Boston — If US transit authorities can trust their riders, at least a little bit, an idea from Europe may save them some money.
Called the ''self-service, no barrier'' or ''honor'' system, it allows riders in many European cities to get on and off buses and subways without presenting their tickets. No turnstiles bar subway riders in Munich or Hamburg, West Germany. And bus riders in Paris can enter any door of the vehicle and don't have to pass before the fare box. Instead, many Europeans use a pass or buy tickets from automatic vending machines. Anyone caught riding for free is charged a stiff fine by roving inspectors.
At a time when many transit systems in the United States are struggling financially, a shift to the no-barrier concept could save money, mass transit experts say. Sophisticated turnstiles and magnetically encoded tickets usually cost more than the ticket vending machines and inspectors needed for a no-barrier system.
Seven months ago, San Diego inaugurated the idea in the US with a 16-mile trolley line. Of 13,000 daily riders, fewer than 1 percent cheat. Even by European standards this is low, experts say.
''Everybody predicted the rate would be very high,'' says Lyn Shepard, North American representative for Autelca AG, a Swiss manufacturer of fare-collection systems. ''In America, it was always felt you had to have a barrier because the public was basically dishonest. But the evidence in San Diego disputes that.''
The San Diego experiment is being watched by transit authorities in other cities. ''Wherever rail development is under consideration, certainly where commuter rail is already in operation, the system makes such sense that it's being considered seriously,'' Mr. Shepard says.
For example: Portland, Ore., inaugurates in September a no-barrier system for its buses, which will be expanded to the light-rail system when it opens in 1985 . In Canada, Edmonton, Calgary, and Vancouver already are using the concept.
The increasing use of articulated (bendable) buses in many US cities makes the idea even more attractive, says WesleyLeas, mass-transit consultant and president of J. W. Leas and Associates. The larger buses carry more passengers per driver, and it saves everyone time if passengers can board through any door.
Still, some transit authorities say they don't think a no-barrier system can work in their cities. Baltimore, for example, rejected the idea for its sophisticated Metro subway now under construction.
''It poses a problem of enforcement,'' says Bill Kirkpatrick, a project engineer for Baltimore Metro. Inspectors can't check everyone, he adds.
Another problem is judicial, Mr. Kirkpatrick says. If a child is caught riding for free and can't pay the, say, $25 fine, ''do you jail him?'' he asks. ''I doubt it.''
Furthermore, the no-barrier idea loses its cost-savings if too many people cheat, he says. Perhaps 7 percent would cheat in a large city such as Baltimore, he says, depriving the system of $7,000 a day. ''I say $7,000 would quickly pay off an expensive fare collection system like ours.''
Baltimore's concern is common of established transit authorities, Leas says. ''I think they have the feeling that it's somewhat uncontrollable. All of a sudden you raise the specter of this thing and people say: 'Oh no! We're going to be cheated. You can't go in and change things overnight.''
Leas suggests that cities try out the idea in small doses, starting perhaps with their articulated buses. Then, as riders get used to the idea, other parts of the system could go no barrier.
The idea also has to be sold to the public. A media campaign ''making the riders aware that it's their system'' is important, he says, because fare evasion will be lower if riders know cheating pushes up the fares.
San Diego has little cheating partly because roving inspectors check 35 to 40 percent of the riders. Offenders are fined $20 for the first offense, at least $ 50 for the second. As with parking tickets, riders send in the money or have to appear in court.
''There's a very, very large benefit we're getting from our inspectors,'' says Maurice Carter, director of planning and operations for the Metropolitan Transit Development Board. ''They've been very well received.'' Riders are proud to show they've paid.