Sarah Sands: YouTube justice is a kangaroo court online

What should you do if someone is foul mannered on public transport? It is spirit-crushing for everyone who witnesses it, yet intervention feels thankless or dangerous. You can report someone swearing or smoking or ranting to an official, but they are as fearful as everyone else. The signs in stations, or hospitals for that matter, warning that staff must not be threatened or abused by passengers or patients, suggest an institutionalised dread of the public. You could call the police, but that means lock-down and nobody getting to work. Furthermore, nobody expects it to lead to a conviction. The result is that we are wretchedly complicit in an uncivil society.

So the filming of the racist mother on a train, uploaded on to YouTube, seemed a brilliant social breakthrough. The Big Brother society assumes the state spies on its citizens. But what if the public take control of it instead? We can police and judge each other. Shame is a powerful social weapon.

Where there are villains there are heroes. The sequel to the YouTube racist woman was the have-a-go husband who hauled a swearing yob from a train. The young man was locked in one of those altercations with a guard that means nobody is going anywhere. It took a member of the silenced majority, a middle-aged man who worked in finance, to say that he wasn't going to take it any more.

Suddenly, we can bypass the expensive and wearisome bureaucracy of the courts. CCTV cameras can identify the criminal or the antisocial, the public turn detective and judge. Newspapers can print photographs of members of the public caught red-handed. Rioters cannot slink back into obscurity. Camera justice also gives us a fascinating social snapshot. Criminals are not only gang members: they can be otherwise respectable old ladies. One granny in a wheelchair was apparently an accomplice in the theft of an elf from a garden centre. If we can shame a violent young looter, imagine the satisfying effect on an old-aged pensioner. YouTube justice looks like the perfect answer to a hard-pressed and clogged-up court system.

So why does this felicitous solution, the YouTube stocks, also make me deeply uneasy? Anyone who has followed a magistrates' court knows that despicable acts have a context. As a local reporter, I watched the trembling caretaker caught shoplifting a present for his wife, or children from what David Cameron calls "troubled families" in fights because violence was the only form of communication in their homes. A crime is a crime, but better judged by a stern but fair magistrate than by internet mobs.

A woman who stole the flowers from a baby's grave was turned in by her daughter and claimed to be suffering depression after a sequence of family deaths. The youth thrown from the train suffered grazes and attributed his ugly temper to being given the wrong tickets and generally having a bad day. On the one hand, we all long for authority figures to sort out uncivil youths. But should we hand over law and order to have-a-go heroes? It is not easy to guarantee proportionate response or accuracy of interpretation. Public opinion is on the Saudi side.

We can catch the crime on camera. The most interesting and complex part is the cause. I am all for seeing and shaming, but would prefer experts to determine the consequences.

Sarah Sands is deputy editor of the 'London Evening Standard'