No one who has ridden recently on a packed New York subway or bus needs to be told: public transit ridership has exploded over the last few years.

But a new transportation study has found a surprising corollary to this trend. In the 1990's, for the first time since before World War II, the growth in public transit ridership outstripped the growth in auto use in the five boroughs, according to the study.

In effect, the reversal suggests that the Robert Moses era, in which highway expansion and automobiles were favored at the expense of public transit, has come to an end.

And the tremendous investment that has been made over the last two decades in subways and buses appears to have paid off not only in increasing transit ridership but also, along with other factors, in discouraging car use, an accomplishment that many other congested cities can only dream about.

"If you had told somebody this was going to happen back in 1990, they would have said you were crazy," said Bruce Schaller, a private transportation consultant and former policy analyst for the city's Transit Authority who wrote the study, which is to be released today. "This kind of change was both unprecedented and unforeseen."

It does not mean, however, that the roads are any less jammed with cars or will be any time soon. In the 1990's, the study found, the number of cars owned by city residents still increased, by 6 percent. But the growth in subway ridership vastly outpaced this, increasing 34 percent over 10 years, in part because of the booming economy and booming immigration, but perhaps also because roads were not being expanded nearly as rapidly as they once were and had become saturated with traffic.

The numbers from the 1990's stand in stark contrast to those for the previous four decades.

In the 1950's, under Mr. Moses, the city's master builder, what would eventually become 627 miles of new highways began to lace the metropolitan region. And elevated train lines were being demolished. (New York, in fact, is the only city in the world that has fewer subway miles now than it did before World War II.)

As a result, auto ownership began to inch up, increasing by 1 percent, while subway ridership dropped sharply, by 19 percent.

In the 1960's, as highway building continued, auto ownership increased even more rapidly, by 13 percent, while subway ridership dropped further, by 7 percent.

During the fiscal crisis in the 1970's, which caused high unemployment and drove people away from the city, both auto ownership and subway ridership dropped. But the subways took by far the biggest hit, with ridership dropping 20 percent, while auto ownership declined only 2 percent.

Finally, in the 1980's, even as capital investments began to breathe life back into the hobbled transit system, subway ridership increased only 2 percent while auto ownership rebounded sharply, increasing 21 percent. (The study focused mostly on subway ridership because bus ridership data before 1970 was unavailable.)

The 1990's actually started out inauspiciously for transit, with the recession in the early part of the decade dragging down subway ridership in 1991 and 1992.

But the billions of dollars in capital investments in the subway that started in 1982 had begun to result in more reliable trains, renovated stations and more frequent service. And those improvements, along with the introduction of the MetroCard system that enabled the first volume discounts and the elimination of two- fare zones, are credited with reviving the city's public transit.

"I think it's not just that people are saying that the automobile is such a lousy option that they are going to take the subway," said Jeffrey M. Zupan, a senior transportation fellow at the Regional Plan Association, a nonprofit group that advises governments on development.

Mr. Schaller, who has served as a consultant to the Federal Highway Administration, the National Academy of Sciences and several cities' transportation departments, said he believed that the other major factors contributing to New York's historic shift were the drop in crime, the city's booming economy and the large increase in immigration during the last decade, which brought the city many more new subway and bus riders than it did car owners.

So many, in fact, that New York City Transit now carries 1.2 billion passengers a year, more than the yearly transit ridership for Chicago, Washington, Boston, Philadelphia and Los Angeles combined.

The only problem with this growth, the study suggests, is the same one that began to face the highways decades ago: When you build it and they come — or, in the case of the subway, improve it and they come — there is the danger that too much success will overburden the system and drive people away.

"The challenge is to stay ahead of the curve," Mr. Schaller said. "Now that they've come, the system has to continue to improve and to expand the service to accommodate the crowds and keep them happy."