By Bob Orr, CBS News

Critics like to point out that three years after 9/11 there's not been one follow-up attack on America. The nation's color code has bounced up and down between yellow and orange and nothing's happened.

Some see this as proof that the warnings are hyped and overblown; and they accuse the media of being complicit in passing on unsubstantiated, uncorroborated raw information that is too vague to be useful. In essence, we have tabloid headlines with no real meat.

Others, though—including officials at the Department of Homeland Security, like to remind us that "we don't know what we don't know." Has the extra vigilance stopped an attack? Have new safeguards deterred terrorists?

I've put those questions repeatedly to Homeland Secretary Tom Ridge and others. They can't answer the questions. Based on interrogations of captured al Qaeda operatives, they've disrupted terrorist planning. But as for knowing if we've stopped an attack, iit's tough to prove a negative.

One disturbing alternative to putting out raw threat information is to withhold it. I've heard more than one person complain that the bulletins are so non-specific, that the average person has no clue how to respond. That may be true but I doubt anyone in this room would vote for more government secrecy. As journalists, we have to be careful what we ask for.

While no one can point to solid evidence that we've stopped an attack in this country, we've made some progress in bolstering our defenses. We have clearly tightened aviation security. Prior to 9/11 aviation security was not geared to stop suicide hijackers. Cockpits were not secured. Knives smaller than four inches in length were legal to carry aboard. There were about 30 air marshals covering only a few international flights. Checked bags were not screened for bombs.

Flight crews—and the rest of us—were of the mindset that we should always cooperate with hijackers. That way nobody would get hurt.

Well, all of that has changed...but it has cost us plenty, in terms of our time, our privacy and our money. The Department of Homeland Security has spent well over 100 billion dollars since 9/11. Cities and states have spent billions more—and the cost to private business and, ultimately, the consumers is incalculable.

Yet critical weaknesses remain. A lot of them.

Terrorism has gone from a subject the average American rarely considered—to one which can dominate our thoughts and feed our worries.

It's hard to keep all of this in perspective. We don't want to live in fear, but at the same time, we can't afford denial.

We must stay informed. And those of us who cover homeland security must be accountable. It's always more important to be right than to be first.

When it comes to terrorism, we have to press for the most complete answers. We have to resist the rumors and the sensational. And we need to strike a balance.

Which in turn can help the public do the same.