IF ONLY IT HAD ALL BEEN STOPPED:

Fifty years ago last week, Governor Francis W. Sargent killed the Southwest Expressway, an 8-mile extension of Interstate 95 from Canton to the South End.

The Southwest Expressway threatened to displace thousands and bisect neighborhoods, forever altering their character and compounding air pollution problems. Advocates and government officials from the time said the decision helped Boston maintain its feel, that it preserved an inner core of neighborhoods that the highway would have been irreversibly sliced up.

Experts also see it as a domino that set in motion many important infrastructure projects that define Boston today, including a third harbor crossing that would ultimately become a reality in the Ted Williams Tunnel.

"The sense that much of Boston has of being a historic city with a very dense downtown fabric would have been destroyed," said Alan A. Altshuler, who was state transportation secretary at the time. "It would have been much more like the cities of the Midwest . . . where everywhere you look there are expressways."

Indeed, it's hard to overstate the impact that Sargent's decision had on modern Greater Boston. Today, part of the proposed highway-that-never-was is the Southwest Corridor Park, a treasured 4-mile greenway that stretches from the Back Bay to Forest Hills in Jamaica Plain. The old elevated Orange Line along Washington Street was torn down and relocated to the former highway corridor, which led to the renewal of neighborhoods in the South End and Jamaica Plain.

Years later, Sargent, a moderate Yankee Republican, would be hailed as a "transportation visionary" for setting the framework for how government thinks about transit and for prioritizing citizen participation in transportation planning.

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