RelayRides has scored a major victory for car-sharing startups. Today the company announced that it will launch free parking for its peer-to-peer car-rental service at San Francisco International Airport, which has been adamant about prohibiting many transportation startups from operating on its premises.

People can offer their cars for daily or weekly rentals through RelayRides’ marketplace. The company has made a big push to get into airports and now has a presence in 180 airports around the country. The airport rental car industry is worth $12 billion a year, according to RelayRides director of communications Steve Webb. He told VentureBeat that the airport option has played a significant part of the company’s growth — it expects airports to contribute up to 50 percent of the business within a year.

“Since we launched our marketplace in 2010, members have been asking us for more airport services,” Webb said. “We have developed a comprehensive airport service that we are ready to expand to other airports nationwide very rapidly. The big picture for us is to provide a viable transportation option that is more convenient and affordable than existing services.”

With RelayRides, people rent out their cars to travelers while they are gone themselves. If you are going out of town, you list your car, and travelers can rent it while you are away. This costs up to 40 percent less than big rental agencies. Previously, RelayRides required car owners and renters to arrange transferring cars/keys themselves (although renters could unlock OnStar-enabled cars with smartphones).

The company announced today that it will offer a free parking service at SFO. Car owners drop off their car at a parking lot for renters to pick up, and all parties get a free shuttle to and from the terminal. The vehicles will also get a complimentary carwash.

Ridesharing services like Lyft, UberX, and SideCar were issued cease-and-desist letters in April because their drivers do not have permission to pick people up at SFO — licensed taxis and limos are required to have permits and pay a fee for each trip to the airport. CBS affiliate KPIX-5 reported that in the past month, airport officials have been making citizens arrests of Internet ridesharing drivers for violating these policies.

RelayRides operates on a different model. It’s an alternative to rental cars, not taxis, so it does not pose the same concerns to public safety. Its main competitor, aside from traditional rental agencies like Hertz and Avis, is Y Combinator startup Flightcar, which launched earlier this year.

RelayRides has raised $13 million and is based in San Francisco.

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