People keep offering Judith Heumann a chauffeur-driven Cadillac. They say it would be so much cheaper than giving her what she wants.

She wants to ride the bus.

Section 504 of the Rehabilitation Act of 1973 gives Heumann and other disabled Americans the right to unhindered access to public buildings and transportation. The act was reaffirmed in 1976. Department of Transportation regulations requiring changes in buses, subways and trains necessary to implement the law were finally issued in June, 1978.

Challenged by the American Public Transit Association, these regulations were upheld some days ago by Federal District Court Judge Louis F. Oberdorfer.

They required changes -- installation of hydraulic lifts, wider doors, elevators and other devices in many existing and all new transportation facilities over the next 30 years -- are extraordinarily expensive. They may cost as much as $7 billion. Perhaps the offer of the Cadillac is not entirely facetious.

"But it misses the point," says Judith Heumann, a feisty activist, moving her electric wheelchair away from the table for emphasis. "The point is not private transportation, or 'separate but equal' transportation, but moving with the mainstream.

"There are 47 million disabled individuals in the United States today, according to government statistics, which I believe to be low -- 47 million Cadillacs?

"Besides, regulations are not appropriations. Changing bus doors does not change attitudes," she argues.

"I was kept out of elementary school because I was considered a fire hazard," she said matter-of-factly.

"Yes, I am bitter about being handicapped," she says. "But I am not bitter about having a disability. It's not my disability that handicaps me. It is society that handicaps me and my disabled brothers and sisters by building inaccessible schools, theaters, buses, houses and on and on and on.

"Many public buildings are still built without wheelchair ramps, for example, despite state and federal laws. All this helps keep us in our place," she explains.

"Having had polio in early childhood and using a wheelchair are not causing undue hardship. The hardship is that people equate my disability with being inferior, sick, deviant and making my life a tragedy. There is this continuing negative bombardment on my self-image."

Heumann, the daughter of a Brooklyn butcher, is, in her own rapid-fire words, "a very outspoken and very verble person." Her intensity is leavened by warm charm and Brooklyn humor.

Her parents tried to integrate her as much as possible despited the wheelchair, which she never managed to move to well, until she got an electric one at age 21. She was in the Brownies and went to religious school and passed all the high-school and college examinations and humiliations. Having won a civil-rights suit against the school board, she became the first disabled teacher in New York City to teach nondisabled children.

Four years ago, at 28, she moved to Berkeley, Calif., to join the Center for Independent Living, where she is now deputy director, developing rehabilitation and training programs and working on legislative concerns. She was just named one of "80 women to watch in the '80s" by Ms. magazine.

Nobody lives at the center. It does not believe in "warehousing people," as Heumann puts it, but in helping people with varying disabilities, such as polio, muscular dystrophy, deafness, blindness or infirmities of old age to become less dependant. "We don't want to protect them from the world but to work for a world they can live in."

Nor is Heumann's rejection of facetiously offered Cadillacs or seriously offered dial-a-bus or taxi services only a matter of pride and Weltanschauung.

"So-called para-transit just does not work," she says.

Cities that have special transportation services for elderly or disabled persons usually make extravagant claims about them, mostly because they are loath to make their regular system accessible. The elderly or disabled do not dare contradict their benefactors for fear of losing the service.

Federal statistics are based on local claims, overestimated costs of complying with the new regulations and underestimated costs of ridership, at least in the view of technical consultants working for the disabled.

It is, in the first place, difficult to calculate whether adapting buses to the use of the disabled is cheaper or more expensive than starting separate minibus services.

Calculations depend largely on the number of people using either service. But nobody knows. The disabled who rarely get out of the house are reluctant at first to use a transit system. They must me trained. "I was terrified when I got my electric wheelchair. I did not know where to look when crossing a street, or even where to put my hands. It takes time to advance from dependency to relative independence," sayd Heumann.

Transportation services, furthermore, are labor-intensive. The primary cost are wages and benefits for drivers and mechanics. Operating a big bus or a small van therefore costs virtually the same. The big, fixed-schedule bus becomes more cost-efficient the more riders it attracts. The small van becomes more expensive because more passengers mean more circuitous routes, more time and more gas.

Contrary to proud claims, "special needs" or "demand responsive" para-transit services generally only meet the special and limited needs of a limited number of certified clients. They rarely respond in less than 24 hours. Many require 48 hours' advance notice, and some demand an application at least three months in advance.

Some services, such as the Delaware Agency for Specialized Transit (DAT), have charged disabled passengers fares that are 10 to 20 times higher than those charged able-bodied passengers traveling the same distance. Most services, such as that of Cleveland's Regional Transit Authority (RTA), operate only from 9 a.m. to 5 p.m. It will take you to work or a medical appointment, but it won't let you work late, take an evening class, have dinner with a friend or see a movie. On weekends you stay home.

If public transportation for the disabled seems expensive, private transportation is exorbitant. Heumann paid $10,000 for her specially equipped van plus $1,000 a year for insurance. In addition, she must hire a driver to use it.

She travels a lot to lecture and consult with other groups, and just the ride from O'Hare Airport to her hotel in downtown Chicago costs her $64.

"Still, I was really delighted the last time I stopped at the Chicago airport," she relates. "It was the first time at an airport that I could use the toilet. Terrific!

"Then I thought about it later. Big deal. I could use the toilet!

"We must still be grateful for whatever little is given us. We have been taught that way since childhood," she says.

"I know, it is going to be a long time until we can decide spontaneously, without careful planning and preparation, that we want to go someplace and just take the bus and go. That is not only because it costs so much money. It is because temporarily able-bodied people don't want to see disability around.

"Our first need is to demystify us."