Highway Trust Fund Improvement Act of 1991

Highway Trust Fund Improvement Act of 1991 - Directs that the receipts and disbursements of the Highway Trust Fund (the Fund) allocable to the transportation-related operations of such Fund not be counted as new budget authority, outlays, receipts, deficit, or surplus for purposes of: (1) the budget of the Government as submitted by the President; (2) the congressional budget; or (3) the Balanced Budget and Emergency Deficit Control Act of 1985 (Gramm-Rudman-Hollings Act). Defines "transportation-related operations" for such purposes as the disbursements, and the receipts allocable to such disbursements, under provisions of the Internal Revenue Code relating to expenditures from the Fund for the Federal-aid highway program and from the Mass Transit Account.

Amends the Congressional Budget Act of 1974 to exclude from the congressional budget the outlays and revenue totals of the Fund allocable to transportation-related operations and the surplus or deficit totals required under such Act.

Amends the Balanced Budget and Emergency Deficit Control Act of 1985 to exempt the Fund from reduction under a sequestration order.

Amends the Congressional Budget and Impoundment Control Act of 1974 to bar congressional consideration of specified changes with respect to transportation-related operations of the Fund.

Directs the Secretary of Transportation to make supplemental allocations among the States of amounts sufficient to ensure that each State receive an amount equal to estimated tax payments attributable to highway users in the State paid into the Fund in the latest fiscal year for which data are available as a result of certain increases in motor fuel taxes. Makes such allocations available for purposes including interstate highway and transit substitute, bridge replacement and rehabilitation, hazard elimination, and rail-highway crossings and construction projects eligible for Federal assistance under the Urban Mass Transportation Act of 1964. Specifies the Federal share (100 percent) of project costs. Authorizes States to use specified amounts allocated to them for transportation planning and research.

Authorizes appropriations.