Cyclists who take part in Critical Mass rides aren't always popular with drivers, and sometimes situations can escalate alarmingly.
A small group of riders in the Welsh coastal town of Aberystwyth is just the latest to suffer intimidation from motorists, but so disturbed were they by the reaction of fellow road users that they have called upon the police to act. One of their number, Kelvin Mason of local organisation Beic Grwp (Bike Group) has written an open letter to town councillors and the area's MP demanding that steps are taken to put an end to the abuse.
"Cyclists in the Critical Mass ride make every effort to accommodate other road users, but whatever the provocation, however long a driver is delayed for, there is surely no justification for cursing, threats, intimidation and dangerous driving," he said.
"We carry signs informing people what we are doing and we stop regularly to allow any tailback of traffic to pass, giving drivers a friendly wave. During the last ride we were assailed with the foulest verbal abuse from some motorists," he continued.
"Moreover, a number of drivers deliberately drove in a way that not only frightened and intimidated cyclists but put us at real risk of serious injury.
"This level of harassment has, I believe, gone beyond the stage of reporting individual drivers to the police, reports that the police are anyway reluctant to take or follow up upon."
Local police have denied a reluctance to prosecute, but say they need enough evidence to make charges stick in court.
In May three cyclists in the English city of Bristol claimed they were hit by cars as they took part in a Critical Mass ride in the city centre.
Police were criticised for allegedly failing to stop one of the drivers from leaving the scene and one cyclist was arrested for obstructing the highway. During the same incident a policewoman was reportedly pushed over.
Some riders complained that the police - who sent 11 squad cars and a riot van to the scene - over-reacted, but other witnesses claimed some of the cyclists had become aggressive and violent. Read an account of the incident here.
Earlier this year the Court of Appeal ruled that Critical Mass rides in England and Wales had to be notified to police at least six days in advance, complete with route details.
The ruling means that organisers of Critical Mass events are now bound by law to inform the authorities of their plans in advance.
This poses a problem because often there is no central or regular organiser of each city or town's Critical Mass.
There have also been problems at rides in the United States. In August, Chicago's 3,000 strong ride ended with seven arrests after police forced a change in the route.
It's not all bad news though. Critical Mass as a movement continues to grow in popularity: Sacramento Cyclists recently reported its biggest ride yet, and spotted new bike lanes being painted along their route. See images from some of their rides here.
Meanwhile, if the San Fransisco riders - who recently celebrated 15 years of Critical Mass - have seen instances of conflict with motorists and the police, they have also seen 126 miles of bike lanes added to the city's streets since their first ride in 1992. And valet bike parking is now commonplace at major city events.
Critical Mass has been an informal way for cyclists to make their presence felt for more than a decade as riders traditionally gather at evening rush hour on the last Friday of the month to monopolise routes normally dominated by cars, lorries and buses.
The events can be simply a celebration of cycling or can be used for more direct forms of protest - in recent years becoming a voice for anti-war and environmental campaigners.