For New Haven’s bridges, the third time’s the charm.

The replacement of the Prospect Street Bridge, which began in May, is running six months ahead of schedule, and is now on track to finish in December. The progress is especially remarkable since the recent makeovers on the sister bridges on Hillhouse Avenue and Temple Street were both delayed.

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But even though the latest project has been the most successful of the trio, it has also been the most inconvenient for the Yale community, severing the main artery to Science Hill. As a result, shuttle lines have been rerouted and classes have been rescheduled.

The $3.5 million project, originally scheduled to be completed in June 2010, involves replacing the entire bridge for the first time in 89 years. The original bridge was crumbling and in “poor condition,” said City Engineer Richard Miller.

Crews are now rebuilding the bridge with stronger foundations but are keeping the original brownstone blocks, said Chief Inspector Russell Tassie of Anchor Engineering Services, which won City Hall’s bid for the project. Yale, the state of Connecticut and New Haven are sharing the cost.

“We’re doing the tedious stuff now,” Roy Oemcke, a supervisor at the site, said Wednesday as workers installed the beams that will support the new roadway. He added that the timetable from now on depends largely on reinstalling the utilities.

The bridge spans the Farmington Canal, which is now a bike and walking trail stretching from the New Haven harbor to Northampton, Mass.

The Temple Street Bridge over the Farmington Canal cost $1 million to replace in 2007, and the Hillhouse Avenue Bridge cost $4.5 million in 2008. Both projects finished so late that, for a brief time, the University Health Services building on Hillhouse Avenue was inaccessible by car on both sides. The expense and delays of those two modest spans has made the Farmington Canal bridges something of a running joke in the city.

No one is laughing, however, about the 9:20 a.m. start time for classes, a direct result of the delays caused by the Prospect Street Bridge’s closure. Classes that normally start at 9:25 have been moved five minutes earlier, as the administration recommended five to 10 minutes of extra time to travel from Science Hill back to central campus.

The blue, green and red lines have been rerouted, said Donald Relihan, the director of support services who oversees Yale Transit.

As classes begin, Relihan said he plans to monitor the exact time it takes to get to Science Hill compared to previous years and inform the Dean’s Office, which will review whether there will be schedule changes for the spring term in October.

The completion of the Prospect Street Bridge anticipates several high-profile construction projects on the northern segment of campus: the two new residential colleges on the adjoining Prospect Triangle, the new University Health Services building on Lock Street and the new School of Management campus on Whitney Avenue.

The replacement of all three bridges was part of a $10 million development agreement signed in October 2006 between the city and the University.

Kristin Stoller, a participant in the 2009 Yale Daily News Summer Journalism Program, contributed reporting.