I love the idea of personal car-sharing, which has been legalized in California since January and now has three companies in the field signing up customers and cars. Since most cars sit idle more than 90 percent of the time, it’s an idea whose time has come. When one of the San Francisco-based companies, Getaround, announced its launch, more than 1,600 people signed up to have their cars shared on the first day.

The basic idea is pretty simple: Make your car, which is just sitting around anyway, work for you by going on cellphone app-enabled trips for customers of personal car-sharing services. The major advantage for operators over regular car-sharing models like Zipcar (which went public in April) or Daimler’s Car2Go is that there’s no need to own and maintain car fleets.

For consumers, personal car-sharing means a) finding a cheap alternative to car ownership; and b) making money from your most expensive asset. RelayRides, with backing from August Capital and Google Ventures, says that the average person can make $250 a month renting out their car.

The three companies list around 200 cars each, with rates that are about $45 a day for a small compact. By the hour, that typically means $5 to $8, but a Tesla Roadster from Getaround is $50 an hour, and a BMW Z4 $15.

The smart phone is key, because it enables not only the transaction but also getting the customer into the car. Getaround has an iPhone app and a “Carkit” that installs in the shared vehicle and combines GPS, Wi-Fi and keyless remote technology.

When I first heard about personal car-sharing, I thought that insurance was the big hang-up. What would happen if a car sharer crashed because the personal car she borrowed had bad brakes? And, indeed, it required changing California’s insurance laws to even allow this new form of car rental to happen. But Getaround now has signed up with industry giant Berkshire Hathaway, and RelayRides has a $1 million policy that covers both the car lender and the driver.

There’s going to be some major adjustment here, because this isn’t how we’re used to renting cars. Ali Fenn, a Bay Area resident who is lending out her big Jeep, told NPR’s Marketplace that she was “slightly awkward” for the first transaction. “The person was like, ‘How does this work? What do I do?’ I said, ‘I guess I just give you my keys and you drive away.’ It was very easy.”

The smartphone apps mean that the two parties don’t even need to meet in person. On video, RelayRides car sharer Angela tells how she makes a few extra bucks by sharing her '04 Porsche Cayenne:

The third San Francisco-area company involved in personal car-sharing, Spride Share, conducted a survey on this new phenomenon with the University of California at Berkeley. Trust is a barrier, with more than 50 percent of the dissenters saying they wouldn’t lend out their car because, “I don’t trust people.” But most people seem to be on board with the idea, and 70 percent in the survey say they’re not worried about the privacy concerns. The good news for the environment is that nearly 60 percent say that if they rented out their car, they’d take other modes of transportation (always an option in transit-friendly San Francisco).

“This approach can reduce greenhouse gas and the need for parking, and improve highway congestion by getting cars off the road,” said Sunil Paul, the venture capital investor behind Spride.

It makes sense that this gained traction in northern California, but it’s growing exponentially. RelayRides has a small pilot program in Boston, and WhipCar is operating in London — with 1,000 cars available to rent. "We never use our car, so now it pays for our holidays," says WhipCar user Maureen, 66. One of her customers was a 34-year-old IT guy, so maybe they bridged a generation gap.

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Jim Motavalli ( @jmotavalli ) writes about cars, technology and the environmental world to anyone curious enough to ask.