Just hop in the car? Not so fast, says one French town.

It takes a while, as you walk around the streets of Nantes, a city of half a million people on the banks of the Loire River, to realize just what it is that is odd.

Then you get it: There are empty parking slots.

That is highly unusual in big French towns, normally clogged with traffic crawling along ancient thoroughfares. But here, Thursday, as one policeman said, "ça roulait bien" - cars were rolling.

Two decades of effort to make life more livable by dissuading people from driving into town has made Nantes a beacon for other European cities seeking to shake dependence on the automobile.

"We are not anticar," says François de Rugy, deputy mayor in charge of transport. "But we send people a lot of signals: If they come into town on buses, on foot, by train, or by bike, we will help them. If they come in cars, we won't."

The effects were clear Thursday, the high point of Mobility Week, a campaign sponsored by the European Union that prompted more than 1,000 towns across the Continent to test ways of making their streets, if not car-free, at least manageable.

"That is an awfully difficult problem," acknowledges Joel Crawford, an author and leader of the "car free" movement that is picking up adherents all over Europe. "You can't take cars out of cities until there is some sort of alternative in place. But there are a lot of forces pointing in the direction of a major reduction in car use, like the rise in fuel prices, and concerns about global warming."

Thursday, proclaiming the slogan "In Town, Without my Car!" hundreds of cities closed off whole chunks of their centers to all but essential traffic. Nantes closed just a few streets, preferring to focus on the alternatives to driving so as to promote "Clever Commuting," the theme of this year's EU campaign.

Volunteers pedaled rickshaws along the cobbled streets, charging passengers $1.20 an hour; bikes were available for free; and city workers encouraged children to walk to school along routes supervised by adults acting as Pied Pipers and picking up kids at arranged stops.

Some critics dismissed the idea as a gimmick. "We live in a society that is organized, like it or not, in such a way that we cannot do without cars," Christian Gerondeau, president of the French Federation of Auto Clubs, told French radio. "Stigmatizing the car is the wrong battle."

Authorities in Nantes, though, are trying to show that there might be another way.

The centerpiece of their efforts is a state-of-the-art tramway providing service to much of the town, and a network of free, multistory parking lots to encourage commuters to "park and ride."

Rene Vincendo, a retired hospital worker waiting at one such parking lot for his wife to return from the city center, is sold. "To go into town, this is brilliant," he says. "I never take my car in now."

Indeed, a poll in April by the tramway authority found that 95 percent of users were satisfied with the service.

It is not cheap, though. Beyond the construction costs, city hall subsidizes fares to the tune of 60 million euros ($72 million) a year, making passengers pay only 40 percent of operating costs.

That is the only way to draw people onto trams and buses, says Mr. de Rugy, since Nantes, like many European cities, is expanding, and commuters find themselves with ever-longer distances to travel.

It's a chicken-and-egg problem, says John Adams, a professor of geography at University College in London. There is no longer room in European cities for cars to park, so drivers must live farther from work, and the traffic increase obliges urban planners to devote more room to roads and parking, which worsens urban sprawl.

The danger, he warns, is that "the further you go down the route of car dependence, the harder it is to return, because so many shops, schools, and other services are built beyond the reach of any financially feasible public transport network."

This, adds de Rugy, means that "transport policy is only half the answer. Urban planners and transport authorities have to work hand in hand to ensure that services are provided close to transport links."

The carrot-and-stick approach that Nantes has taken - cutting back on parking in the town center and making it expensive, while improving public transport - has not actually reduced the number of cars on the road. But it has "put a brake on the increase we would have seen otherwise" and that other European cities have seen, says Dominique Godineau, head of the city's "mobility department."

City Hall has other plans afoot to keep up the pressure. This week it launched a website to connect potential carpoolers. It has put out tenders for a car-sharing system that would allow city dwellers to rent a car for a few hours from curbside locations with the swipe of a magnetic card.

The authorities are also encouraging big employers to match up to 15 percent of workers' monthly bus and tram passes.

"There is no single solution, but the important thing is to be coherent," says de Rugy. "There is no point in spending money on these sorts of things and still building more roads."

"You get what you pay for," adds Mari Jussi, a transport analyst at the Estonian branch of the Stockholm Environment Institute. "If you put money into tarmac, you get more cars. If you invest more in cycle lanes and safer streets, that's what you get. We have solutions now, but politicians are not eager to apply them."

Alain Chenard knows that only too well. As Mayor of Nantes in the early 1980s, he began building the city's tram line, the first modern tramway in France. The inconvenience of the construction and the price cost him his job in 1983.

"It seemed mad at the time," says Thomas Renaud, a cook, as he rode the tram to visit his parents. "But it paid off royally."