All across Europe tomorrow, citizens are being urged to give the environment a break by leaving their cars at home and taking the bus or walking.
But there's one big exception: Paris, despite pollution that is so severe alerts are often issued in summer months for children to stay indoors.
Nobody, it seems, wants to broach Parisians with the subject of the annual En ville sans ma voiture! (In town without my car!) day. Not after what happened last year.
"Last year, Paris participated and it was not at all successful -- there were traffic jams everywhere," said Christophe Fréring, of the French agency of the environment and energy in Paris, who is co-ordinating the Europe-wide car-free day.
"The city of Paris closed four districts to car traffic without publicizing it at all, so people had no idea what was going on and there was chaos everywhere."
To make the problem worse, motorcycles were still allowed, making streets overcrowded with pedestrians "very dangerous," said Mr. Fréring.
The Parisian experience completely undermined the purpose of the program, which is to prove to citizens they can get by without their cars on a work day.
Despite the campaign's backfiring in Paris, 744 other towns and cities will be taking part in the first Europe-wide car-free day, which grew out of a 1998 initiative from the French Environment Ministry.
Last year, 156 French cities took part, 92 in Italy, as well as Geneva. Bolstered by the results, the European Union, plus 11 other countries, including Malta, Cyprus, Romania and Israel, are taking part this year. Participating towns must undertake to limit traffic in certain areas to only bikes, pedestrians, public transport or other "clean" vehicles.
There have been some outstanding successes and long-term urban changes resulting from the program.
"Last year, the city of Tour in the Loire Valley of France -- a city of about 200,000 people -- prohibited all car traffic from the main street, and limited it to buses only. That is still the case today," said Jacqueline Denis-Lempereur from the French Environment Ministry.
"In the Atlantic coast city of La Rochelle, bikes, electric cars and scooters are all hired out for token sums, and there have been up to 900 per cent increase in the number of cyclists."
Last year's event found the ecological fruits enormous.
Results of environmental monitoring showed a 50-per-cent drop in noise levels in areas where traffic was restricted, and drops in traffic-related pollution of between 20 per cent and 50 per cent, depending on the pollutants and the towns.
EU statistics show 70 per cent of Europeans place air pollution at the top of their list of environmental concerns, and identify car-traffic problems as the main cause.
Levels of support for the car-free day have been such that 50 per cent of respondents to one survey, Parisians aside, wanted to see the same thing happen one day every week.