A Boston engineering firm has advised the University and the City to cooperate in a major building program to relieve traffic congestion in Harvard Square.

In a 75-page report to be released later this month, Bruce Campbell and Associates propose a three-phase system of improvements designed to avoid "major landtakings or disruptions to the economic, social, and educational life of the Square."

The University, the Harvard Trust Company, and the Harvard Cooperative Society sponsored the report, which is based on the first large-scale study of the Square's traffic problems, without endorsing specific proposals.

One of the proposals is that Harvard build a parking garage opposte the Broadway Market and a pedestrian bridge between the Yard and Memorial Hall. The engineers also recommended closing off Kirkland St. north of Mem Hall; making Massachusetts Ave., Mt. Auburn St., Boylston St., and Brattle and Eliot Sts. one-way; extending Cambridge St. at its western end; and putting through a new road between Garden St. and Memorial Drive.

Charles P. Whitlock, assistant to the President for civic affairs, said yesterday that the University already had plans to build the garage, which he estimated would hold at least 200 cars and be located at the corner of Broadway and Felton St.

He said the bridge was "an idea that has been talked about for at least ten years" but that no specific plans had ever been prepared for it, largely because of its high expected cost. The traffic report warned that "as student facilities are increased . . . and normal traffic growth increases vehiculas volumes, the conflict between pedestrian and vehicle will create more congestion and increase the accident potential" along Cambridge St.

The engineers found that "contrary to popular belief, the volumes of traffic going through the Harvard Square area are relatively small." About 6000 motor vehicles traveled through the 75-block study area during the evening peak, half of them passing the MTA klosk.