IN ITS FIRST warnings of bare cupboards ahead for the halls of governments around this region, the special task force headed by Robert S. McNamara has much to say about Greater Washington's most politically delicate, administratively complex and frighteningly expensive "cost item": the Metro system, bus and rail. The question, not at all now but still generating both deep thought and strong reaction, is: how high can fares go before riders go -- to some other form of transportation?

If the governments are serious about addressing the general red-ink alert sounded by this task force -- namely, a projection that regional spending could outstrip revenues by $605 million in another four fiscal years -- they are going to have to scrutinize Metro's fare policies, to squeeze from the riders what the traffic will bear. On this, the task force is clear, citing the grim dimensions of the tab to keep buses and trains running in the future: under current policies, the figure is projected to rise from $113 million in the fiscal year just ended to $287 million in the fiscal year that will be under way four years from now.

Higher fares could reduce this subsidy by $100 million, the task force notes, causing -- and herein lies the controversy -- a projected drop of 8 to 12 percent of Metro's riders. This rate of fare increases would be between 40 to 60 percent faster than Metro's current plan, which calls for fare increases roughly following the rate of inflation.

What about poor people? Subsidize them, the study suggests, with special cut-rate tickets, the way Arlington County is doing, by buying Metro Flash-passes and reselling them at a discount to qualifying families. Mr. McNamara has noted that "we are sensitive to the suggestion that poor people cannot afford to pay higher fares. This argument has been used to hold down all fares, even though the overwhelming majority of area riders -- particularly on the rail system -- could and would pay higher costs."

But what about middle-income people? It is here that the task force suggestions may need tempering.Cost is one thing, cost and convenience another; raise cost and lower or fail to improve convenience, and today's Metro customer becomes tomorrow's car traveler.

Just as this might prove damaging to Metro, so might another task force suggestion -- that local governments pay for rail construction based on what would be built within their borders in the next few years, instead of based on what each jurisdiction is supposed to get under a full 101-mile system. Even if the current formula could be easily scrapped -- unlikely -- this would invite pullouts, revisions, bobtailed routes and heaven knows how many impact statements, delays and cost overruns as each jurisdiction decided whether to up the ante, stay in or fold.

What the region's governments will have to do -- and what the task force has served to point out well -- is come up with a fair balance between what can be reasonably extracted as users' fees and what should be underwritten by the public at large as a matter of equitable transportation policy.