Smartphone ride services Lyft, UberX and Sidecar are tackling a long-sought green goal: make carpooling a viable alternative to owning a car.

All three companies are launching on-demand carpool options that will let passengers share rides - and costs - with other people going the same way. All are initially trying out the services in San Francisco, where they are based and have thousands of riders and drivers.

With costs averaging 30 to 40 percent less than solo rides, the shared services may compete with public transit as well as car ownership.

"Instead of public transit, we're building what we call personal transit," said Lyft co-founder and CEO Logan Green. The company elaborated in a blog post: "This is a transit system with infinite routes - and it becomes stronger, more affordable, and more efficient the more it's used."

The benefits of fewer cars on the road are obvious: less congestion, fewer emissions, cheaper costs, reduced parking hassles. But carpools have never become widespread for a variety of reasons. People don't like to wait. People don't like to commit to exact commute times. And people don't like to share space with strangers.

But the ride services may have the critical mass, along with the technology and social engineering, to overcome those obstacles.

Lyft said 90 percent of its riders take a trip that someone else requests within five minutes.

"These services have reached a level of demand and popularity that may address one of the largest obstacles to real-time ride-sharing: having enough supply to meet demand," said Susan Shaheen, co-director of the UC Berkeley Transportation Sustainability Research Center. "There's the potential to expand their platforms for greater social benefit. It's an exciting development."

Shared-ride option

The shared services work similarly. Riders can select a shared-ride option in the app, enter their destination and be shown a fixed discount price. They'll get a discount even if the app can't find another passenger.

The companies will use technology to ensure that the extra pickups and drop-offs don't add much time to the trip. Passengers are expected to do their part by being ready to go as soon as a shared ride arrives. Drivers are likely to make more money because they'll be driving longer routes.

Gabriel Metcalf, executive director of SPUR, an urban think tank, said the new services could be groundbreaking.

"It offers point-to-point transportation without needing to own or park your own car," he said. "The potential here to drive down car ownership rates and free up urban space for better uses than parking is extraordinary."

Lyft Line started on Wednesday for iPhone users, with an Android version coming soon. Uber said UberPool is in a private beta test with a broader test starting on Aug. 15. Sidecar said it has quietly been trying its Shared Ride service since May, but has only a few dozen drivers available. It will do a bigger test during this weekend's Outside Lands music festival, and a broader rollout in coming weeks, said CEO and founder Sunil Paul.

Shared rides obviously carry a social component, although that may be old hat to Bay Area riders accustomed to the region's long-standing casual carpool system, as well as travelers familiar with countries where shared jitneys and shared cabs are common.

Lyft's laid-back culture of riders sitting in the front seat and exchanging fist bumps with drivers already breaks the ice on sharing rides with strangers, it said. Uber, which has had more staid interactions, called the UberPool "a bold social experiment" and said it would give passengers a heads-up with the names of their fellow riders.

A small company called Hitch that has offered paid on-demand carpools in San Francisco for several weeks goes a step further and shows passengers Facebook profiles of their car cohorts.

"With any new service, it's always fascinating to see who are the early adopters," Shaheen said. "One would presume they'll attract even more people who are price-sensitive with this product."

$5 per trip?

John Zimmer, Lyft co-founder and CEO, said many shared rides might cost $5 - for a trip from downtown to the Mission District, for instance.

"That (price) would attract a whole new socioeconomic demographic," Shaheen said.

Lyft got its start in 2007 as a carpooling service for long-distance shared rides, called Zimride. Two years ago it adopted the current Lyft model of paid solo rides, primarily intra-city, by community drivers in their own cars.

"We planted the seeds with supply and demand," said co-founder and CEO Logan Green. "Lyft Line is our biggest step in bringing down prices. ... We've been thinking about this ever since we launched Lyft. We always intended to do it."

The carpool options, like the services themselves, are limited to people with smartphones and credit cards. Those on the wrong side of the digital divide, and those who don't use banks, are left out. Disabled activists already criticize the ride companies for a lack of accessible vehicles.

"The services may need to be modified to address the needs of other populations," Shaheen said.

But overall, she said, the development may be "the recipe to get people to get into cars with strangers and to carpool more."

"If you couple (these existing services) with a discounted fare through ride-matching, then it becomes more efficient on multiple levels: more cost-effective, higher level of occupancy, higher passengers miles, reduced emissions per traveler and fuel savings," she said.

©2014 the San Francisco Chronicle