By now we’ve all had that feeling. You’re waiting in a crowded underground station, about to step onto a train. Suddenly you think: What if it happened here? What if this station, or this escalator, or this train, is a target? If you’re like most people, the feeling passes. You shrug, get out your newspaper, get on the train, and go to work.

As it happens, that intuitive estimate of risk is the right one. Most people probably don’t sit down and calculate the chances of being blown up by bombs like the ones that exploded last week in London, or last year in Madrid. But if they did, they’d have to reckon that there are 7 million people in London, not counting the millions more who commute into the city every day. If 50 of them died last week, then the odds are … very low. So low that it’s still safer to ride the London tube than it is to drive, just as it’s still safer to ride Washington’s Metro or the New York subway.

Yet, whatever the intuitive and mathematical calculations, the political calculation of risk is sure to be different. After the London bombings, it was inevitable that someone would call on the government to “do more” to protect commuters in this country, to “beef up” mass transit security – and indeed various people did.

An aide to New York’s governor said the attacks should be a “wake-up call” to Washington. Among others, Senators Charles E. Schumer and Hillary Rodham Clinton (of New York and New York, not surprisingly) called for more money for public transit systems to be added to the Homeland Security spending bill wending its way through Congress. Schumer talked of $200 million. Clinton spoke of $1.3 billion. That, of course, is chicken feed compared with what the public transportation lobbyists say their industry needs: $6 billion to “upgrade” security.

Just to be clear, I’m not saying that the nation’s various public transit systems weren’t right last Friday to lay on a little extra security, which doubtless made everybody feel better. Nor am I saying that a relatively small fraction of the stunning $18 billion the nation has spent on airline security since September 11, 2001, might not have been better spent on extra video cameras in underground tunnels, or some extra emergency response training for bus drivers, or some backup communications capability, which anyway might be useful in case of a hurricane or robbery as well as a terrorist attack. But the chances of terrorist attacks on a typical cross-town bus are too small and the realistic options too limited to merit extraordinary new expense.

Here’s the truth about mass transit security: There is no technology that can guarantee it. There are no machines that can reliably detect the presence of a backpack filled with homemade explosives in an underground tunnel. There is no point in putting metal detectors at every single subway entrance or at every single bus stop.There is no amount of money, in other words, that can guarantee that subways and buses will be completely safe from small-time bombers, suicidal or otherwise. It’s going to be a temptation, especially for Washingtonians, New Yorkers, and others who regularly ride mass transit, to lobby their politicians for more spending. Don’t do it.

If that makes you feel queasy, or powerless, or claustrophobic, then do the one thing that passengers actually can do: Keep your eyes open for unattended packages, and if you see one, say so out loud. The Madrid bombs may have been left by terrorists who exited the trains. Maybe a sharp-eyed passenger could have called attention to the packages before they exploded. Maybe someone could have stopped the train. By the same token, maybe a sharp-eyed train driver could have spotted a suicide bomber’s erratic behavior in London, just as Israeli security guards have more than once spotted suicide bombers before they actually entered shopping malls. A little forethought goes a longer way, in the current climate, than a load of technology.

But if you don’t particularly mind, and the subject doesn’t particularly interest you, don’t worry about that, either. It’s tempting fate to say so, but the chances of being one of 50 in 7 million are very, very small. Read your newspaper, enjoy your subway ride, and think how nice it is not to be stuck in traffic.