Assuming you are the average Washington worker, you either use public transit or you drive to work, spending about 90 minutes a day going back and forth.

And if you're a driver, you are spending a lot of time sitting in your car cursing about traffic along Constitution Avenue or crawling bumper-to-bumper on the John Hanson Highway. Roughly speaking, that's 15 days a year wasted, unless you consider just staying alive on the Beltway an accomplishment.

Fifteen work days equals a three-week vacation, and who wants to blow that much time dodging Metrobuses?

Increasing numbers of Washington commuters are finding ways to take advantage of those hours, turning what previously was a boring, routine struggle into productivity. Thanks in large measure to the audio cassette -- and man's basic creativity -- commuters are mastering new skills, solving office or personal problems, or catching up on a good book.

Commuting time isn't just for breakfast.

Paul Frank uses those rare moments alone to plan office staff-meeting agendas or to dictate memos into his cassette recorder. "I jot down a few words on paper, too, something that will remind me later when I get to the office. A lot of businessmen do that."

But Frank, who heads Paul A. Frank Associates, a personnel firm, is also the Rachmaninov of Rockville, composing music, writing lyrics and scoring his own compositions as he drives along. He is also musical director of "Deaf Dimension," a professional group of some 20 area performers, most of whom are deaf.

"The deaf performer must have the music on tape because it must be exactly the same every time it is performed," says Frank. "There can't be the slightest variation because he or she cannot hear the music and adjust to it. So I work very hard coordinating the arrangements and I find I get lots of ideas while I'm driving."

He says he sometimes takes visual phrases that include a sound, such as "sand sifting through the fingers," or "the flap of soaring eagles' wings" and tries to write a song around them.

"In effect, I -- a hearing person -- am asking what is it like not to hear? Being alone in the car helps me work out the problem."

Another commuter to profit from the empty miles is Caryl Weiss, a former teacher of interior design at Mt. Vernon College. A natural left-hander, she taught herself to write with her right so she could drive and take notes.

"I kept a book open on the seat next to me and jotted down what I needed. I also read a lot when I was stuck in traffic. I happen to be a good speed reader, but it's fairly easy to do when you're familiar with the material." And the road.

For many of us, however, the morning commute has never been so productive. We don't have the ability to scan Bellefleur and the Beltway at the same time; nor can we compose a concerto on Connecticut Avenue.

The audio cassette is changing all that, turning our Chevys and Fords into instant learning centers on wheels and at very moderate costs.

Almost without warning, a new "magazine" industry has sprung up with the cassette replacing the glossy paper of news magazines. In the main, this new medium is geared toward the busy executive who doesn't have time to pore over Time, Forbes, Fortune or U.S. News & World Report, but who needs current business information.

Larry Butler, radio manager for the U.S. Chamber of Commerce and publisher of the Washington Audio Journal, says an economic cassette "magazine" is subscribed to by hundreds of businessmen around the country. The bimonthly cassette is usually divided into four sections: a "lead" story on a current issue in the news, an economic update, legislative issues and a special report or interview.

Butler says the average subscriber, who pays $150 a year for the cassettes, is the corporate middle manager living outside the D.C. area.

"The cassettes are a fast-paced 30 to 35 minutes with lots of different voices. We have to create word pictures and we use outstanding voices regularly."

Butler admits that the audio-cassette magazine did not sweep the nation overnight like Rubik's cube. "It was difficult to sell the idea, but where it's caught on -- in the business community -- it's doing well."

The Harris "Sound of Business," a Chicago-based monthly cassette magazine featuring three leading economists discussing current financial issues, has over 1,300 subscribers around the world. Several of the country's leading banks, such as Manufacturers Hanover, produce their own audio "magazines." Citibank's "Sound of the Economy" is narrated by former TV news anchorman -- and watch seller -- John Cameron Swayze.

The Newstrack Executive Tape Service claims 3,500 subscribers to its business-oriented cassettes. The series, says publisher Rudy Savage, is aimed at top-level business executives. The bimonthly tapes, $195 a year, contain readings from a wide variety of publications. A recent issue included "Newport's Bad News Bear" from Forbes, "How to Avoid the Pitfalls of Computer Contracts" from Inc. and "Reviving the Middle Class" from The Christian Science Monitor. Four times a year the 90-minute cassette magazine includes a special tape on an outstanding business or personality.

But if business is not your thing, other cassette manufacturers are around to help fill your empty minutes and make you a better person. Caedmon claims to have the largest catalog of tapes in the country, everything from Richard Burton reading the love poems of John Donne to Gertrude Stein reading her own works. And imagine the impression you can make if you have carpoolers and you flip on your four-cassette set of Beowulf (in the original old English, of course).

During a particularly slow drive home you might enjoy hearing Eugene O'Neill's epic, Long Day's Journey Into Night.

Stuck on Rte. 395? Try Sartre's No Exit.

The most popular cassettes in the catalogs, says Robert Knox of Caedmon, are poets and dramatists reading their own works. James Michener was just in to do his Tales of the South Pacific and every living poet within the past 30 years, he says, is represented on the Caedmon tapes. The catalogs are free.

Jeffrey Norton Publishers is another leading manufacturer of cassettes, marketed under the Audio-Forum label. Some very high-brow tapes are available, such as "A Study of Persian Literature Over the Past One Hundred Years," "Shakespeare's Tragic Structure in King Lear" and the noted science-fiction author Robert Heinlein delivering the Forrestal Lecture at the U.S. Naval Academy.

One cassette particularly appropriate for the harried commuter is Audio-Forum's "On the Psychology and Esthetic of High-Speed Race Driving" narrated by Stirling Moss.

Then there are the self-help cassettes, useful for fighting depression and aggression in the car. You might like "The Ins and Outs of Therapy: A Consumer's Guide" (to go with the Ins and Outs of Rock Creek Park?), "How to Handle Depression" (on the Whitehurst Freeway?) and "Fathering" (a practice frowned upon by the Auto Safety Council while behind the wheel).

If you happen to be shuttling a teen-ager to school on your way to work, Jeffrey Norton has just put out a two-cassette set on how to prepare for the Scholastic Aptitude Test. The tapes and an accompanying book How to Take the SAT sell for $21.95, a potentially worthwhile investment if 1) it helps get the kid into college and 2) provides a scholarly substitute for his favorite all-rock music station.

The most popular line of cassette tapes in the Jeffrey Norton stable is the foreign-language series, the idea being that learning Old Church Slavonic or Middle High German should be a snap if you can just sit there and force yorself to take it all in. But Listening Library general manager Tony Ditlow warns that it isn't so easy to parle francais or habla espanol.

"It requires a great personal effort to actually learn from an audio cassette. And most people -- particularly if they're in their cars -- just cannot concentrate with the intensity it takes to master a new skill such as a foreign language."

The Listening Library has been turning out tape recordings for a quarter century already, but Ditlow says the audio-cassette industry is still in its infancy with a good future ahead, thanks in part to the new, very portable cassette players, such as the Sony Walkman ($90 to $150) and to the growing availability of audio cassettes from the public library.

Ditlow predicts that within the next several years four-track cassettes which play at half the present speed of 1 7/8 IPS will allow a 300-page novel to fit on one cassette. That translates into a four-hour reading.

That also may mean that one day in the not-too-distant future when a weary D.C. worker finally wends his way home and his wife asks, "Where have you been? It's 10 o'clock!" he can answer with some justifiable pride, "I've been in the world's worst traffic jam, three hours just to cross Key Bridge.

"But I got through all of War and Peace in one sitting."