Critical Mass (CM) started as a cyclist's way of sticking two fingers up at their car-dominated environs.

CM is when a group of cyclists ride slowly through city streets for enjoyment or to block the traffic. In cities where cyclists are few, CM is an empowering and profile-raising activity. It reminds everyone that cyclists have as much right to use the road as the gobby car-driving majority.

CM rides traditionally have no leader and no set route, and most are held on the last Friday evening of every month.

It all started in San Fransisco 15 years ago. A bunch of militant cyclists, who were sick of the usual highway pecking order, decided to reclaim their streets.

On Californian CMs, frustrated drivers take the law into their own hands. Apparently, it's one thing to be stationary in a long queue with all the others who (like you) decided to drive. But being delayed for a few minutes by cyclists is red rag to a bull.

This road rage isn't a simple a hormone imbalance caused by Americans' excessive beef consumption - the same also happens in Oxford. A driver is happy to wait for a few minutes behind a rubbish truck or a delivery van. But if a cyclist causes a moment's delay, it's open warfare.

On my way home from Oxford's own reincarnation of CM last Friday, I was sworn at, swerved at, and forced into a parked car by the drivers and occupants of three cars in the space of 50 metres on the narrow road in which I live.

This kind of behaviour exposes the base, bullying moron in many drivers - and prompts any cyclist with a self-preservation instinct to CM.

In some places, CM is needed. In Melbourne, the least bike-friendly city in the world, it is an important part of the cyclists' toolkit. London's CM, which started in 1994, has a chequered history. The police tried unsuccessfully to ban it in 2005 and today it is enjoyed by thousands every month, including many MPs.

Fifteen years ago, Oxford had a shortlived CM ride which went round and round the Plain roundabout and wound motorists up quite pointlessly. In Oxford, we don't need CM to raise our profile for we are legion, and in a small city, there are better ways to assert our rights.

A few years ago, a CM restarted in Oxford, doing the traditional Friday evening trawl of city centre streets. I went on one and was immediately turned off by some aggressive participants. I vowed never to return.

So why did I? Because, new to Oxford, on the last Friday of every month, starting at 6.30pm in Broad Street, we have Carnival Mass. Carnival Mass is all about enjoyment. It celebrates cycling in Oxford by taking traffic-free or low-volume routes around the city centre and Jericho.

There's even a tricycle sound system pumping out bicycle classics. Carnival Mass is friendly and humorous, the perfect antidote to a stressful month at work.

Riding around the city in a peloton of 30 or more riders to the strains of Queen's I want to ride my bicycle, I want to ride my bike is a real blast.