America's 1,500 intercity bus lines carried an estimated 375 million passengers to more than 14,000 cities, towns and country crossroads in 1981--more, says an American Bus Association spokesman, than airlines and the railroad combined. (Amtrak has about 525 destinations and carried 20 million passengers in 1981; the combined domestic airlines have 628 terminals and carried 277 million). Although about 55 percent of the bus passengers were aboard charter trips, the figures point out that scheduled bus travel is still relied on heavily by many Americans.

The chief market of today's bus line is the trip of 200 to 500 miles that can be made in a day. Greyhound is the biggest line; Trailways, with a fleet of about 1,700 coaches, is about one-third as large.

On my 35-hour ride from Washington to Dallas and an eight-hour extension a day later to San Antonio, I found really nothing to complain about. When I boarded here, the bus was clean, and though we passengers managed to scatter plenty of debris, it was swept out thoroughly at least three times. You are not going to dine well on a bus, but the food was no worse than what you get at a national fast-food franchises.

Only once was there the briefest of disputes between passengers, who overwhelmingly were quiet and polite. on one leg of the trip, when the bus was almost full, a woman who was boarding apparently was denied a place by a young man who wanted to spread out across a double seat. She simply marched up to the front of the bus and announced loudly to the driver (and the rest of us): "I don't have time to put up with this foolishness. Please tell that young man to let me be seated." The man shrunk immediately into a single seat, and that was that.

Radios, except those with earphones, are not permitted; alcohol is prohibited, and cigarette smoking (no cigars or pipes) is limited to the back rows. Each new driver reaffirmed these posted rules and they seemed to be obeyed, though once I spotted a beer can rolling forward.

You are better off if you have a double seat to yourself, especially overnight, because of the extra space to stretch out in. My bus was uncrowded for the first 24 hours to Memphis, but then filled up and stayed that way for most of the day into Dallas and the next day to San Antonio. Two in a seat is tight, but less claustrophobic than the window seat in a six-across 727.

A few suggestions:

* Take a map to trace your route. You'll be able to find out what that big river was you just crossed. Pick up a schedule, too. Surprisingly, many people didn't and had no idea how close to their destination they were.

* A small pillow is helpful (they're not furnished), and so is a pocketful of change for the candy machines. Potato chips become irresistible when everyone around you is eating them.

* If you are a nonsmoker, pick a seat as close to the front as possible to escape the fumes. It also offers a wider view, front and side instead of side only. Aisle-right is best so the driver isn't blocking the window.

* Be sure you find out where and when you transfer when you buy your ticket. To her chagrin, one elderly woman learned she would have to spend a couple of late-night hours waiting for a connection in a busy Burger King because the regular terminal closed at 6 p.m.

* One night on a bus is not so bad; a second night without a break could ruin your disposition.

On both Greyhound and Trailways, a $99 one-way fare, good for 15 days, will take you coast-to-coast, the most you will have to pay to any destination in the country (children 5 to 11, one-half fare; under 5, no charge but must occupy the parent's seat if the bus is filled). Through Aug. 31, the maximum roundtrip fare is $150, good for 30 days.

For vacationers, both bus lines offer unlimited mileage passes (to be used as you wish) over their routes. The rates are as follows: 7 days, $186.55; 15 days, $239.85; 30 days, $346.45. The 30-day pass can be extended at the rate of $10 a day for an additional 30 days.

Neither Greyhound nor Trailways takes reservations, but both guarantee a seat (though it doesn't always work that way). At one point in Texas, my Greyhound filled and a couple of passengers stood for about a half hour until a second bus was added to the run for them. The next day in Waco, the bus to San Antonio filled fast, but a second bus was immediately added to take the overflow.

One last tip: Take a good book or magazine just in case things get slow, but if you are like me, you will be so interested in what's going on you won't have time to read a page.