Profits and traffic volume for the nation's 100 largest trucking companies as a group declined in 1979, the Interstate Commerce Commission reported yesterday.

The rate of return on equity for the to 100 firms dropped to 14.7 percent in 1979 from 21.2 percent in 1978, the agency said in its quarterly report. The top 100 firms generate about 52 percent of the total revenues reported to the commission by about 3,900 trucking firms. Another 13,300 ICC-regulated companies with revenues of less than $500,000 a year do not have to file the detailed reports.

As a group, the 100 companies hauled 216.8 million tons in 1979, down 5.1 percent from 1978. While operating revenues rose 11.8 percent to $18.8 billion in 1979 from $16.8 billion in 1978, net income of the carriers declined 24.3 percent to $376.2 million from $497 million.

Adversely affecting trucking operations last year were strikes by the Teamsters Union and the owner-operators, fuel shortages and general economic conditions, the ICC said.

Earnings and traffic volume also declined in the fourth quarter, the ICC said. Revenue tons hauled declined 12.7 percent to 55 million tons from the same quarter a year earlier, due primarily to the weakening of the economy, the ICC said. Operating revenues rose 11.1 percent to $5.3 billion in the October-December quarter, compared with the same period of 1978, as a result of rate increases.