SideCar, an on-demand ride-sharing app, lets users request a ride by indicating where they would like to be picked up and dropped off. The app matches them with nearby drivers - prescreened by the company - who are willing to give rides.

"We're trying to set up a situation where people share the burden of transportation with each other," said Blake Wirht, director of marketing for San Francisco-based SideCar.

When someone requests a ride, nearby drivers can decide if they want to respond. The passenger can compensate the driver via the app at a suggested fare that often undercuts the cost of a taxi.

"Longer trips tend to be significantly less than taxis and shorter trips are more on par, but generally it's a better deal," Wirht said.

Compensating the driver is completely voluntary, according to Wirht, who said the policy conforms to state ride-sharing regulations.

"We have instituted a community average which shows people what others in the community have been paying for similar rides," he said.

He added that donating less than the recommended fare could cause a passenger to get a low rating on the app, and deter drivers from accepting the passenger's ride requests in the future.

Wirht said drivers span a range of ages and professions, from part-time waitresses to managers at technology companies and even retirees.

"It's fascinating the people you meet, and a lot of people are drawn to it for that reason. But there are a lot of people that drive for SideCar to offset their costs of vehicle ownership," he said.

Some critics have questioned whether accepting a ride with a stranger is wise, but Wirht said the company has made the safety of passengers and drivers its core focus.

"In order to be approved as a driver you have to go through a vetting process, a background check, proof of your valid insurance, proof of driver's license and proof of registration," he said, adding that rides are also tracked via GPS.

All passengers need to register with a valid credit card.

"We know who everyone is," he said. "It's the visibility that creates a very safe environment."

Users can also rate each other based on their experiences.

The app, launched at the end of June, is available for iOS and Android devices across the United States. (Reporting by Natasha Baker; editing by Patricia Reaney and John Wallace)

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