When I was 13 or 14, my parents let me take the train from central New Jersey to New York's Penn Station and then transfer to a subway heading to Queens so I could watch the U.S Open at Forest Hills. Before I went, they gave me a lot of safety tips: Don't look lost. Don't get in empty subway cars. Don't talk to strangers. Don't take your wallet out of your pocket. Etc., etc.

One thing my Bronx-raised parents never thought to warn me about was what to do if a bomb went off in the train.

These days, the old anxieties about taking the subway seem almost quaint. In contrast to the perils we worried about in the 1970s (during the peak of New York City's crime wave), the danger of getting blown up is more remote but harder to guard against.

That's why Thursday, July 7, the day of the London subway bombings, found me and my wife wondering whether to send our 11-year-old daughter and two of her friends off to day camp on the Metro, as we had done every day for the previous two weeks. On the one hand, as the mother of our daughter's friends said, the higher level of alert here in Washington meant that the kids would probably be safer than on any other day of the year. On the other hand, after hearing the London news on the radio and then seeing the images on television, we couldn't help imagining a similar disaster here. (Only a week earlier, our biggest worry was whether she and her friends could avoid getting squooshed or separated by the rapidly closing doors.)

In the end, we opted to drive. It wasn't an inconvenience and as my wife, who decided to take the subway, said later, "The idea of her on the Metro with no adults and having to fend for herself if something happened was too terrible to contemplate." Most commuters, however, shrugged it off. According to Metro statistics, ridership the day after the bombings was 10 percent below what it had been on the day before them -- but July 6 had seen Metro's ninth-highest ridership, and ridership usually dips on Fridays, especially when the Nationals aren't in town. Last week, ridership was back to normal.

After the train bombings in Madrid and London, Washington and its mass transit seem natural targets. Our own government officials often speak of the inevitability of a cataclysmic attack on American soil and talk about the need for more homeland security. But sometimes it isn't clear what those of us in prime target zones are supposed to do about that.

Our daughter was disappointed to get a lift. When we told her that there might be police with submachine guns on the Metro, she wanted to see them. "I haven't seen guys with machine guns and it's kind of cool," she said later.

Wasn't she afraid? I asked. "A little, but not really," she said, "because I knew they wouldn't shoot me." Her 12-year-old sister, a child of the post-9/11 era, explained why the guns weren't scary. "High security stuff is nothing new for us," she said. "We're used to seeing police with big trucks and guys in black suits and armored cars."

On Friday, we again packed the kids onto the Metro for camp. Since then, I've wondered: Who was right on the day of the London bombings: The parent who has reported from 20 countries and written and edited dozens of articles about terrorism and national security, or the curious 11-year-old?

It comes down to a calculation of risk. If I were being scientific about it, I might say something like: risk multiplied by frequency divided by life benefits yields a number with an inverse relationship to advisability. Except in this equation, every variable is unknown or unquantifiable.

That doesn't stop us from making guesses, just as we guess about the risks of many other, more mundane activities. My late father was an orthopedic surgeon, so I avoid skateboarding and ice skating and only recently tried skiing. On the other hand, as a journalist I have flown on a variety of doubtful airlines, covered South Africa's violent black townships during two states of emergency in the 1980s, and breathed Beijing's heavily polluted air for four years. None of that seemed unduly dangerous. I wouldn't quarrel with anyone who felt otherwise, though.

Sometimes risks are only apparent with hindsight. For 31/2 years I worked two blocks from the World Trade Center towers. Later, I lived in London and took the underground to work from the Edgware Road stop that was blown up on July 7.

Looking ahead is harder. Since Sept. 11, 2001, my wife and I have talked about whether we needed to recalibrate the risk factor in our calculation about our lives here. It isn't ideal for both of us to be working within five blocks of the White House, a possible terrorist target. Plus I take the Metro to work every day. And a variety of other locations we frequent around town seem to be attractive targets in ways I'd rather not mention.

Yes, people reassure themselves with the cliche about the dangers of crossing the street and other everyday activities. And we take all sorts of risks for the same reason we cross the street: because we have to get to the other side.

So when I get on the Metro this week and head downtown, it won't be because I'm oblivious to the risks of living in the nation's capital. But I've weighed the likelihood of anything happening against other factors: the privilege of working for a major newspaper; the ability to live in a nice house and run in the park; and the intellectually and culturally stimulating urban environment Washington offers.

The Metro is an important part of that environment. To me, having my kids on the Metro when they're in high school, even with a degree of terrorism risk, seems like it will be safer than some of the times I had to rely on the bad or drunken driving of high school friends in New Jersey. This city's public transportation system will give them a mobility and access to learning and entertainment possibilities that I didn't have until college. And it will teach them self-reliance.

We're not the only parents, of course, trying to figure out our comfort level. The other day I called Courtney O'Connor, the mother of the girls who were riding on the Metro with my daughter, to talk about what risks are reasonable. "I'm not your average parent," she said, explaining why she wouldn't have minded if our kids had taken the Metro last week. That's an understatement. She worked for the United Nations in El Salvador during the war there during the 1980s and has advised refugee agencies in hot spots in West and Central Africa. She's driven by blown-up cars, seen military abuses and was nearly kidnapped once. "I've lived with violence and have a sense of when you're at risk and when you're not," she said.

When does she think we're most at risk? "When we're not watching," she said. She's not oblivious to the risks of simply living here, though. "I worry that we're raising our kids here in Washington and that it's such a key target." Still, she and her husband are busy renovating their house for a prolonged stay. "I certainly say to myself that in order to live here and invest in this renovation, you have to be in a little bit of denial about what can happen at any moment."

A little bit of denial can be a helpful thing -- a balm in troubled time. But too much denial can be harmful.

That's the way another friend, fellow journalist and Washingtonian Jeffrey Goldberg, thinks of it. He was talking about the "We're Not Afraid" campaign in London. A Web site called We'renotafraid.com has been swamped by more than 4 million hits and more than 3,500 photos. The site proclaims to "show the world that we're not afraid of what happened in London, and that the world is a better place without fear."

Goldberg, who lived in Israel for six years, says that, on the contrary, we should be afraid. He says, "To say you're not afraid is a form of denial. Admitting you're afraid doesn't mean you don't go outside. But it's foolish not to act on justifiable apprehension."

A little bit of fear -- and caution -- could make us much safer, he says. He cites the different ways that people in Israel and the United States react to unattended packages or bags, or to suspicious people. "In Israel, when you see a package unattended, almost instantly someone will call the police and the police will respond and whole bus stations will be evacuated," Goldberg says. "Ninety-nine percent of the time it's some dope who went to the bathroom. But that person is the one who's scorned. The person who made the call is the hero even if the bag is old underwear. But in America still, I don't think these are behaviors considered acceptable."

Another friend, who worked for the 9/11 Commission, agrees. People here eventually began to make fun of Code Orange alerts issued by the head of the Department of Homeland Security. But while the color codes may have been a stupid way of expressing an idea, the idea is still valid: Against a supple and ingenious foe, the most effective defense is a population that's alert in airports, on subways, on streets to strange packages or strange people. "Subway riders need to think of themselves as passengers on the flight with the shoe bomber Richard Reid," this friend said.

New York City Transit Authority is promoting that new attitude with an ad campaign that says: "If you see something, say something." The Massachusetts Bay Transit Authority urges people to report others "behaving oddly" and "people showing visible signs of nervousness, such as excessive perspiration." But those categories could cover just about everyone in Washington on a typical summer day.

As much as President Bush or British Prime Minister Tony Blair say we won't let terrorists change our lives, this could be the start of a new era, and not in a good way. There is something unsettling about the idea of turning America into a nation of snitches and amateur spies. Is the guy taking photos of the George Washington Bridge a terrorist or the next Henri Cartier-Bresson? Are the folks playing Frisbee in the park near National Airport enjoying the weather or checking out the planes? Are people wandering in front of national monuments scoping out targets or are they tourists? And do you trust the strangers around you to make those judgments based on looks and feelings?

All the same, on the Metro in the mornings last week, I departed from my usual routine of simply reading the newspaper, or looking over some manuscript, or daydreaming. I found myself glancing up to look at the other passengers and their bags, or to gauge the distance to the stairs, or read the instructions on the emergency exits. Reassuring? Maybe.

In any case, this week you'll be able to find me on the platform waiting for the next train.

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Steven Mufson is deputy editor of Outlook. He has reported on national security issues as a member of the newspaper's national and foreign staffs.

Risk assessment: Does a more visible police presence on subway platforms lower the likelihood of an attack or reassure riders? That's just one of many questions facing Metro officials in the wake of the July 7 London explosions.