> say you're cleaning if anybody asks

Why would anybody asks? But, your point stands: I've been absolutely amazed that nobody had blown up our (Parisian) metro at 8:30 in the morning already. If you want a maximum casualty count, just bring a bunch of stuff and shake it in a pressure cooker. Just synchronize payload over multiple lines, and boom, you got something around thousand of casualties, plus a massive panic mouvement, and a genuine fear of public transportation for a while.

Exactly about the fear of public transportation. Terrorism is meant to terrify people. Israelites have been dealing with terrorism for decades and it's usually met with a sober reaction reminiscent of the Japanese getting the trains back to running the day after Hiroshima. Terror attacks do happen, but there is hardly the kind of freak out one might find in, say, America.

It sounds like you're saying that because it can't be 100% effective, we shouldn't do it at all. You could say the same about having a lock for your front door, but I bet you've got one.

No, I think you're taking it too far. We only need to conclude that the minimal benefits from the proposal (i.e., few attackes would actually be averted) are vastly outweighed by the extreme cost to our civil liberties.

There do exist more reasonable compromises. For example, we as a society have already said that you can search someone's property if you get a court order. That's a smaller cost, because the authorities are forced to go through a supervised process before anyone's privacy is in jeopardy, and it's still sufficient to find the evidence in a huge proportion of cases.

But the proposal on the table is extreme. It affects everyone's privacy, before there's even a notion that a crime has been considered.

Other people - probably the majority of the UK population based on opinion polls and elections - weigh the balance differently. We (HN readers) are probably in the extreme minority, and we are not necessarily right.

The opinions of laymen only carries weight with those proposing idiotic solutions.

Burning down the house to kill a spider is stupid no matter how many people vote for it.

The lock on my door is basically a legal seal: the door itself is often so flimsy, you could probably kick it in, but because there is a lock, you can't then claim you entered by accident or that you assumed it was ok to enter. This is why effectiveness is relatively secondary.

This sort of surveillance, on the other hand, is supposed to be all about effectiveness... But it ain't.

I have a lock so that if I am burgled my insurance will pay out.

You don't even need to mix anything. Gasoline can be purchased freely in any country and is deadly when poured under doors, especially those doors are blocked.