What are the police doing?

Any time I walk through a Tube station these days I see at least four police officers (and sometimes more). What are they doing? They're chatting, studying their nails, shifting from foot to foot, or simply looking bored and pissed off.

I don't blame them: as an approach to dealing with the terrorist threat I have to wonder who thought this was a good idea.

What precisely are they supposed to do?

Let's imagine an actual suicide bomber is approaching them - presumably in peak hour, when the entrance hall to the Tube will be packed with people.

First they have to somehow - perhaps by mental telepathy? - identify the bomber from among all of the other people dragging suitcases, lugging large backpacks, or boxes of what appear to be electrical items from the sales.

Then what do they do? They challenge the person, approach, then are the first to get blown up, together with all of the people around. If it is a less confined space than a Tube carriage I guess the carnage would be slightly less, and help would reach the victims more easily, but does the degree of improvement justify the expenditure of resources? I doubt it.

The theory goes that they are supposed to "reassure" the public. But anyone with half a brain will have reached the same conclusion as I - and the apparent emergency state of London, with police very visible everywhere, can only be adding to the panic among those of nervous disposition. (Reports are suggesting Tube usage is down 15 per cent - or to put it the other way around, 85 per cent of people are going about their business perfectly normally.)

After what is increasingly looking like the most horrific misjudgement in the shooting of Charles de Menezes, the police doing nothing might be a good thing, but surely they could do it more cheaply and sensibly by getting back on to a normal footing.

What they are now doing, as Matthew Parris points out in The Times today, is glorifying the suicide bomber.