Everybody loves Zipcar, as their recent IPO attests. In the 11 years they’ve been around, the car sharing service has kept at least 120,000 cars off the road and revolutionized the way we think about car rentals. But even though it’s available in over 50 cities and 100 universities, the service still doesn’t have anywhere near the number of cars or locations as a conventional car rental service. And despite their impressive IPO, Zipcar has yet to turn a profit. Buying, leasing, and maintaining an 8,000 car fleet that turns over about every three years isn’t cheap. But what if you could turn any car on the street – even yours – into a Zipcar? And then turn it back into a privately owned car when its owners needed it?

That’s the thought that occurred to Shelby Clark a few years ago as he was pedaling his bike through Boston to pick up a Zipcar. “Along the way, I passed dozens of cars that hadn’t been driven in weeks, and it dawned on me that there are these high fixed costs, so perhaps we could leverage this resource,” he told The New York Times last fall. “I began to research how to connect people in their communities.” He went on to create RelayRides, a peer-to-peer car sharing service now running in Boston and San Francisco. He’s not alone. There’s also Getaround and Spride Share, both based in San Francisco. All of them are trying to take advantage of the fact that the average car is only in use 8% of the time. That means 92% of the time your car is just sitting there like a deadbeat, when it could be helping someone else get around and making you some money in the process. Clark says the average RelayRides car owner makes about $250 a month renting out their own car.

Although the technology that each system uses is slightly different, most of them operate along the same lines as Zipcars. Car owners have a device installed in their car that allows them to be accessed by approved renters through either a membership card or their smartphone (Getaround users have the option of simply arranging a key drop-off with each renter). Renters search the web for cars nearby that are available when they want them. And all of them depend on a peer-rating system similar to eBay or Amazon. If you’re the kind of renter who leaves cigarette butts and empty Big Gulp cups rolling around in the back seat, it’ll affect your rating and you may even be charged a penalty fee. Likewise, if your car smells like a wet dog, users will rate your car accordingly and you won’t get much business in the future.

As encouraging as the development of these peer-to-peer car sharing networks are to the bigger “Shareable Economy” movement (if people are comfortable lending others the most valuable object they own, then it bodes well for the success of other share services), their success would be an enormous boost to the sustainable transportation movement. Since they don’t require the purchase of any new vehicles, they’ll be infinitely cheaper and easier to roll out in cities where Zipcar isn’t yet available. Specifically, cities with populations under 100,000 that are dense enough to be walkable but lack enough public transportation to make forgoing car ownership possible. Albany, New York; Charleston, South Carolina; Boulder, Colorado; and Davenport, Iowa all lack access to Zipcars. Two dozen or so shareable rides in any of those cities would make an immediate impact. And as cool as the idea of finding and unlocking a car in your neighborhood with your smartphone is, it still requires more infrastructure than the old-school technology of simply meeting a stranger in your neighborhood and handing them your keys. If enough people are willing to do that, there’s no reason peer-to-peer car sharing networks can’t effectively cover the whole country.