Traffic conditions in the Square, especially during rush hours, will improve with a proposed extension of the MTA, Thomas Lenthall, Cambridge public transportation expert, claimed last night.

Two bills, now before the State Committee on Metropolitan Affairs, call for the extension of the subway to Porter Sq. under Massachusetts Ave. and, from there, to either Bedford or Weston.

Commuter parking and traffic through the Square will be reduced by making Harvard "just a stop along the way," Lenthall said. In addition, the car barns near the Charles River will become virtually useless, an MTA official disclosed, and the property will be sold. Last month the University offered to purchase the property for the ninth and tenth Houses.

Parking facilities would be provided at North Cambridge, and the trackless trolley line running along Massachusetts Ave. eliminated with the proposed $20 million extension.

Mrs. Mary B. Newman and Lawrence F. Feloney, state representatives, are sponsoring the bills with Lenthall. Legislative approval is required for extension of the transit system, while the cost of the new subway would be borne by the 14 cities and towns served by the MTA.

Lenthall hoped that work on the underground extension could start in July, when Massachusetts Ave. is scheduled to be widened. Under plans drawn by the Department of Public Works, the street will be widened six feet on each side. A five foot safety island will be added and unused trolley tracks removed.

Similar expansion plans for the MTA were defeated several years ago since the 14 municipalities could not agree upon a method of finance and payment.

The extension to Bedford or Weston would utilize Boston and Maine branch lines now used by two trains daily.