Mayor Bill de Blasio announced Thursday the expansion of the city's car-sharing program.

It will receive 309 reserved parking spaces across the five boroughs, the mayor said. Of those, 230 are on the street and won't be subject to alternate-side parking rules. Another 55 are in municipal parking lots. The remainder were previously assigned to NYCHA.

The city has already installed signs informing drivers of the change at the Parkchester train station, where 20 spaces are now reserved for Zipcar and Enterprise car-sharing programs. By Monday, the signs will be in place in more than a dozen neighborhoods.

De Blasio says car-sharing will reduce traffic and air pollution.

"If people only sometimes need a car, let's make it easier for them to get the car only when they need it," he says. "And not have to pay all those other costs all year long for something they don't need a lot of the time."

Critics say the new rules aren't fair for people who have their own cars -- and that car-sharing likely won't be as popular as the city has predicted.

The program begins Monday and will run for at least the next two years.