More than a year after expanding out to San Francisco, Cambridge-based RelayRides announced today it would make its peer-to-peer car sharing service available nationwide and get people other than city dwellers interested.

At the same time, the startup said their new “key-exchange program,” which lets car owners quickly sign up on RelayRides’ marketplace and start renting their car out to neighbors. Renters reserve the car on and get the keys from the owner or a lockbox.

“Since the day we first launched in Boston, we have received thousands of requests to expand into more communities,” Shelby Clark, RelayRides founder and chief community officer, said in a news release. “Today, we are thrilled to be able to finally meet this demand. Now that RelayRides’ marketplace is available across the nation, every eligible driver can participate in this green, simple and economically sensible answer to the traditional car ownership model.”

Unlike ZipCar or some similar services, RelayRides doesn’t keep a fleet of cars for people to rent for short periods of time. Instead, users pick from a selection of cars owned by individuals. Basically, you could be borrowing your neighbor’s car for a trip to IKEA, or something like that, for as little as $5 an hour. Owners are allowed to set the hourly price between $5 and $20, based mostly on the kind and condition of the car, the area it’s parked and if there’s a designated space to park it at the end of the rental. RelayRides keeps 40% of the rental fare, while the car owner gets the rest. Drivers are backed by a $1 million insurance policy.

Car sharing has traditionally been targeted at cities with good public transit systems and where fewer people tend to have cars. By launching nationwide, RelayRides is going after customers who sometimes need a second, third, or fourth car on short notice.

“A lot of families always need one car and sometimes need two,” Clark said in today’s Los Angeles Times. “Right now, their only option is to round up. The only way to access that car when they need it is to own one.”

The company, founded in Cambridge in June 2010, has raised more than $13 million in venture capital funds from investors like Google Ventures, August Capital, Shasta Ventures and General Motors Ventures. Last year, RelayRides announced it would partner with GM to allow OnStar to let users find nearby rentals.

While fellow Cambridge company ZipCar is probably the name most people think of for car sharing, numerous other companies are emerging. JustShareIt and GetAround are also peer-to-peer car sharing outfits. Car2Go, which is owned by German automotive giant Daimler and operates fleets of Smart cars internationally, announced last week it would expand to Washington, D.C.

It’s too early to tell if RelayRides will change the way people use cars away from the dense cities – the LA Times stated fewer than 1 million people use car sharing in this country. But there is an upside for people who want to raise some money in the era of $4-a-gallon gas: RelayRides said owners can raise upwards of $7,000 a year by renting their car out. User Curtis Chong listed his Honda Civic on the site about nine months ago and has netted about $5,300 since.

“The last time I checked Kelley Blue Book, my Civic was worth about $4,800,” Chong said in the release.