Cycle helmets: that ole chestnut . . . Once a year, a cyclist has a collision which prompts a neurologist and/or the police to pronounce that the victim would have died had he or she not been wearing a helmet - ergo, helmets should be compulsory and we'll all live happily etc.

April's warming up and those whose bicycles have hibernated for six months are trying to remember where they last saw their helmet. Then, bang on cue, a cyclist has a collision in which "a helmet saved his life".

The owners of the hibernating bicycles decide that cycling's dangerous, and they grab the car keys instead. Result: cycling in Oxford becomes that nth of a per cent less safe to cycle. Why? Because the best evidence suggests that the more cyclists there are, the safer it is to cycle. With bikes, there's safety in numbers, but not necessarily in helmets.

There are two prongs to the pro-helmet argument: Wearing a cycle helmet should be compulsory Cycle helmets are good and we should encourage people to wear them.

The compulsory helmets argument is rubbish. Helmets are required by law in Australia, New Zealand, Canada and the USA (21 states), but the laws have achieved almost no reduction in the number and severity of head injuries sustained by cyclists, much to the frustration of the helmet lobby. This may be because cycle helmets are designed to protect the wearer's head in low speed (12mph) in impacts which do not involve another vehicle. When you throw a half-ton lump of metal into the equation at 35mph, a helmet doesn't really help.

For me, the problem with compulsory helmets is how many people are completely put off cycling because of it. As soon as Australia made helmets compulsory, 35 per cent of cyclists gave up. All they'd achieved was to shove the less-than-fully-committed cyclists into their cars.

There is one grain of goodness lurking in the pro-compulsion argument. After cyclist Oliver Adams's incident was reported in the Oxford Mail last week, I read a brilliant insight appended to the story on www.oxfordmail.net/news/ Referring to the problem of cyclists abandoning their bikes rather than wear a helmet, "C" says: "To be fair, aren't these the sort of people you want to deter from cycling? The sort of cyclists who view helmets as an optional inconvenience are most likely the very same ones who have the same views as regards stopping at red lights and not cycling on pavements. Without that lot making a nuisance of themselves, the roads (and pavements) would become a safer place overnight." I am almost swayed.

Helmet or not is cycling's hot potato. Oliver Adams told me: "Wearing a helmet probably saved me in this instance and I was lucky. But I don't think helmets should be compulsory. On balance, you have to weigh the evidence for and against."

Even the apparently sensible suggestion that we should encourage helmets at all is contentious - find out why in my next column.