It's 9am in the centre of one of the busiest, most traffic-clogged cities in the world, and I am cycling, entirely alone and without a car in sight, along its central, tree-lined, four-lane boulevard. I brake as a roundabout approaches, but a police officer is waiting, whistle between his teeth, to beckon me across: he is holding up a barrage of traffic on the intersecting road, entirely for me.
I sail past, registering as I do the hundreds of vehicles backed up to north and south. Only when I am safely across the roundabout does the policeman give them the go-ahead to inch their way along the overburdened minor avenue, while I continue freely along my generous expanse of empty, exhaust-free highway.
What is this, an ecowarrior's dream? Well, it could be: but no, it was a recent Sunday morning in Mexico City. I was cycling along the main traffic artery, Avenida Reforma, a road built by the Emperor Maximilian during a spell of French rule in the mid 19th century. Usually, the scene on Reforma is of nose-to-tail cars, most of them clapped-out, pre-1990s models. Vehicles move slowly, exhaust fumes cast a pall over the road, and there is a constant backdrop of noisy horns and aggravated shouts from angry drivers, punctuated from time to time by the sickening crunch of car on car as a roadway altercation goes awry.
On a normal morning, this road is an environmentalist's worst nightmare. But not so at the end of each week, because, for the past few months, traffic has been banished from Reforma each Sunday between 7am and 2pm. It's a bold move, and the brainchild of the city's mayor, Marcelo Ebrard, who has gone green big-time (certainly by Mexican standards). In another headline-making move, the mayor and his closest advisers now cycle to work on the first Monday of every month - no mean feat for Ebrard, a 48-year-old smoker who, by his own admission, doesn't exercise as much as he might.
Ebrard is talked of as a presidential candidate in 2012, but the Reforma cycling initiative certainly can't be dismissed as populist. Local people, on the whole, hate it. "It's all very well for tourists like you, wandering out of your hotels on Reforma and enjoying the rare smell of fresh air and the eerie silence because there's no traffic," says Mirella, who lives in Mexico City. "But for a mother like me, based slightly out of town in the suburbs, what it means is I can't bring my kids in to the city-centre museums on a Sunday the way I used to. The traffic in the smaller roads off Reforma is simply too chocka."
Mexico City - DF, for Distrito Federal, as the locals call it - has grown quickly. In 1950 it had roughly three million inhabitants; today there are more than 19 million. And, tragically, that mushrooming population has been starry-eyed about the benefits of car use, as demonstrated par excellence by their North American neighbours. The Mexicans might be scathing about the folks who live in the country next door to theirs, but when it came to cars they swallowed the American dream hook, line and sinker. Which is doubly sad given that their city centre is so compact and would - were it not for all those cars making the place dangerous and unpleasant for walking and cycling - be perfect for ambling and pedalling round.
But car overuse is etched on the fabric of DF: environmental groups have long pointed to this city as one of the most polluted on the planet, and the chief culprit in that pollution is exhaust fumes. It's not just about the high number of cars there, it's about the fact that most of them are decades old, burn poor-quality fuel, and are being driven at 7,000 feet above sea level (thinner air makes pollution worse).
Soberingly, it's not so long ago that Mexico City was known for the purity of its air. The novelist Carlos Fuentes's first book, a portrait of DF published in 1959, was called Where the Air is Clear, a title the author has since called ironic. Back in the 1950s, though, it was known for its blue skies and picture-postcard views of nearby snow-topped mountains: mountains that (you can almost guarantee) youngsters growing up today will never glimpse, through the haze hanging above their inner-city playgrounds. In a recent piece of research, children at primary schools in DF were asked to draw the sky. Almost all of them chose grey, brown or black as its colour: none used blue.
According to conservative estimates, roughly 2.5 million working days were lost in Mexico City last year due to respiratory problems caused by pollution. Mayor Ebrard would like to see that figure slashed - but he realises, in a city where life is more about short-term gain than long-term strategy, that he's pedalling uphill. All of which makes his Sunday traffic truce on Reforma all the more impressive. It is, in a sense, one of the most unlikely locations on earth for so pioneering a green statement.
But then again, maybe it's inevitable that the most ambitious initiatives will come from the world's most polluted places. There is a similar scheme in Bogotá, Colombia, another traffic-throttled city. And one scheme begets another - the World Bank recently agreed to give Mexico City US$100,000 to make the space more bike-friendly. A hundred and eighty-six miles of bike lane are now planned.
A few years ago this would all have looked like madness to DF dwellers, less than 1 per cent of whom use bicycles to get around. Even now, if you walk down Reforma on a weekday, as I have been doing regularly for the past month, you can see why it might strike any reasonable soul as suicidal to encourage bicycle use here: if the pollution doesn't get you, the bad driving surely will, eventually.
Come Sunday, however, the scene is very different. Small children are cycling beside their parents, rollerbladers whizzing along the quiet expanse of tarmac, and walkers strolling hand in hand. To be sure, the drivers backed up on the side roads off Reforma are honking their horns and shouting loudly in derision at the lunacy of the scheme. But maybe, just maybe, one or two of them are sitting behind the wheel of their stationary car and thinking quietly to themselves. And maybe they're thinking: Hang on a minute! How mad is this? It all looks rather nice, on that big wide road up there with just a few bicycles on it. Maybe next week I'll leave my car at home and join them instead . . .