This all began as a leaflet issued to CM participants by police at the start of the ride on September 2005 which stated:

"...Police can impose conditions on processions, demonstrations and other assemblies, and participants render themselves liable to arrest if they fail to comply with those conditions. These cycle protests are not lawful because no organiser has provided police the with the necessary notification. Your participation in this event could render you liable to prosecution. Police policy in facilitating these events is currently under review..."

After that the High Court decided in favour of CM but the police then appealed against that decision and won. The legalities involved are quite complicated and seem to hinge on whether the ride is customarily held and therefore not subject to certain police restrictions.

On Monday 20th October 2008 an appeal by Des Kay and Friends of the Earth, against the Commissioner of Metropolitan Police, was heard by five Law Lords and their decision is that the appeal is allowed and CM is a commonly or customarily held procession without organisers and therefore does not need to inform the police of each ride. So, still we ride too! Though whether we are traffic or not is debatable.

 http://www.publications.parliament.uk/pa/ld200708/ldjudgmt/jd081126/metro-1.htm

"21. Section 11 does not require notice to be given of every procession that is capable of creating a disturbance. The fact that, on their natural meaning, the words of section 11(2) are wide enough to exclude some processions in respect of which the police do not have all the information that they would wish is no reason to give those words an unnatural meaning. They should be given their natural meaning so as to apply to Critical Mass as a procession that is commonly or customarily held. For this reason I would allow this appeal."

 http://www.criticalmasslondon.org.uk/