Mayor Michael R. Bloomberg, warning that a transit strike would seriously hurt the city's economy, announced yesterday that only cars with at least four people would be allowed to enter Manhattan south of 96th Street on weekday mornings if bus and subway workers walk off the job on Friday.

Starting times for city schools would be delayed by two hours to allow children time to get to school, and several streets, including much of Fifth and Madison Avenues, would be closed to all but emergency vehicles, according to an affidavit the city filed in court yesterday.

Wading forcefully into the labor dispute, Mr. Bloomberg said he hoped that a walkout would be averted and urged transit workers to follow the example of many municipal unions by exchanging productivity increases for bigger raises.

"A strike would not be good for the city, a strike would not be good for the union," Mr. Bloomberg said during an appearance in Manhattan. "It will cost an enormous amount of money in economic activity. There will be a lot of people who would lose their jobs during a strike."

In court papers filed in support of an injunction against a strike, slowdown or sickout, the Bloomberg administration estimated that the city's businesses would lose $440 million to $660 million per day in business activity during a transit strike.

Roger Toussaint, president of Local 100 of the Transport Workers Union, representing 33,700 subway and bus workers, said the mayor should not interfere in his union's talks with the Metropolitan Transportation Authority, a state-controlled agency.

"Mayor Bloomberg is not part of these negotiations, and it should stay that way," Mr. Toussaint said as he entered the Grand Hyatt hotel for another negotiating session.

Last night, the authority altered its earlier proposal of a 3 percent increase one year and 2 percent the second, proposing instead a 27-month contract with a wage increase of 3 percent in the first year and another 3 percent effective March 16, 2007. In addition, instead of linking the second year of wage increases to reductions in sick leave use, as it proposed last week, it would impose new restrictions on sick leave unless workers reduce their use of sick leave to 2002 levels. The union did not immediately respond to the offer.

Earlier in the evening, Mr. Toussaint had restated his demand for an 8 percent wage increase in each of the next three years.

Local 100 has threatened to shut down the transit system if the two sides fail to reach a settlement by 12:01 a.m. Friday, when its contract expires.

"I'd still say there is a 50-50 possibility" of a strike, Mr. Toussaint said, "but there is still plenty of time to work things out."

In an appearance at Parsons the New School for Design, Mr. Bloomberg told reporters that the city would be firm in barring cars with fewer than four people from entering Manhattan south of 96th Street from 5 to 11 a.m. weekdays. He said the city had not completed enforcement details. "When we say cars coming in, we mean every car," he said. "One of the things we learned out of 9/11 was if you start making exceptions, it becomes unfair and unenforceable. And you're going to have four in a car the same way I'm going to have to."