Officials in Mexico City have been ordered to leave their cars behind and cycle to work once a month.
The plan was put forward by Mayor Marcelo Ebrard (pictured)
The new city regulation that has just come into force is aimed at reducing traffic and pollution in one of the most congested cities in the world.
About four million cars travel every day through the city of more than 18m people - and officials say their aim is to cut pollution as well as disease.
Tax incentives are envisaged for firms encouraging alternative transport.
Those officials who cannot cycle because of health reasons, or because they live too far from work, will be allowed to use public transport, but not their vehicle.
Mayor Marcelo Ebrard proposed the programme last year - and was the first to get on his bicycle from his home south of the city to his office in the central Zocalo.
His Corridors of Unmotorised Movement: Pedal your City comes complete with a guidebook on "urban cycling" - including security-related details like what to wear and how to get spotted by motorists.
Only 0.7% of all journeys in the capital are by bicycle - and Mr Ebrard aims to increase to 2% in three years' time and 5% in six years.
At the same time, the mayor says he will improve public transport, including building more special bus lanes.