As a general rule, although we'd never admit it, I suspect the public has a fairly high tolerance for laddishness. This explains the success of programmes such as Top Gear. Political incorrectness and bad taste sustain a successful guerrilla warfare against the offend-no-one-at-your-peril attitudes which prevail in public life.

There are two lines to cross, if you like. There's the public line, largely reflected by the government of the day and reinforced by public broadcasters, which forbids us to laugh at things which we find funny in private.

And there's the private line, whereby individuals decide for themselves the point at which bad taste ceases to be funny and becomes offensive.

On some subjects, the two lines may be quite close together - drink driving, domestic violence, bullying and bigotry, for example. The furore over Shilpa Shetty showed that private intolerance of racism has moved much closer to the public stance than previously.

On other issues - women, religion, physical status - there is a much wider gap between public and private attitudes towards bad taste. No matter how proper our authorities may be, private subversion still takes place in the form of, say, sexist, Muslim and handicapped jokes - and we grant semi-institutionalised status to safety valves such as lads' mags, late-night comedians and bad-taste e-mails. There's a kind of natural cultural balancing act, whereby what needs to be expressed is expressed until ordinary people decide otherwise. Which is healthy.

And then we come to Top Gear, which is an interesting case because it is popular with all those whose private love of speed, fast cars and consuming global resources is rather out-of-kilter with the eco- and safety-conscious public view. But the BBC's vehicle for Jeremy Clarkson's shlock-jockery still falls within acceptable limits of bad taste, and until now I've rather enjoyed it for that reason. The dark side is always more fun.

Until Sunday night it was, anyway. Then Top Gear crossed my personal bad taste line; in seconds it went from being naughty entertainment to deliberately, puzzlingly unpleasant. And the reason, I contend, was their failure of taste, not my failure of tolerance.

Clarkson, Richard Hammond and James May travelled to Florida to prove that fly-drive holidays are more fun if you buy instead of rent. So they bought three cars, each for no more than $1000, and went off to be as silly as possible.

My first sense of humour failure came when Clarkson had a dead cow strapped to his car roof in some kind of parody of the hunter gatherer. To get it off, the boys performed a driving stunt so the creature was thrown into the air and thumped into the ground to cheers and laughter.

I don't have a problem with the sight of dead animals. But I do have a problem with using them as entertainment. In death, as in life, they are creatures that deserve dignity. Those of us who thought Top Gear was still distinguishable from Jackass felt our mouths begin to fall open.

It got more infantile, less sophisticated. The more they tried to be ironic, the less they succeeded. They decided to drive into redneck country to provoke trouble. So they painted gay-rights slogans - "Man love rules OK" - on their vehicles, in pink paint, and "We hate country and western", and headed for Hicksville. Sure enough, when they stopped for petrol, they managed to provoke the locals, were involved in a violent incident along with the TV crew and had to drive off at speed, shrieking with laughter.

It was at that point that I decided this was not what I pay my television licence for. I do not wish to subsidise a public broadcaster which provokes illegal acts as entertainment. I do not think the BBC, under any guise, be it Newsnight or be it Top Gear, should entrap people into violence or homophobia.

Clarkson's attitudes to gay people are well known. Last December, after a complaint was upheld (he referred to a car as "gay"), the presenters and the production team were reminded of the importance of avoiding derogatory references to sexual orientation.

So yes, my private bad taste watershed has been passed - and my threshold is a high one. Interestingly, all the laddish people I've spoken to who saw the programme feel the same as me. It went too far. It wasn't funny. I wonder if anyone is offended enough to complain. I wonder if the lines are shifting.

Harry's case is closed
Last week I ventured a column criticising (very mildly) the Harry Potter colossus. I questioned the way in which the books, written for children, have been colonised by adults. The resulting online reaction on The Herald website was a perfect QED: a sub-literate stream of barminess from, yes, adult Harry Potter fans.

Among the hundreds of dreary entries were some gems. I loved the person who logged on pretending to be me, and wrote: "Why are you so mean to me I mean cut me some slack i am just trying to earn some cash to buy beer. so please i apologize for offending any one im sorry" (couldn't have put it better myself). I am indebted to the kindly words of Hans, from the Netherlands, who said: "Hidden deeply under all Harry Potter's excitement and suspense is a spiritual message of the purest and most sublime power and beauty. Harry Potter is in fact a symbolic story of complete human liberation, a story of the kind of spiritual enlightenment as achieved by people like Gautama the Buddha, Zarathustra, Mani, Krishna, and Jesus the Christ." Guys, you've made my case for me.