When cars and people collide, the cars usually win out. But when Harvard students and Square traffic were at odds with each other several years ago, the University stepped in to build a $3.5 million underpass that would allow its students to get to classes and let commuters get to their jobs.

C.E. Maguire, Inc., the same firm that said the John F. Kennedy Memorial Library would have a "minimal impact" on Harvard Square, conducted a feasibility study on the tunnel and the De Mateo Construction Company built it. Afterwards, the University turned it over to Cambridge.

The problem is that the tunnel has sprung a $200,000 leak, and city manager James L. Sullivan said this week he will initiate a suit against Harvard to recover the costs of the repairs.

Conrad C. Fagone, commissioner of public works, said Tuesday that the defects in the tunnel were present in the original construction. Because the tunnel was improperly sealed, he said, water seeps into it when it rains, freezing during the winter and creating hazardous driving conditions.

The head of the construction company last week denied blame for the leakage, shifting it instead to Harvard. "Harvard's engineers were on the job all the way through," he said. "They must have been satisfied, otherwise they would not have paid us."

Donald C. Moulton, assistant vice president for community affairs, shifted the blame to the city. Cambridge's engineers were present and approved the work, he said.

But Fagone denied that his men had anything to do with the project. And his judgement that Harvard is responsible for this gift horse gone lame will send the matter to the courts for resolution.