If The Post's editorialists had been as thoughtful in mathematics as in English on that day, the faint praise they recently granted the FAA for opening the Dulles access road to car pools would have faded away entirely. By insisting on a four-person pool, that agency has turned on neat bureaucratic trick. They have received credit for sensible, if belated, action while ensuring that nothing much will happen.

Consider, for example, a simple world: 24 typical commuters each driving to work alone, as most of us do. If the objective is to get the most cars off the road, nothing works better than the two-person pool. Perfect pairing would cut traffic in half. But tripling-up only eliminates four mre cars, while the FAA's ideal of four people per auto buys just two less vehicles on the road than three per car.

But perhaps the FAA's big objective isn't to get cars off the road. The commuter's certainly in't: He wants to save money. And just as the number of cars removed from the road diminishes by one minus the reciprocal of the total number of pool members (two reduces one-half, three two-thirds, etc.), so does the financial incentive. One good buddy can cut your daily $3 gas bill in half; the fourth fellow contributes a quarter to offset the time and gas it takes to collect and deposit him each day.

He may not be worth it. The mechanics of getting four people from home to office and back again in one vehicle are, like most things, more complicated than they appear. Consider the variables.

Starting point: People going to the same office complex seldom start from the same neighborhood.

Destination: Washington may be a one-in-industry town, but we and our neighbors don't all go to the same plant.

Regularity: People, including reporters and FAA employees, take business trips and vacations, get sick or laid off, oversleep, run errands, keep appointments, work overtime, change jobs, etc.

Temperament: You can ignore seatmate on Metro, but travelling fanny-to-fanny in a Fiat two hours a day, five days a week, constitutes an intimate relationship. It isn't always enough just to be friends. Consider variations in musical taste, smoking habits, response to heat and cold and politics in a political town.

Now contemplate the interplay of these variables on the task of forming a car pool and keeping it going. Assume for simplicity that each variable is equally important. Chances are 32:1 that any two people chosen at random will be an ideal match -- the reason, perhaps, why so many ride alone. Raise the entrance bar to a four-person height, however, and the odds are 1024:1 -- in the FAA's favor. Red tape doesn't have to be complicated to be effective.

Car pools aren't formed by people selected at random, of course. But count cars and heads for an hour some day and see if there aren't roughly four times as many cars with three people in them than there are cars with four passengers.

Press accounts of the FAA's plan do not specify how it would work, so my assumption that four people in a car could use the Dulles access road, but three could not, may be incorrect and unfair.If that's true, however, things will get worse as cars get smaller. It takes at least five members to yeild four for the road consistently, and already one car in five only seats four. When airlines overbook, the FAA (Or is it the CAB? Who can remember?) cries naughty.

How can FAA lend just enough and not too much encouragement to car pools? I suggest a nontransferable permit, financed by a nominal charge to each car pool with three or more members, that, when displayed on the dashboard, gains entrance to the road for cars carrying two or more people. Then some day, if those of us who only use Dulles access to access Dulles really need it, we could build a third land going to the airport. Folks coming from the airport are in no more of a rush than the rest of us.