After more than a decade of ever-shifting traffic detours, the last major piece of roadway in Boston's Big Dig opened Friday, officials said.

The opening of an off ramp from Interstate 93 south means the $14.6 billion project to ease congestion in downtown Boston is substantially finished, said Matt Amorello, Massachusetts Turnpike Authority chairman. Heavy construction began in 1991.

For drivers, the ramp's opening signals that the city should finally have a stable street pattern.

"We're starting to see the success of this project," Amorello said. "There's been an incredible engineering achievement."

There is still work left, including construction of parks and museums where the elevated Interstate 93 used to sit. Also, a major leak that flooded a tunnel in 2004 led to the discovery of wall panels that needed repair. Grouting to patch leaks is expected to wrap up around the end of the month. All work should be done by spring 2007, Amorello said.

The dig has been plagued by soaring costs and long delays. The cost ballooned from $2.6 billion to $14.6 billion.

Formally called the Central Artery and Third Harbor Tunnel project, the Big Dig buried Interstate 93 in tunnels beneath downtown and connected the Massachusetts Turnpike to Logan Airport with a third tunnel beneath Boston Harbor.