HAVE YOU EVER wondered why mass-transit projects, like Metro, always seem to have financial problems and interstate-highway projects don't Part of the answer lies in a set of numbers. If a state puts up, say, $1 million to build a superhighway, Uncle Sam responds by providing $9 million. But if the state wants to spend that same $1 million on mass transit, Uncle Sam will put up only $4 million. Logical? We suppose it was, a quarter of a century ago when gasoline was cheap and plentiful, when air was clean and the nation was in love with the automobile. But it does not seem so now. Most of the unbuilt segments of the interstate-highway system are in urban areas, which need mass transit more than they need highways. Yet the federal government still tilts heavily in favor of the big roads.

To remedy this imbalance, the Carter administration has proposed that all federal financing of surface-transportation projects be at the same level. It wants Washington to contribute $4 for every $1 raised locally. Interstate highways, secondary roads, new bridges and mass transit would all be treated alike. Thus, the availability of federal matching funds in varying amounts would not be a factor when a community decides how to spend its money.

Given the power that the highway lobby has demonstrated in the past to beat back efforts to curtail the interstate system, we are not optimistic about the chances of this proposal in Congress. But it does seem to us that the time for some such major change has come; substantial inroads have been made in recent years on the inviolability of the highway trust fund. And this proposal has a sweetner in it for those states in which the interstate systems are complete or almost complete. It would increase the federal share of secondary-road improvements and bridge-replacement projects. That could provide the incentive for some, mostly rural, states to join with urban areas in supporting the change.

There is a touch of irony in this proposal, however, when it is compared with the administation's current coolness toward completion of the nation's biggest new mass-transit project - Metro.If this proposal had been government policy nine years ago, Mertro would not be facing a major financing crisis now. The first $574 million raised by local government for Metro was matched on $2-for-$1 basis. If the $4-for-$1 ratio now recommended had been in the place then, Metro would have enough money in sight to complete the full system. Indeed, if the federal government had matched local spending for mass transit the way it has been matching spending for interstate highways, Metro would be giving money back. Our point is simply that while the administration is right in trying to eliminate the pro-highway tilt of the federal government, it ought to recognize the penalties that have already been paid by those areas, like this one, that eliminated the tilt in the past.