GAO discussed the Highway Trust Fund, focusing on unexpended balances and trust fund revenues that could be made available to meet highway needs. GAO noted that: (1) the Highway Trust Fund's highway account balance was approximately $10.6 billion; (2) Congress placed limitations on states' use of funds and states had not used an estimated $1 billion in available trust fund revenues; (3) state officials believed that they did not use funds primarily because the funds provided a cushion against an uneven flow in federal funds; (4) projections indicated that the final trust fund balance would exceed outstanding commitments by $6.4 billion; (5) to prevent future trust fund buildups, Congress should limit trust fund revenues to more closely approximate expected obligations; (6) since 1987, the advance construction balance has ranged between $2 billion and $2.6 billion; (7) the amount of funds not subject to obligation ceilings ranged from approximately $700 million to $2.2 billion, and minimum allocation funds and demonstration projects accounted for 98 percent of the exempt funds in 1990; (8) from 1982 through 1988, highway system bridge deficiencies increased from about 70,000 to 77,000, primarily due to the rapid growth in deficient interstate bridges; (9) through fiscal year 1991, the mass transit account (MTA) is expected to receive income of about $13.5 billion; and (10) Congress did not authorize the Urban Mass Transit Administration to obligate about $3.8 billion because of appropriation process limitations.