EVENTS of the past week have shown again that there is probably no aspect of transport that polarises public opinion more than cycling.

What should be innocuous, if not altogether encouraged (cycling is a way to keep fit and cut greenhouse gas emissions), seems to send even the most level-headed motorist into paroxysms of anger. Time and again, I hear the same complaints: cyclists cut through red lights; they hog the road; they weave in and out of traffic; they are a danger to themselves.

Put that to any cyclist and they will give you half a dozen anecdotes of cyclists clipped, cut up or harassed by drivers who appear to be enveloped in a red mist at the mere sight of bike spokes. At their worst, both sides of the debate are guilty of self-righteousness, but the level of anger provokesd is sometimes bemusing.

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It came as little of a surprise, then, that the decision by the Advertising Standards Agency (ASA) to ban an advert with an image of a cyclist without a helmet on the grounds that this was "socially irresponsible" sparked a flurry of comment online.

In the blue corner were those who accused the ASA of insanity and overstepping its powers, noting that wearing a helmet was not a legal requirement and that no-one batted an eyelid at cyclists commuting in Holland without a "foam hat".

In the red corner came the familiar ripostes that cyclists should be banned from the road until they pass a proficiency test. Also, anyone who refused to wear a helmet was clearly devoid of "reason, sense and logic".

The offending advert, part of a campaign by Cycling Scotland, urges drivers to give cyclists as much space as they would a horse. However, the sequence showing a young woman without a helmet attracted five complaints from viewers.

ASA ruled that this was "likely to condone or encourage behaviour prejudicial to health and safety". The watchdog said that, while helmets were not a legal requirement, they were "recommended as good practice", and pointed to additional concerns that the cyclist's positioning meant the driver "almost had to enter the right lane of traffic" to overtake.

Both points were disputed by Cycling Scotland and, within days, the ASA's ruling was withdrawn pending the outcome of an independent review. An outsider might wonder what harm can come of wearing a helmet. Why oppose something intended to save lives and reduce the number of head injuries? The answer is that there is no simple answer.

As Cycling Scotland points out, Britons are more likely to be killed in a road crash while walking than cycling. No-one campaigns for pedestrians to wear helmets. At the same time, the body fears enforcing helmet use risks putting people off by making it seem overly dangerous when the potential health benefits outweigh the risks.

Some international studies have shown that, where helmets are enforced, deaths decrease at roughly the rate serious head injuries increase but, even where overall head and brain injuries are shown to decrease, this is often linked to a reduction in cycling than the effectiveness of helmets.

Whatever the outcome of the review, it is unlikely to settle opinion.