Cab drivers know it, cab passengers know it, and now Thomas Moffett knows it. If you're black, you're going to have a much tougher time hailing a cab in Washington than you would if you were white.

Thomas witnessed yet another replay of this long-running drama on 18th Street in Adams-Morgan. He and a friend, Gordon Vaeth, had just left a restaurant when they noticed a black woman trying to hail a cab. She wasn't having any luck.

So Thomas and Gordon, both of whom are white, offered to help her with her hailing. Sure enough, the next empty cab stopped. But when the driver noticed the black woman getting into the back seat, and not either of the two white men, he suddenly asked the destination.

No sooner did Thomas reply, "20th and D NE" -- a recognizably black neighborhood -- than the cabbie zoomed off, muttering that he needed gas. If Thomas hadn't let go of the door handle, his arm might have been yanked out of its socket.

The next cab agreed to take the woman, and she was delayed by only a few minutes. However, Thomas describes himself as being furious over the incident, and it's not hard to understand why.

I've often asked D.C. cabbies of both races why so many of them duck blacks. The answer is always the same: Blacks don't tip; blacks live in dangerous neighborhoods, and blacks often live so far from downtown that the driver will have to "dead-head" back to "civilization" without a fare.

If there's anything encouraging in this attitude, it's the fact that no driver has ever cast it in racial terms. It's not that we duck blacks because they're black, drivers say. It's a matter of simple economics. If whites tip, and whites want to go to neighborhoods where another fare is likely to be found, any fool would choose a white passenger over a black, the drivers always say.

How to fight back? The solution is the one Thomas chose. First, he jotted down the number of the cab that ducked the black woman. Second, he filed a complaint with the D.C. Cab Commission, 300 Indiana Ave. NW., Room 2077, Washington, D.C., 20001.

Certainly D.C. cabbies have the right to make a living, and to be free from the fear of holdups. But they are violating the law when they bypass blacks, and they are poisoning the atmosphere of a city that already has enough social pollutants flying about. Brother Moffett was right to complain. So should every white who sees a black ducked, and every black who's ducked.

While we're dishing out raspberries, here's a barrelful for Dart Drug.

On March 27, there was a serious house fire on Buckman Road in Fairfax County. Several very young children were left homeless.

Alan Vandenberg of Alexandria was one of the Red Cross volunteers called to the scene. He realized that, in all the confusion, there'd be a shortage of diapers. So Alan headed for a nearby open-all-night Dart Drug to buy some. He planned to pay with Red Cross emergency vouchers, as Red Cross volunteers always do.

But the store wouldn't honor the vouchers. And Mary Heister, Dart's administrative assistant for operations, says no Dart outlet does, has or will.

"Our accounting department is not equipped to accept them," she said.

Well, last time I checked, there were accounting departments at Zaire's, Bradlee's, Giant Food, Magruder's, Holiday Inns and Days Inns, to name just a few major local businesses. And none of them have any trouble processing Red Cross vouchers.

According to Rose LaRocca, director of emergency services for the Fairfax County chapter of Red Cross, an emergency voucher is the same as a check. It is signed over to the store where emergency supplies or services are bought, and processed through a bank, just like a check.

"Usually, the vouchers are processed within a week," Rose says. And since there are relatively few emergencies around here, Dart accountants would not exactly be swamped with vouchers.

Time to rethink the no-voucher policy, Darters? I'd say: High time. Providence Hospital celebrates its 125th birthday this year, and it plans to salute itself in imaginative fashion: by throwing a party in honor of every baby ever born there.

That figure is about 40,000, says Providence spokeswoman Jennifer Cromartie. But many are dead, and others wouldn't want to spend May 17 on the Mall eating birthday cake with strangers. Providence figured 2,500 people would show up.

Surprise! The Varnum Street NE institution has already received 7,000 responses. They include yesses from Robin Weir, the First Lady's hairdresser, and Desiree Keating, the current Miss D.C. Jennifer promises cake for all, despite the huge turnout. Call 269-7296 if you started at Providence, and would like to celebrate that fact.