If New Yorkers don’t pay a fee to use the police and fire departments, they should not have to pay to use the city’s mass transit system.

That’s part of the thinking of Theodore Kheel, who last Thursday donated $100,000 to the Institute for Rational Urban Mobility to study how a free mass transit system could save money for the city. Mr. Kheel, a 92-year-old philanthropist, environmentalist, and labor relations lawyer, says charging a fee to drive on the city’s most crowded streets would create an incentive for drivers to switch to mass transit. The revenue earned on the streets could be used to subsidize free subways and buses.

“Drivers would get a great benefit, too,” Mr. Kheel said. “Instead of getting stuck in traffic, they’d be able to move. We must treat traffic and transit as inseparably related.” Incentives to switch to subways from cars would improve public health and reduce time lost due to traffic congestion, as well as the price of all goods and services delivered in the city, he said.

Mr. Kheel’s critics argue that the already crowded subway system could not handle the influx of passengers if it were made free, but he describes the subway system as an underutilized facility. In 1943, when the fare was five cents, average weekday ridership was more than 8 million, almost double what it is today.

The president of the Institute for Rational Urban Mobility, George Haikalis, said no one is studying the cost implications of mass transit. “Vertical transportation systems like elevators are free,” he said. “The horizontal transportation system is as much a part of our system as the vertical.”

Mr. Haikalis’ nonprofit organization, which focuses on reducing automobile use in the metropolitan area, is also the group behind vision42, a proposal to build an auto-free Light Rail boulevard along 42nd Street.

Mr. Kheel, who represented Christo and Jean Claude in their legal battle with the city to erect “The Gates” in Central Park in 2005, founded a nonprofit organization, Nurture New York’s Nature, in connection with the art installation. Under the symbol of the orange gate, Mr. Kheel through his foundation funds environmental projects, such as studying the effects of a free subway system.

The four-month long study, titled “Price Matters,” will be conducted by professional transportation consultants Brian Ketcham and Carolyn Konheim. Mr. Kheel, a lifelong New Yorker who describes himself as a “catalyst on a hot tin roof,” said he has always been an advocate of mass transit because “it is the single most important step that can be taken to improve the quality of life in our city.”

While Mr. Kheel said he is chauffeured through the city in a private car because of a back problem, he said he is willing to pay for that privilege. “I’m exercising a privilege that’s costing other people money,” he said. “Free mass transit will be very costly for me, but I will get the privilege of being driven around in my car.”