The really bad thing about this little universe of ours is that the value of many deeds can be determined only after they have become an accomplished fact and, by then, it may be too late.

The notion of transforming 42nd Street into a pedestrian mall along the 1.9 miles from the East to the Hudson River, serviced by light-rail, has been discussed now and then over the years but is being taken more seriously than at any other time in recent memory. Last week, a group called Vision42 even released plans, studies, and renderings for what they hope will be a transformed city center.

The proposal is certainly doable, but is it a good idea? This question is in fact two questions: Are pedestrian malls inherently good and, assuming that they are, will one work on 42nd Street?

As to the former, there are two kinds of answers, but both ultimately go back to urbanist Jane Jacobs. Yes, pedestrian malls are good: They emphasize girlie things like people and fresh air and ambulation, thus curbing the male impulse – which, as we all know, is to drive a Humvee through the Vatican if it went unchecked.(Ms. Jacobs is too intelligent to be so unnuanced in her condemnation of cars, but try telling that to those who invoke her name.)

On the other hand, cities have their own kind of density and vitality. New York more than any other city, and 42nd Street more than any other part of New York, derives its pulse and vitality from that syncopated, clamorous, anarchic ebullition of energy that makes the Big Apple one of the wonders of the world.

I have some sympathy for both views. There are some very fine pedestrian zones in some of the finest cities in the world, and New York is all but unique in not having any of them. Think of Covent Garden and Leicester Square in London’s West End; Avenida Florida in Buenos Aires; Pitt Street and the Circular Quays in Sydney; Amsterdam’s Kalverstraat and Spui; and the Via Condotti in Rome (which is a de facto pedestrian zone).

At the same time, I have never felt the traffic on 42nd Street was in any way detrimental, either in its density or its implicit danger. And removing automobile traffic would, in a very material way, neuter and tame one of the most vibrant parts of New York. Only a fool or a cynic would deny that Times Square has changed for the better in the past decade. The oft-cited fear of Disneyfication is exaggerated, but it could be all too real if the project announced byVision42 ever became a reality.

There is also something of midcult in the reflexive assumption that pedestrian malls are always an unmixed blessing or any sort of blessing at all. Many pedestrian areas are failures, either because they are poorly designed, like Les Halles in Paris, or because the planners assumed that malls, by their mere existence, have the power to invigorate an area that otherwise lacks vitality.

I know of one mall in Buffalo, N.Y., that has nothing to offer pedestrians and so has the feel, even at midday, of a ghost town. If it had cars coursing through it, it would at least have that much vitality, rather than a chilling, unnerving stillness that mocks the pretensions of the city planners.

Surely, Times Square, by its very nature, is so vibrant that there is little risk that it would become a dead zone. But there is the distinct possibility that its vitality would be diminished. Certainly, it would be changed, and in no small measure, New York and New Yorkers would change with it.

Several of the renderings that appear on Vision42’s Web site (www.vision42.org) look enticing enough, even allowing for the fact that, in renderings of this sort, the sky is always blue and the pedestrians always chipper. The most promising of the renderings are, as you might expect, around Grand Central and Times Square. Whether it is merely the art of the designer or something else, these promise a beguiling, vibrant public space vaguely European in its feel.

But these renderings fail to account for the fact that 42nd Street is not the same thing as “42nd Street.” When we think of the latter, we really mean the Great White Way – which, in the most generous assessment, stretches from Fifth Avenue to Eighth. But then you have half a mile or more on either side to the east and west.

Though it is possible that this new proposal would transform those other areas as well, it is hard to envision many happy strollers along, say, 42nd Street between Ninth and Tenth Avenues. Even the renderings provided by Vision42 do not succeed in giving any persuasive sense of vitality, especially around the Port Authority building on the West Side, or certain crude, project like residential buildings on the East Side.

In all likelihood then, a pedestrian mall need not extend from river to river, but only along the few crucial blocks east and west of Fifth Avenue. At that point, we would not need the light-rail system, either. But even then, there is little urgent need for such a transformation, which I highly suspect would contravene the spirit, the genius, of New York City.