Boston — Thousands of billboards from Maine to California, which had been threatened with extinction, may yet survive. Their future rests on two key and potentially far-reaching Washington decisions within the next few months:
* Continued federal funding of the highway beautification program, currently a target of Reagan administration budget cuts.
* Outcome of a billboard industry challenge of a San Diego ordinance outlawing all off-premises advertising signs.
The case, argued Feb. 25 before the US Supreme Court, involves whether the measure violates free speech guarantees contained in the First Amendment to the US Constitution.
Although the decision, expected in June, will directly affect the San Diego ordinance, similar billboard bans in at least four states -- Alaska, Hawaii, Maine, and Vermont -- also could be in jeopardy.
So too might be standards established in some of the 3,206 cities and towns in 39 states, which during the past 15 years have adopted billboard controls tougher than federal highway beautification regulations.
From the billboard industry's standpoint, a victory in the San Diego case is especially important because the city, which includes some 320 square miles, now is California's second largest municipality and the nation's eighth biggest with 8,707,000 residents.
Metromedia, in seeking to overturn a March 1979 California Supreme Court ruling upholding the ordinance, argues that billboards carry the types of messages -- political and public affairs -- that traditionally have been accorded First Amendment protections.
While those close to the scene decline to speculate how the nation's highest court might rule, it is noted that in recent years the justices have extended First Amendment protections to various forms of advertising.
A ruling in favor of San Diego would almost certainly encourage other cities and maybe even a few more states to impose similar billboard bans or at least tighten zoning restrictions on large commercial signs within their boundaries.
Maine already has on its books a law aimed at ridding the state of billboards. Portions of the controversial statute, however, were struck down Dec. 22, 1980, by a three- judge federal appeals court.
Under the program, though temporarily halted, at least 600 of the more than 1 ,500 signs targeted for removal have been dismantled. The federal appeals court judges held that the state's interest in protecting the beauty of the landscape is not legally strong enough to justify the law's restrictions on free expression.
Judge Bailey Aldrich, in concluding that portions of the measure established an unconstitutional infringement of free speech, said that "First Amendment freedoms are on one side of the scales, the balance must be struck by the courts and not by the legislators."
Critics of the Maine law cite what they consider a major inconsistency: that political candidates are permitted to mount signs along roadways three weeks before the election, but at no time can noncommercial, socalled public service messages such as "Save the Whales" be posted.
Under the 1965 Federal Highway Beautification Act, states and municipalities removing billboards from interstate highways or access routes are eligible for reimbursement of 75 percent of the cost removal.
But these funds have diminished substantially in recent years. Those close to the federal highway scene are pessimistic about funding under the Reagan administration.
Since the billboard reduction program began, the Federal Highway Administration has spent $141.9 million to remove 38, 000 roadside billboards across America.