(FORTUNE Magazine) – One moment you glide the highways safely in your sleek Mercedes. The next, grit from an ordinary motorist fouls your headlights' tiny wiper blades. Should you proceed? To answer such questions, Mercedes-Benz provides Roadside Assistance, a program that lets car owners telephone specially trained Mercedes mechanics. If the problem demands more than advice, a mechanic goes to the scene. In 80% of such cases the car is under way in 45 minutes or less. Though fewer than 2% of Mercedes owners use the service, introduced nationally in 1984, it has proved so potent a marketing weapon that Volvo, BMW, Jaguar, Cadillac, and other luxury car makers have devised competing programs. Membership is typically included in a car's purchase price and lasts three years. Conventional auto clubs aren't nervous. AAA, the Automobile Association of America, has nearly 30 million members, compared with perhaps 2.5 million in the assistance programs. AAA spokesman Richard F. Hebert says, ''We're not convinced this is anything more than a passing fad -- another accessory.'' He doubts the programs ever will expand beyond the luxury segment -- but Chevy drivers shouldn't despair. They might keep in mind that they can get roadside assistance from AAA for $17 to $56 a year.