It was on last year’s Earth Day that Mayor Bloomberg unveiled his far-reaching plans to make New York City more sustainable, with congestion pricing as one of the centerpieces. For some reason, making Central and Prospect Parks car-free did not make the list of 127 announced initiatives. With congestion pricing off the table for now thanks to some profiles in fecklessness in Albany, the Bloomberg administration has more reason than ever to remedy that oversight.

Congestion pricing would have quickly resulted in a palpable drop in traffic, but it is hardly the only strategy for removing substantial numbers of cars from city streets. The administration simply has to switch city models, moving from the London one of making it costlier to drive to the Paris and Copenhagen ones of making it more difficult to move around by car and pricier to park, while creating infrastructure that encourages alternatives like biking and mass transit. The approach is more incremental — close a road here, build a dedicated bus or bike lane there — but eventually you get to the same place: less traffic, cleaner air, and a more livable and functional city.

If you were a traffic engineer at the Department of Transportation, you would now be looking for opportunities to close roads that are not essential arteries, particularly those that, by their mere existence, serve as enticements to drive. Can anyone think of such roads? Wait a minute! Isn’t there one in some famous park just north of Midtown, and another one in a gorgeous park in Brooklyn?

In short, the stage seems to be set for at least a trial closing of both parks’ bucolic loop roads to car traffic this summer. In fact, the logic of this seems so self-evident that if it fails to happen, it will be a clear sign that powerful and sinister forces are blocking it.

Here’s the new, post-congestion pricing case for car-free parks:

The amount of traffic affected by closing the bucolic loop roads of Central and Prospect Parks would be small thanks to previous cutbacks in the hours that cars are allowed to invade them. But closures would nevertheless play a modest role in reducing traffic. With the loops no longer available, a significant percentage of the drivers who use them would switch to other transportation or significantly modify their driving patterns so that they would effectively disappear from the grid — a well-documented phenomenon called "shrinkage."

What percentage would do this? Estimates vary. The Regional Plan Association has said that closing Central Park’s loop would induce 20 percent to 60 percent to get out of their cars or drive when they won’t be contributing to congestion. In testimony before the City Council in 2006, transportation consultant Bruce Schaller (now with the DOT) predicted that this figure could be as high as 100 percent. This is not as outlandish as it seems. After the collapse of the West Side Highway in 1973, almost none of the traffic that had used it turned up on surrounding streets. Drivers evidently found other ways to get around a city that has a notably flexible and diverse transportation system.

We also know that the availability of the Central Park loop is drawing hundreds of cars a day into Harlem that otherwise would stay on peripheral roads like the Henry Hudson Parkway or would opt for alternative transportation modes. A recent study by Transportation Alternatives of private car drivers entering Central Park from the north found that 57 percent began their trips outside of Manhattan. TA estimates that closing the park to traffic would remove at least 3,107 private vehicles a week from Harlem streets during the morning commute.

For an administration searching for politically painless ways to cut traffic congestion, the Central and Prospect Park loop roads are ripe for the picking. In fact, making our two crown jewel parks the car-free spaces they were meant to be could be the signature initiative of the mayor’s shift to "Plan B." Some political pundits have said that the demise of congestion pricing may have cost Mayor Bloomberg a significant piece of his anticipated legacy. While small in its contribution to reducing traffic, permanently eliminating cars from both parks would be huge in symbolic value and help secure the mayor’s historical standing as a gutsy environmental innovator.

Ken Coughlin is Chair of the Car-Free Central Park Campaign.