GOV. HARRY HUGHES was on the right track the other night when he said that bolstering Maryland's transportation trust fund should be among the "top priorities" in the next legislative session. Maryland's highway program, like those of most other states, is being badly squeezed. Road construction and maintenance costs are rising quickly. Trust-fund revenues are not. The gasoline tax, which generates about 30 percent of Maryland's money for transportation, is a fixed amount per gallon (now 9 cents). It does not rise with general inflation or gasoline price increases. Moreover, supply problems and the trend toward more fuel-efficient cars have caused gasoline consumption and tax revenues to level off or drop. Maryland's fuel tax brought in only $48 million this summer, compared with $52 million in summer 1978.

This is, of course, the painful side of the trust-fund, dedicated-tax approach. Relying on user taxes to support roads is easy as long as use is growing more than needs and costs. When the equation changes, however, highway and related programs get pinched even though a state's general fiscal situation may be fine. That is happening all over the country. Although fewer new highways are being built, most states are looking at large agendas for repairing and rebuilding bridges and roads -- and wondering where the money can be found.

Paradoxically, the problems are worse for states that have been leaders in the transportation field. Maryland, for one, has responded to its urban areas' needs commendably by setting up a unified transportation fund and making generous commitments to mass transit -- including Metro, the Baltimore subway and commuter trains. But transit costs are also soaring; by one estimate, the state's share of mass transit operating deficits is expected to rise from $215 million now to $355 million by 1985. Inclusion in the trust fund, which was a victory for transit so recently, has become a harsh constraint.

To loosen the squeeze, Gov. Hughes and a legislative committee are looking at ways to make the gasoline tax responsive to rising costs. One possibility -- a tax that goes up automatically as fuel prices rise -- has been adopted in Washington state and New Mexico. Other states are considering switching to a percentage (sales-type) tax. That makes good sense. Yet even a stiff increase in Maryland's user taxes will not provide enough for roads and trains and Metro and Baltimore's and all. That's why Gov. Hughes also emphasized the need to "devise a mechanism for funding" the two subways' operating deficits. Special taxing districts are one possibility. Many points remain to be wrestled with. Still it's encouraging that one of the region's governors has such a sound grasp of transportation challenges -- and such interest in responding to them.