Why does a good road go bad?

A road that has carried presidents in their moments of triumph and on their way to the tomb, that has felt the footfalls of the Rev. Martin Luther King Jr. and 200,000 marchers, that trembles each May under the roar of the Rolling Thunder motorcycle riders?

Constitution Avenue, a pathway to America’s most cherished monuments, is crumbling.

Fixing it will snarl traffic for the rest of the year.

The National Park Service, which owns the avenue, is about to begin the most significant overhaul to the eight-lane thoroughfare in more than 60 years. Working one block at a time, between 15th and 23rd streets NW, it will rip out the whole rotten shebang, replacing pavement, curbs, crosswalks, lights and sidewalks.

The work will require closing four lanes of traffic on each block under construction, beginning at 23rd Street on April 11, after cherry blossom season ends. The estimated 50,000 drivers who use the avenue daily will probably not remember this as a fun experience.

“There’s no easy way. There’s no good time to do this, so we will try to make it as painless as possible,” said Charles N. Borders II of the Park Service.

To minimize the mess, the Park Service has put out a list of alternative routes and plans to update its Web site whenever it designates a new one. Overhead traffic signs in Virginia and the District will display warnings and news.

Work will cease on big traffic days: the White House Easter Egg Roll, Rolling Thunder, July 4, the Marine Corps Marathon, the National Christmas Tree lighting and national holidays.

The $10.3 million project has been needed for years, the Park Service said, but the agency waited until other road jobs on its turf and in the District were completed, for fear of giving the impression that every street in town was under construction.

The reason for the need — the explanation of why good roads go bad — applies to millions of miles of roadway across the nation. Understanding it provides one key to comprehending the country’s crumbling infrastructure, the repair of which has been likened to the urgency of addressing the deficit: The longer it’s ignored, the worse it gets.

Constitution Avenue, literally and figuratively, is a slice of America’s problem in microcosm.

The road — like most, including virtually the entire interstate network — was laid decades ago. At least 60 years ago, and probably closer to 80, the underlying roadbed of Constitution Avenue was placed as a series of concrete slabs, each 20 to 40 feet long.

A smooth ribbon of asphalt, 5 to 6 inches deep, was poured on top of those slabs. Ever since, that asphalt has been milled away and replaced whenever potholes got the upper hand.

But concrete is not forever. The earth moves under it. Those concrete slabs shake, rattle and roll minutely, crunching into one another at the joints. Think of it as highway arthritis: You can’t see or hear it, but it causes pain to the asphalt above. The concrete’s crumbling gets worse as water seeps in and the freeze-thaw cycle bursts things apart.

The concrete gives way, causing the asphalt layer to crack and crumble, too. Soon potholes sprout across the surface, and routine milling and repaving won’t solve the problem for long.

In a sense, Constitution Avenue is getting a knee replacement. The road will be dug down to bare earth, and new concrete slabs will be laid. These slabs will be connected by rods that will dampen the up-and-down movement while allowing necessary horizontal shifting.

Back when the White House had a Potomac River view, a canal ran where Constitution Avenue is. So what lies underneath shifts more than Broadway, which sits on the granite of Manhattan. But regardless of whether concrete sits on rock or mud dredged from a river, it has a life span — and much of it, put down as the nation grew after World War II, is at death’s door.

“You can overlay asphalt on a failed concrete road, but it won’t last long,” Borders said.