The Supreme Court yesterday gave the trucking industry a green light to run giant trucks with twin trailers on the interstate highways, striking down an Iowa law limiting the length of trucks using that state's roads.

Saying "Iowa's law is now out of step with the laws of all other midwestern and western states," a sharply divided court ruled that the state's limits ontruck size "unconstitutionally burden interstate commerce."

The ruling immediately allows big West Coast-style trucks that pull two trailers at once to use the interstate highways as far east as Pennsylvania. The decision also could cast in doubt truck-size limits in virginia, Maryland and the District of Columbia, warned Supreme Court Justice William Rehnquist, who dissented from the ruling.

Arguing that states' rights were at stake, Virginia had intervened in the case on the side of Iowa highway officials who for nearly two decades have fought trucking industry efforts to operate longer and heavier trucks.

Most states west of the Allegheny Mountains permit use of twin-trailer trucks up to 65 feet long on at least some highways. Like most eastern states, Virginia and the District prohibit twin trailers, and Maryland restricts them to designated highways.

Iowa limits truck lengths to 60 feet, and the trucking industry has likened the effect of this limit on Interstate 80 to a roadblock. Truckers heading east with 65-foot twin trailers were forced either to detour around Iowa or to split up the two trucks for the 300-mile trip across the state.

After two successive Iowa governors vetoed measures to increase the truck length to 65 feet, Consolidated Freightways Corp. sued the state, claiming the 60-foot limit was an unconstitutional restriction on interstate commerce. Iowa's 60-foot limit cost Consolidated $2 million a year in additional operating costs and cost the trucking industry $12.6 million a year, the truck line claimed.

The U.S. District Court in Des Moines, the Eight Circuit Court of Appeals and finally the Supreme Court all ruled against the state. Yesterday's Supreme Court ruling upheld the initial district court judge, who ordered that Consolidated and other truckers could run their 65-foot double bottoms on Iowa interstate highways and within five miles of them.

The three decisions, however, give little clue as to what extent a state constitutionally can regulate interstate trucking.

"The other state with truck-length limits that exclude Consolidated's 65-foot doubles would not at all be paranoid in assuming that they might be next" to be challenged in court, predicted Rehnquist in a dissent joined by Chief Justice Warren Burger and Justice Potter Stewart.

Four members of the court, led by Justice Lewis F. Powell Jr., decided the Iowa length limit was unconstitutional because the state could not prove that 60-foot trucks were any safer than 65-foot trucks when operated on four-lane highways. If the state could prove shorter trucks were safer, it might be able to limit lengths, they suggested.

Two other justices, William Brennan and Thurgood Marshall, also ruled against the Iowa law, but they said the safety issue was irrelevent. What Iowa really wanted to do, they said, was to keep interstate traffic off its highways and that's an unconstitutional restriction on interstate commerce.

The Supreme Court limited its ruling only to the use of Iowa's interstate highways, noting that legitimate safety concerns could justify restricting double-bottom trucks on narrow two-lane roads or city streets.

With the Iowa roadblock removed, truckers crossing the United States on Interstate 80, the main east-west route, now can operated double trailers between the Pacific Ocean and the Pennsylvania Turnpike.