On a clear, cloudless afternoon, Enrique Peñalosa, former mayor of Bogota, leaves his office early in order to pick up his 10-year-old son from school. As usual, he wears his black leather shoes and pinstriped trousers. As usual, he is joined by his two pistol-packing bodyguards. And, as usual, he travels not in the armoured SUV typical of most public figures in Colombia, but on a knobby-tired mountain bike.

Mr. Peñalosa pedals through the streets of Santa Barbara in Bogota's well-to-do north side. He jumps curbs and potholes, riding one-handed, weaving across the pavement, barking into his cellphone with barely a thought for the city's notoriously aggressive drivers.

On most days, this would be a radical and perhaps suicidal act. But today is special.

Ever since citizens voted to make it an annual affair in 2000, private cars have been banned entirely from this city of nearly eight million every Feb. 1. On Dia Sin Carro, Car Free Day, the roar of traffic subsides and the toxic haze thins. Buses are jam-packed and taxis hard to come by, but hundreds of thousands of people have followed Mr. Peñalosa's example and hit the streets under their own steam.