Falling ridership could redirect mass-transit planning

America's public transit systems are in trouble. But the troubles are forcing some cities to try creative new responses to the changing needs of commuters and shoppers, according to transit experts.

Over the last two years ridership on the nation's public transit systems has slid about 7 percent (or has at least leveled off by other estimates). Bus service has been cut back in many cities, and fares have been rising. By contrast, between 1972 and 1980 the number of riders had increased.

The average daytime fare jumped from 35 cents in 1979 to 52 cents this year, according to the American Public Transit Association, which represents the transit industry. Rising labor costs and declining government subsidies have spurred such increases, says an APTA official.

''We simply don't have the funds,'' says Kenneth M. Gregor, general manager of the Metropolitan Atlanta Rapid Transit Authority (MARTA). Labor must accept a slowdown in raises, he adds, or the system will face further fare hikes, service cutbacks, and ultimately fewer passengers.

According to a number of transit system officials, scholars, and others, a ''downhill'' cycle has begun to affect many transit systems: higher costs, forcing higher fares and cuts in service, followed by further declines in ridership.

As a result of such problems, a number of ideas that have been around for quite some time are beginning to get a new look, according to these experts.

One of these is greater use of privately run transit. While most experts see a continuing need for public transit, they say that privately run systems may be the best choice for serving more distant commuters.

Generally, these experts agree, it's not the fare but the lack of convenience that's keeping most Americans off public transit.

Changes envisioned to address this shortcoming and lure more riders out of their private cars include:

* Plush, rapid, private commuter bus lines running express service, and perhaps offering refreshments, from distant suburbs to urban centers. Only a few cities, such as Boston and New York have such service now.

* Shared taxis along crowded city corridors - taxis run by private companies or independent cab drivers. Many cities currently have legislation against such service.

* Greater use of company van pooling and car pooling, and greater use of special lanes for these methods of transportation. Such methods are on the increase already.

* Government-paid travel vouchers for the poor. Such vouchers, it is said, would avoid shut-out prices for the poor in the event greater use is made of more expensive, privately run transit systems. So far this idea is not getting anywhere, but the current economic crunch may force a reconsideration, say some transit scholars.

Some transit systems are already trying nontraditional methods to cut operating costs and still provide service.

In Norfolk, Va., the public transit system already leases buses and vans to private operators who provide more personalized commuter service than fixed-route bus service could. The system also leases vans to taxi operators who charge up to $1.50 a ride.

The transit system in Phoenix, Ariz., subsidizes taxi operators to provide low-cost service when buses are not running.

Huntsville, Ala., is leasing, at no charge, vans to neighborhood organizations which then provide transportation to residents on a contribution basis with volunteer drivers.

At the moment, most public transit systems are ''providing a service people don't want,'' says Martin Wohl, professor of transportation systems planning at Carnegie-Mellon University in Pittsburgh. Less than 3 percent of urban passenger trips in the United States are made on public transit, he says.

But the 3 percent figure is only part of the picture, says an official with the Urban Mass Transportation Administration (UMTA), the federal agency dealing with mass transit. As many as three-fourths of rush-hour commuters use public transportation in many cities, including New York, Chicago, and Philadelphia. says the official.

But commuter trips today are no longer just to and from downtown, they are ''all over the place,'' he says. Public transit, he adds, has an ''inability to serve urban sprawl.''