A novelist friend wrote an article recently confessing she was becoming scared of cyclists. She told a story of crossing a slow-moving four lanes of traffic near her house, with the cars slowing to allow her to pass, only to be nearly taken out by a figure "hunched over his handlebars" who steamed up between semi-static cars and bore down on her at speed. She tripped and fell. The cyclist carried on leaving car drivers to pick her up.

The point was that she had been frightened, and many pedestrians in London could tell a similar story. In the same week, on a steep hill where I was crossing at a zebra crossing with the dog, a very-well equipped male cyclist going too fast downhill went into the back of the car that had stopped to let me pass. He survived but was furious with everyone else.

Of course, as a pedestrian, I am more scared of skip lorries, boy racers and Charedi Volvos (though I am told the Volvo is no longer the ubiquitous frum-chariot that once it was) than of cyclists. They're the killers. I stopped cycling because of them. But, over the years, I have become good at listening out for, and spotting the danger. I can't hear cyclists and, if they're not obeying red lights at crossings (and in London this happens often) they can easily take me by surprise. At the very least, there is a case to be made for reminding cyclists that pedestrians, many of whom are elderly or with children, feel vulnerable, too.

That was not the reaction of cyclists responding to my friend's piece. She was guilty of a range of sins. First, she had not properly enumerated the death toll of cyclists at the wheels of motorists. Second, she had not provided full statistics for her claims concerning cyclists jumping lights. Consequently, her article could only be seen as a "hit-piece" on Britain's leg-powered two-wheelers, and one that would give comfort to the murderous motorists' lobby. Some accused her of inciting attacks on cyclists as though maddened drivers would mow down anything in lycra while shouting "THIS IS FOR LINDA!!!" One man compared what she had done to the hate-articles which accompanied gay-bashing in his native Ireland back in the old days.

But even the "more in sorrow than in anger" critics of her piece could not admit, even for a second, that she might have a point. To do so would simply be to concede too much to the other side, to the enemy, to the four-wheeled cyclophobes and their allies. A line had been drawn: all virtue on this side, all sin on the other. To blur the line was to betray the cause.

It reminded me of something that happened during the Jewish Film Festival recently when I chaired a discussion following a documentary called The Zionist Idea. The film had seemed to me to be balanced and not uninteresting. When we began the discussion, one of the panel was mildly critical of the film's attitude, suggesting that it had been too critical of Zionism. By the time we finished, half-an-hour later, this same panellist was being vigorously applauded by around a quarter of the audience for suggesting, in effect, that the movie might as well have been made by the Sidon chapter of the official Hamas Fan Club.

I couldn't work out how we had got there, except to note a kind of self-polarisation going on. The more we talked (as chair I restrained myself), and in the absence of any great argument, the more binary the proposition became. If we'd carried on till morning we'd have bombed Gaza ourselves. I'm exaggerating of course. But not by much.

David Aaronovitch is a columnist on 'The Times'