Call it the Richmond Park test. Yesterday morning, amazing to report, it wasn't belting down with rain, and the park was crammed. It was a joyful sight – families, dog-walkers, runners. But what would have amazed a time traveller from a couple of decades ago would have been the thousands and thousands of cyclists. Something has happened.

There are the dolphin-like schools of muscular men and women in matching Lycra, teams from Britain and the continent, moving as one; there are the men of a certain age, still on racing bikes, but swaddled in woollens; there are the kids with their parents; and there are the stately amateurs, sitting up straight on their three-gear Raleighs. But the main thing is the sheer number of them, a constant, gleaming, metallic stream.

Richmond Park is unusual, granted. It's a huge urban park, with hills and views that attract the big cycling clubs as well as many individuals from the rest of London. It now has a 20mph speed limit. But the something that's happened there is hardly unique, and is spreading, and should spread.

And it's simply that cycling has reached a tipping point, a critical mass, that has turned the motorists into a minority, and an increasingly boxed-in, nervous minority at that. On most of Britain's roads, if a couple of cyclists are side by side, causing cars or vans to brake, there will be shouts and hoots and waved fists. Here, drivers simply have to accept that the world has changed. Drivers who lose their temper find it isn't so easy in the park. They can't zoom off. And they are outnumbered by cyclists.

I've written about the cycling revolution before, just after the Olympics, but it seems a particularly appropriate subject for the year's end, as we struggle with questions of the real legacy of 2012. Bradley Wiggins, who I called cycling's patron saint back then, is now to be Sir Bradley. It's interesting that it was the cool mod who was chosen to be the number one front-page star at the year's end; and I think it's about his sport as well as about him.

This isn't a year, after all, the legacy of which will be fondly remembered in many areas. It's been a year of economic failure and public spending cuts. Civic leaders from left and right – the Labour-run cities in the north and rural areas run by now-rebellious Tories – are up in arms about the prospects for 2013. Even the overall sporting legacy of the Olympics is dubious. Ofsted's report into levels of sport in schools has been delayed, leading to suspicions that it would show a dramatic drop-off since the coalition took office. It is easy to look ahead with a certain amount of grim cynicism.

So the cycling revolution is worth clinging on to, a rare bright exception, which shows the country becoming a better place in at least one way. With Sustrans championing the National Cycle Network, cities from Scotland and Northern Ireland to the English south promoting safer cycle routes, and perhaps around 15 million people regularly cycling, it's a genuinely national story. At a time when there isn't much money around for investment in major projects – not enough, but that's an argument for another day – investing in cycling is relatively cheap and highly effective.

The most high-profile case remains, inevitably, London, where the re-elected mayor Boris Johnson, is promising to plough £640m of extra money into the capital's cycling network. That will mean new east-west links, probably involving new cycling bridges to bypass bottlenecks and even – perhaps – overhead cycleways, using parts of the old railway network.

Sustrans, which campaigns for public transport and safer pedestrian routes as well as for cycling, believes that it's possible for London to have the world's biggest cycle network by 2020. That would involve more than 600 miles of "safe, quiet cycling routes … at the cost of just £10m a year, less than the cost of just one new underground train", the charity says.

Johnson isn't committing to cycling because he's secretly a greenish leftie but because he's an ambitious politician. He's seen the critical mass of cyclists coming together, and he's realised that with little money around it's one of the ways he can make an impact. It can't be long before national politicians properly get the same message. But there's a lot to do. A few days ago, it was announced that the number of cycling deaths this year was 122, a five-year high. I happily cycle round the park; I'm much queasier about chancing rush-hour city traffic on wet, dark afternoons.

So let's go back to the park, and that multicoloured, all-ages, bikes-of-all-kinds spectacle; because although I've been part of it, I also drive there. And like so many people, I'm hypocritically divided in my head. On two wheels I think like a cyclist. On four, even though I should know better, I feel frustrated and cross. Richmond Park is a wonderful green lung but it's also a vital commuter route and a place huge numbers of people can only reach by car.

The Richmond Park test is how to push more of Britain's roads to the critical cycling mass, which means cycling is safer (safety in numbers works) but which acknowledges that what Margaret Thatcher called "the great car economy" isn't going to simply vanish.

Some cyclists are in danger of treating cycling as a cult, or even a secular religion – on your saddles, brethren, and be saved! But cycling isn't for everyone. There are people with small kids, older people, people whose journeys are too long, people who don't like it. They have rights too.

In this particular park, a transport microclimate, there is a quiet, undeclared war of attrition against cars – fewer parking places, lower speed limits, regular gate closures for special events – which readers may recognise going on in many other parts of the UK too. We are going to see more congestion charging, increasingly pricey and restricted parking and lower speeds. And this is tolerable – if proper help is also given to those who need to drive, or who can be offered good and reliable buses instead. It's the intricate balancing act of a proper transport policy.

What we can't have, however, is a country in which this balancing act fails to push forward the cycling boom. A quarter of us, roughly, are obese, children as well as adults. Our urban air is still filthy. We are using far too much carbon. But the great thing is, millions of us are getting the message. Real revolutions come from below, and this one is too. That's perhaps the brightest message from 2012, the year of the bike.