SAN FRANCISCO, Aug. 2— Randolph Collier, ''the father of the California freeways,'' died of chronic pulmonary obstruction today in Sacramento, where he had lived since losing his State Senate seat in 1976. He was 81 years old.
Mr. Collier's reputation as a powerful lawmaker grew out of his more than 20 years of service as chairman of the State Senate Transportation Committee. He was the principal author of the Collier-Burns Act of 1947, which led to a surge of highway contruction in the 1950's and 1960's that made California freeways known throughout the world.
For 38 years, the first 20 of them as a Republican, Mr. Collier represented a large rural district in the Siskiyou Mountains of northern California. With his thick shock of white hair and his mastery of legislative wiles, he was also known as ''the silver fox of the Siskiyous.''
Mr. Collier was elected as a Republican in 1938, but switched to the Democratic Party in 1959 when he opposed then Senator William F. Knowland's campaign for the governorship, a campaign that Senator Knowland subsequently lost.
While Mr. Collier's political base was in a mountain area where freeways had little political significance, he used his chairmanship of the Transportation Committee to wield a combination of automobile clubs, public works contractors, construction workers' unions, oil companies and automobile manufacturers and dealers, to authorize California's extensive freeway system. Opponent of 'Rabbit Transit'
This group, which Mr. Collier called ''the freeway establishment,'' could count on him to pass bills they wanted and, as ''the fastest gavel in the West,'' to smother those bills it disliked.
He was disliked by many California legislators from the cities because of his practice of withholding mass transit bills from what they believed was a fair hearing in his committee. But Mr. Collier contended that rapid transit systems would never replace Californians' fascination for the automobile, and once referred to a system of buses and railroads to carry people in and out of cities as ''rabbit transit.''
In the end, it was reapportionment that toppled Mr. Collier. He had to campaign in 1976 in a reapportioned district of 15 counties, 13 of which he had never represented before, and lost to the Republican nominee.
Mr. Collier was born in Etna, Calif., the grandson of a former Governor of Alabama. His mother, a suffragette, was born in Tahiti. His father, who had been Attorney General of Alabama, was 60 years old when the younger Mr. Collier was born.
He was himself a father at the age of 73 and again at 76, when his fourth and fifth children, both daughters, were born to his second wife, the former Barbara Hamoui. Mr. Collier's first marriage, to the former Ada Pillsbury, ended in divorce in 1971. Their three children also survive him.