study.

The analysis, to be completed within three months, will look at such questions as how restricting cars on Market Street would affect traffic on nearby streets and how it would affect businesses and neighborhood quality of life.

It will look at limited restrictions and an all-out ban.

The idea is not new. It was broached by Willie Brown more than a decade ago when he was mayor, and it has been studied and debated since.

But now the timing might be right.

The Department of Public Works is planning a major Market Street repaving project, creating an opportunity to remake the streetscape. The Municipal Transportation Agency is moving forward with an overhaul of the city's transit system in which Market Street would play a major role.

Add to that a growing awareness of how cars aggravate global warming and the effect that rising gasoline prices can have on getting people to rethink driving, and some of the opposition to a car-free Market Street has started to thaw.

"There has been a change in attitude over the last five years," said Carolyn Diamond, executive director of the Market Street Association, a nonprofit organization that promotes the beautification and economic development of the central corridor.

"This may be a way for the city to become better," she said.

Rather than a ban, Diamond said restricting private autos during certain times of the day could be more palatable.

No matter what, she added, city officials need to provide assurances that restricting cars on Market Street would not hurt businesses.

"That's been our biggest fear," she said.

Linda Mjellem, executive director of the Union Square Association, agreed. "Our concern all along is that a ban - whether full or partial - would harm the economic vitality of the street."

A spokesman for Mayor Gavin Newsom said the idea of banning cars on Market Street is worth studying.

"If it doesn't have a detrimental effect on businesses and is done the right way, then the mayor would be open to it," said Newsom spokesman Nathan Ballard.

The new study will consider possible impacts on businesses. It also will determine whether removing autos would add to or detract from the unsavory elements that pock much of the mid-Market area between Fifth and Eighth streets.

Does having cars travel up and down the corridor provide an element of safety, particularly at night? Or, could the creation of a pedestrian-friendly corridor bring more life and vibrancy to the struggling area?

Andy Thornley, project director for the San Francisco Bicycle Coalition, said revamping Market Street shouldn't be approached too narrowly. If cars are restricted, the city should make sure that improved amenities are in place for transit users, cyclists and pedestrians, he said.

That would mean investing in new street furniture, better lighting, new bus stops and dedicated bike lanes. More expansive landscaping also could improve the ambiance.

Diamond said the city also would have to commit to increased police patrols and traffic enforcement so people would feel safer.

"If we're going to take cars off of Market Street," Daly said, "we're going to have to find a way to remake Market Street to make it more inviting."

E-mail Rachel Gordon at rgordon@sfchronicle.com.

This article appeared on page A - 1 of the San Francisco Chronicle