Nearly half of the nation's interstate highways are in "barely tolerable or worse condition," the General Accounting Office told Congress Tuesday.

The GAO, a congressional watchdog agency, said some states have not performed needed road maintenance, which has led to slow deterioration of some interstate highways.The agency reviewed interstate highway maintenance and repair programs at the request of the House Public Works and Transportation Committee, which heard testimony Tuesday on future highway needs.

"Due to funding difficulties, some states have not performed needed maintenance to slow pavement deterioration and reduce (repair costs)," the agency said.

GAO based its assessment on a review of Federal Highway Administration data and on its own examination of highway maintenance programs in California, Louisiana and Michigan.

The 42,500-mile interstate highway system is nearing completion, and lawmakers are debating how future highway funds should be spent. Congress also has begun consideration of a new five-year highway funding bill, which will set priorities beginning with the 1991 fiscal year.

GAO said the condition of the interstate highways has been deteriorating for years, partly because of deferred maintenance.

The percentage of highways classified as "good" - those providing a smooth ride - has shrunk to 57 percent from 61 percent in 1981, the agency said. Moreover, interstate highways rated only "fair" increased to 31 percent from 25 percent. "Poor" highways - those providing an uncomfortable ride - stayed even throughout the 1980s, at roughly 12 percent.

"The bottom line is that the ride on 43 percent of the nation's premier highway system may be barely tolerable or worse," the agency said.

GAO said highways are also deteriorating because funds earmarked for repairs are being used instead to widen roads.

The Interstate Resurfacing, Restoration, Rehabilitation and Reconstruction Act of 1976, or 4R program, originally provided the states with federal money for major roadway repairs. Routine maintenance is funded by the states.

In 1981, however, the 4R program was modified, allowing the states to use federal funds add new highway lanes or to widen existing lanes. Less money, therefore, was available to repair damaged road surfaces.

About 13 percent of 4R funds were spent on lane widening last year and as much as 50 percent may be spent on similar programs in the future.

"If about 50 percent of of 4R funds are spent for major interstate widening, this spending would represent a significant departure from historical spending trends," the GAO said.