More than 1,000 cities, most of them in Europe, took part in the sixth annual car-free day. In London, accordion players provided entertainment in streets closed to cars; in Paris, people toured electric-powered buses and tested environmentally friendly Segway scooters.
The annual event started six years ago in France as a way to push drivers to think about air pollution and their role in creating it. This time, the problem seemed even more pressing: Bad air worsened the suffering of thousands of elderly people who died in August's soaring temperatures.
But people had a hard time leaving their cars in the garage, and many streets were as clogged as usual.
"People are too addicted to their cars," said London cab driver Joe Steele, who said he hadn't seen an effect on traffic, though some streets were closed. "They're too used to them."
In Paris, police blocked most cars - except for taxis and low-emission vehicles - from the city's tourist core surrounding Notre Dame cathedral and the Louvre Museum.
Streets in the area were free of the usual angry honking. But bottlenecks were worse than most days just a few blocks away, on the wide boulevards heading to the suburbs.
Emiline Chonville, a 20-year-old medical student, spent more than an hour on the bus to get to city hall, where she borrowed a bike for free. The trip from her suburban home usually takes 20 minutes.
"There are traffic jams everywhere," she said.
The event was part of "European Mobility Week," whose organizers say that 40 percent of the transport sector's carbon-dioxide emissions come from private cars in cities.
As the heat soared in August, air pollution skyrocketed in many places. Some cities were on alert for high ozone levels and even forced drivers to slow down.
High ozone levels can worsen problems for people with breathing and heart ailments. A report from France's Health Watch Institute urged more studies on the link between dirty air and the thousands of heat-related deaths this summer.
In France, the government put the heat wave death toll at 11,435. More than 4,000 elderly Italians may have died, according to official estimates.
Most efforts Monday focused on encouraging people to take public transport. In Helsinki, Finland, a single ticket allowed travel on subways, buses, trams or commuter trains for the whole day. In Geneva, the usual $8 day pass was valid for a week.
In Dublin, few people switched from their cars to the Irish capital's patchwork of buses and trains. Many griped they had seen no advertising about the day - particularly the plans to hand out free bus tickets for a few off-peak hours.
"What am I supposed to do with this? Go home and kiss the wife?" said Frankie Meehan, 42, when offered a free ticket at lunchtime.
Some cities launched the operation over the weekend instead. Officials in Copenhagen, Denmark, said it would be too expensive to close the capital on a weekday.
Bratislava, the Slovak capital, participated on Saturday, with organizers offering rides on rickshaws or a historic tram. In Sarajevo, Bosnia, about 300 people rode bikes together through the calm streets on Sunday.
In Athens, many people had no choice Monday but to take public transport. Taxi drivers and gas station owners were on strike, protesting government plans to force them to issue printed receipts.
Rome, notorious for its chaotic streets, didn't take part because it was short on funding. Still, city officials urged people to leave cars at home, and schools organized bike rides - an idea that one Roman said was just too dangerous.
"As a city, Rome is constructed for cars," said Mario Catanzaro, a bookshop owner. "If you cycle, you are risking your life."
By Angela Doland