The interstate Commerce Commission formally adopted a new policy yesterday that will make it easier for trucking and intercity bus businesses to get started or expand.

Although the commission has become more liberal in the last few years in granting motor carrier applications, the new policy represents a significant formal change in the agency's traditional attitude of protecting existing carriers from new competition.

Under the new policy, the 92-year old agency said it will give greater weight to the benefits of competition and less weight to protecting existing companies in licensing interstate bus and truck firms.

The commission specifically dropped a 13-year old test that required an applicant to prove that the service it proposes cannot be performed as well by existing carriers.

The agency will be looking at whether an applicant can demonstrate that the "new service proposed will serv e a useful public purpose, responsive to a public demand or need."

If an applicant is deemed fit to perform that service, authority will be granted to fill the demonstrated need unless companies opposed to the application can establish that the entry of a new carrier into the field "would endanger or impair the operations of existing common carriers to an extent contrary to the public interest."

To carry that burden of proof, an opposing party would have to put forth "specific factual information" that includes a description of the equipment and facilities it has available to meet the purposes of the application, the specific service it provides those who would utilize the proposed services, the amount of traffic it handles that would be diverted if the new authority is granted and how its profitability would suffer, and a statement concerning other adverse impacts of a grant of authority on its business generally and on the public. This would include what facilities or or terminals would be closed, the number of employes who would lose their jobs, resulting inefficiences, and how its ability to continue its existing service would be affected.

The far-reaching policy statement, issued on a 6-2 vote, disposes of a proposal that has been pending at this commission for almost 11 months. It goes into effect 30 days after it is published in the Federal Register.

The majority noted that the truck industry is "vastly different" now than it was when the ICC began regulating the industry in 1935, when it was characterized by one-truck operators providing short-haul trips. It is now "one of the largest businesses in America." the ICC said.

"We are convinced that we have the duty as regulators, charged with administering in today's world a statute that was written and originally construed under vastly different economic and social conditions, to change the standards we use in interpreting and applying the terms and concepts reflected in that statute," i said.

A spokesman for the American Trucking Associations said yesterday that the trade group hadn't seen a copy of the new policy and therefore couldn't comment on it. However, it is a safe bet to assume that the ATA will challenge the policy in the Court of Appeals.

In their submissions to the ICC on the proposed policy, the ATA and the regulated carriers generally said the proposed changes involve the basic purposes of the original statutory plan and the ICC couldn't make these changes administratively. They required congressional action, they contended.

In its decision, the commission disagreed. Acording to its reading of the law and the legislative history, the statutory standards were "extremely broad and flexible.

"They do not suggest that the commission should be either open-handed or tight-fisted in dealing with applications," the agency said.

The three new members of the agency were in the majority; the dissenters were George M. Stafford and Robert C. Gresham. Both dissenters said the policy statement means little because the current trend has been toward more liberal entry. Gresham said applications still will be denied, but now because the ICC will find that no useful purpose will be served by a grant of new authority.

But commission watchers -- and clearly the regulated industry that is so opposed to the change -- disagree.