Ask Lionel Bernstein, M.D., what kind of a doctor he is, and the immediate reply will be: "Mediocre."

Ask why he is obsessed with easing Washington's commuting woes, and he will reaply: "I'm just a public-spirited American."

But each of these Bernsteinisms is as false as the other.

Not only is Bernstein director of the Lister Hill National Center for Biomedical Communications at the National Institutes of Health (mediocrities need not apply), but he is coauthor of a clever rush-hour idea that just might work.

It's called the semi-taxi.

Taxi because it would ferry you from point A to point B just like any other cab.

Semi because the conveying vehicle would be a private car at all other times beside rush hours. Nor would it look like a cab -- no dome light, no incomprehensible zone map, no name painted on the side.

The driver would be a working shloonk just like you who:

Had been licensed by a board.

Would be allowed to charge a fare somewhere between what a bus and a cab cost.

Had agreed to drive from A to B along main thoroughfare C at time-of-day D -- which he probably always did anyway, but by himself.

Let's consider an 8 a.m. weekday journey from Chevy Chase Circle to Farragut Square as an example.

Mr. Shloonk fires up his car and begins to drive down Connecticut Avenue. In his front window sits a sign that reads: "Connecticut-4-Farragut."

The words mean that the Shloonk-mobile's route will be along Connecticut to a final destination of Farragut.

The number "4" means that this semi-taxi will stop to pick up passengers only in the fourth mile of Connecticut north of the White House. A "3" would mean third mile only, a "2" second mile only, and so on.

At evening rush hour, the process simply reverses.

Bernstein is well aware that his plan is really car pooling in sheep's clothing. So he has devised features to "attack" the ways in which car pools consistently fail.

For one thing, Shloonkmobiles would run along main routes throughout both rush hours. Goodbye to the car pooler's complaint of having to adhere to someone else's schedule.

For another thing, atmosphere inside the Shloonkmobile itself would be a vast improvement over a normal bus or typical cab. People might even find themselves talking to one another if they're not careful.

For a third thing, significant amounts of fuel would be saved. So would significant amounts of money -- not only because semi-taxi fares would be relatively small, but because a large semi-taxi fleet would make it unnecessary for the city to buy more buses.

But Lionel Bernstein would'nt be a card-carrying bureaucrat if he hadn't anticipated delays, opponents and hassles.

What would the cab companies say to the Shloonkmobile idea?

Nothing too polite, certainly. So Bernstein proposes to let cab companies run the system.

What about liability insurance?

Good question, Bernstein says. Perhaps group policies for groups of semi-taxi drivers could be worked out.

As for administration, Bernstein concdes that an office would have to be established where routes and licenses could be issued and where disputes could be settled. That could snarl matters in bureaucratic bubble gum; suggestions as to streamlining are welcome.

But as for time, Bernstein argues that more of it is now wasted in waiting for buses than would ever be wasted under a well-managed semi-taxi system.

Finally, if you have guessed that Bernstein conceived his system out of self-interest, go back two spaces and forfeit your turn.

He lives in Chevy Chase -- which means that, to get to NIH in Bethesda at rush hour, he commutes against the flow. No semi-taxi is necessary.

Dr. B's idea seems, if not exactly necessary, at least intriguing. Onward, Shloonkmobile Soldiers.

* * * * *

Like Me, Gene Miller, of Washington, gets nearly homicidal whenever he hears that Lite Beer has "less" (not "fewer") calories than other beers.

Unlike me, he has now moved on to antacids.

No, no, he isn't taking them to soothe the pain of Lite commercials, although maybe we both should. Miller simply heard an ad the other day in which some tabletmaker claimed: "All antacids are not alike."

That's backwards, Miller says. Should have been: "Not all antacids are alike."

But the error touched off a question. "Up with this how long shall we have to put?" Miller wants to know.

I say Gene should count his blessings. At least some aging slugger and some blind umpire aren't guffawing over Lite Antacid Tablets at a bar somewhere.