IT WAS well disguised in President Bush's domestic checklist for the State of the Union address, but the administration is proposing a significant reorganization of federal financing for highways and mass transit in the years ahead. Mr. Bush proposes to keep the government in the highway building business after the completion of the Interstate system, but to pay a lower share of the costs. There are bound to be differences in Congress over formulas for federal-state financing, for allocations to the various states and for the total amount of federal money that should be committed to the policy. But the president's new Surface Transportation Act would at least mesh mass transit and highway financing in a better fashion by providing new ways for shifting funds from roads to transit or the other away around.

The proposal, which will be outlined in the fiscal 1992 budget today, would boost federal highway and mass-transit financing to nearly $90 billion over the next five years. States, however, would have to come up with bigger shares of their own money. A large portion of highway funds could be used for transit, and all transit money could be shifted to highways. Congressional Democrats already are talking about a push for a larger bill altogether or at least for more mass transit money. Other members in both parties may seek changes in proposed allocation formulas to protect the interests of states with lots of land but not lots of people.

The administration proposal would establish a 150,000-mile system of "highways of national significance" that would include the Interstate highway system. The few remaining segments of the Interstate system would, like the rest of the system built over the last three decades, be done with 90 percent federal funds. The rest of those "national significance" highways would for the most part be financed through a 75-to-25 federal-to-state matching ratio for improvements or construction. Yet another category -- an "urban-rural" system -- would be designated for 60 percent federal financing, the same formula applying to mass transit construction funding. This way, any switching of road money for transit money wouldn't result in federal dollars lost because of differing federal-state formulas.

Given the shortages of state money these days, the pressures for greater continuing federal participation are likely to be intense. But the administration has done some sensible thinking about its approach to a transportation policy -- and about the necessity of setting highway and transit priorities as America's great age of the Interstate highway system winds down.