It was 20 years ago this week that the Williamsburg Bridge was closed after the discovery of a 6-foot-long crack caused by severe corrosion.

As then-chief engineer of the New York City Department of Transportation, I had little choice but to order the roadway closed, and trains traveling over the bridge stopped.

I'm glad I did; ultimately, inspectors found more than 400 locations with corrosion serious enough to declare them potential hazards.

The bridge's closure became a national symbol of neglect. After all, this was the largest bridge shutdown in U.S. history - affecting eight lanes of traffic, two subway tracks and two bike/walkways. That presidential election year, infrastructure became a hot topic. One candidate, Jesse Jackson, walked across the closed bridge to spotlight the problem. In the midst of another presidential campaign year, our infrastructure is in crisis again. We must ask: Are America's bridges any safer today than they were 20 years ago? The troubling answer: No.

Fortunately, here in New York City, where most of us travel over bridges every single day, our crossings are healthier than before - thanks to money and effort. In the wake of the Williamsburg scare, Mayor Ed Koch approved a sweeping package of reforms. More than $3 billion has been spent to rebuild and maintain our bridges since, with about half going to the flagship East River crossings. The number of poor bridges has plummeted by 96% - from 63 of the 840 bridges in the city in 1988 to just three today.

But the nation's bridges, starved by a lack of attention and investment, are another story - a very scary story.

We all know about the Interstate 35 bridge collapse that killed 13 in Minneapolis last August (I am leading one of the forensic engineering teams looking into the tragedy) and the I-95 emergency closure in Philadelphia last month. They weren't anomalies. The I-35 bridge was among about 12% of the nation's bridges rated structurally deficient last year. That's a whopping 72,500 of our 600,000 bridges.

And oddly enough, it's not the oldest spans that ought to worry us the most. The I-35 bridge was just 40 years old; the I-95 bridge viaduct in Philadelphia was just 35 years old.

It is our baby boomer bridges that are getting sick or hitting retirement age. You see, the bridges built in the first half of the 20th century, like many of New York's major crossings, were strong and solid, with lots of redundancy - meaning, if one support gave way, plenty of others would pick up the slack.

But post-World War II, as the Interstate Highway System ramped up, we took to building much sleeker bridges, with dangerously little redundancy. These are allover the country, and they're starting to deteriorate precariously.

We know how to fix this. The only question is whether we'll take the necessary steps.

My proposed course of action: Start by increasing the gas tax, as recommended by the national surface transportation commission. Use the new money to create a national infrastructure bank that could issue bonds to finance projects. Both Sens. Hillary Clinton and Barack Obama have proposed some version of such a bank. As a side benefit, those projects would generate thousands of jobs in an economy that could use them.

Then, encourage user pricing on highways and bridges nationally - like the congestion pricing that regrettably just tanked here in New York - to pay for their maintenance. At the same time, use high technology to monitor bridges, much as doctors use MRIs and EKGs (disclosure: I'm affiliated with one of the monitoring companies).

Finally, set a clear goal of spending 3% of our Gross Domestic Product on infrastructure going forward. Today we spend about 2%. China spends 9%, India 5% and much of Europe 4% to 5%.

The choice before us is a question of generational ethics: Pay more now or have our children face even higher costs and more risk tomorrow.

Sam Schwartz, aka Gridlock Sam, runs a transportation consulting firm, Sam Schwartz Engineering PLLC.