Emboldened by a three-week experiment to shut streets to car traffic on the East Side of Manhattan, bicycle activists are preparing to press Mayor Bloomberg to rid Prospect Park of car traffic by arguing that a car-free park would become part of his lasting legacy.

Advocates of a car-free Prospect Park say they have successfully whittled away at the number of hours that cars can use the park each day and are within striking distance of having them banned altogether.

With vehicular traffic allowed only for two hours in the morning and two hours at night Monday through Friday, they argue that ridding the park of cars at all times wouldn’t be too much of a stretch.

“We think it’s time. Now it’s to the point where the cars are in the park for so few hours that we can’t do too much more than go car-free and see what happens,” a spokesman for Transportation Alternatives, Wiley Norvell, said. “It would really touch the lives of millions of people.”

Crafting a strategy designed to appeal to the mayor’s penchant for experimental pilot programs and collecting data to determine if a new policy is successful, Mr. Norvell said his organization is asking for a three-month, car-free summer in 2009, which he said would give the city the opportunity to study the effects of the street closures on traffic patterns and park usage.

Critics of a car-free park have argued that it would lead to traffic congestion around the park and clog neighborhood streets during morning and evening commute times. The president of Brooklyn, Marty Markowitz, has said that the current arrangement strikes the right balance between the needs of park visitors and neighbors of the park concerned about traffic buildup.

In 2006, after an earlier push for the park to go car-free, the city agreed to reduce the number of hours that cars could use it. The compromise seemed to please many of those involved in the debate over use of the park, and Mr. Norvell said there has not been too much movement on the issue since then.

The group is aiming to capitalize on the mayor’s openness to experimental street closures, such as the closure of seven miles of Manhattan streets for three Saturdays this month for the Summer Streets program.

Next month, a new campaign to promote a car-free Prospect Park will kick off, when a number of teenagers who worked with Transportation Alternatives this summer will present the mayor with some 7,000 postcards signed by park users in support of a car-free summer next year. The students also tracked the speed of cars using the park this summer and found that more than 90% exceeded the speed limit, often by more than 20 miles an hour, Mr. Norvell said.

A spokesman for the city’s Department of Transportation, Scott Gastel, said the city implemented a partial closure to vehicles a year ago, but has no plans at this time to completely close the park to vehicles.

The president of the Prospect Park Alliance, Tupper Thomas, said she’s in favor of closing the park to cars, but said she understands the reservations of residents who are concerned that it would lead to congestion around the outside of the park.

She noted that even though there are few hours that cars can use the park, they coincide with the times that many New Yorkers also want to head to the park for a jog, a walk, or a bike ride — in the morning before work or in the early evening, when they get home.

Ms. Thomas said she rides her bicycle in the mornings, but leaves the park by 7 a.m. because she doesn’t trust her cycling ability around cars.

“From a park’s perspective, the more the park can be available to relax, recreate, enjoy yourself, the better it will be,” she said.