San Francisco transportation officials plan to drive more cars off Market Street, paint and enforce red transit lanes and ticket drivers who block intersections. The changes to the city's main drag could begin within months.

The traffic diversions, which would force private vehicles to turn left or right off of Market, depending on the direction of one-way streets, are essentially an extension of the program that pushed cars off Mid-Market at Sixth and 10th streets in 2009. But while that effort was premised on speeding up transit, these changes aim to reduce collisions, particularly with pedestrians.

"This is primarily a safety project," said Timothy Papandreou, director of strategic planning in the sustainable streets division of the Municipal Transportation Agency.

Some elements of the plan are already moving ahead, including posting "Don't Block the Box" signs at Market Street intersections, retiming traffic signals at Sixth and Market and Sixth and Mission streets, and painting the existing transit-only lanes red. Those changes will be completed by late spring or early summer.

The changes announced Friday include stepped-up enforcement of existing transit-only lanes and turn restrictions. Early next year, additional mandatory turns are to be installed at Third, Fourth and Fifth streets and transit-only lanes would be extended eastward down Market.

Market Street between Eighth and Montgomery streets has twice as many collisions as parallel Mission Street despite having only a third of the traffic, Papandreou said. It also includes four of the city's 20 worst intersections for collisions that injure or kill pedestrians - Fifth Street, Sixth Street, Eighth Street and Main Street. Two of the worst intersections for bike collisions are also on Market at Third and Fifth streets.

The MTA will focus first on Montgomery to Fifth streets before considering whether to head farther down Market.

"That's where the worst of the worst is," said Papandreou. "There are a lot of people heading from the downtown area to the bridge."

Paul Rose, an MTA spokesman, said the agency will be talking with merchants and the public in the next couple of months about pushing more cars off of Market Street. Plans will be completed by fall.

The agency is pushing ahead with changes to Market after holding off while a variety of city agencies work on a plan to redesign the city's main boulevard. During a workshop earlier this year, MTA officials said they couldn't wait any longer.

Part of that urgency is San Francisco's growing problem with pedestrian collisions and the city's commitment to increase safety for pedestrians and bicyclists. Market Street is a major transportation corridor for transit, pedestrians and bicyclists, who outnumber cars. Market serves 24 Muni routes plus several regional transit agencies and shuttles and on an average weekday, an average of 200,000 people walking.