When Sierra Club members and former student activists John Zimmer and Logan Green, above, met two years ago, fresh out of Cornell University and U.C. Santa Barbara, respectively, they found they'd both been pondering the same question: Why doesn't carpooling play a bigger role in helping cut back on our gasoline consumption and the resultant carbon dioxide emissions?

The impediment, they decided, was two-fold: There was no simple way to hook people up, and there was the issue of safety and trust-are you going to jump into a car with a perfect stranger?

Flash forward to Spring 2007 and the launch of Zimride, an online social rideshare and carpool matching system that uses Facebook to hook riders up with drivers going to a common destination. (The name Zimride, incidentally, derives not from Zimmer's last name, but from a trip Green took to Zimbabwe in 2005.)

"The big dream," says Green, "is that there will be so many rides posted on this system that online ridesharing becomes a reliable form of transportation, where you can log on at any time, enter any city, and find a ride. Once it hits that level of reliability, it can really fill an important role in our culture."

Zimmer, who was interested in combining business with sustainability on a large scale, was influenced by reading "The Ecology of the Automobile" by Peter Freund and George Martin (no, not the Beatles' engineer) in a Green Cities class at Cornell. "It's phenomenal the impact car culture has had on American society and patterns of development," he says. "Our hope with Zimride is that we can have an impact on our automobile culture simply by increasing efficiency, without adding any more resources."

The key is using online social networking tools as a way to hook riders up with drivers. As an undergrad, Green had no car, his girlfriend lived two hours away, and every other weekend he found himself trying to find a ride down to Los Angeles. "There are probably thousands of people driving from Santa Barbara to L.A. or San Diego every weekend," he says, "so I figured I should be able to find a ride with someone."

That's when it occurred to him to use Facebook as a way of matching people up who were taking the same ride. By letting them connect with one another via social networking, they could review each other's profiles before deciding to get in the car together.

Green built the prototype for Zimride using the U.C. Santa Barbara and Cornell campuses as prototypes. "Universities were a natural place to start," he says. "There are lots of people with no cars in need of rides, and so many people use Facebook. We thought if we could capture a good chunk of students' weekend trips, it would be more appealing than Amtrak or Greyhound."

Zimride is now used extensively on six college campuses, and has served more than 300,000 users, mostly in the U.S., but some in Great Britain, Australia, and Singapore. After six months of use at Cornell, more than 15 percent of students and faculty are using the application, a five-fold increase over the carpooling generated by the previous rideboard. "Universities are the natural early adopters," Zimmer says. "Our next step is to move on to corporations, and eventually municipalities."

Zimmer sees the application as something that will enhance a community's quality of life while doing right by the environment. A calculator at Zimride.com can project money saved, CO2 emissions saved, and driver miles avoided for a given campus or community, based on actual usership. The company currently licenses its social ride-sharing and transportation software to universities for an annual fee.

"The goal is to create a positive feedback loop where the more riders there are, the stronger the transportation system grows, while simultaneously getting cheaper and more reliable," he says. "And the great thing is it creates a new sustainable form of transportation out of thin air, leveraging existing grassroots momentum."

In June, musician Jack Johnson, who is incorporating a "greening" theme into his summer tour, partnered with Zimride via a carpool link on his website so fans can save gas and money by carpooling to concerts. And on July 16, Zimmer embarked on a Carpool Across America road trip (below) from New York to San Francisco in a Honda Prius hybrid donated by Zipcar to raise awareness about community-oriented travel options in times of rising gas prices and concerns about global warming.

Carpool Across America ties together the themes of energy conservation, the power of social networks, and environmentally-conscious business solutions. "We'll be rotating passengers and blogging about the trip the whole way," Zimmer says. "We'll also be interviewing young eco-entrepreneurs en route who are making a difference with creative business, art, and non-profit business solutions."

The journey will be documented by filmmaker Trevor White and covered by green consultant and eco-writer Margaret Teich. Below, Zimmer's girlfriend Cristina Garcia Rivas is pictured at the start of the trip in New York City.

Follow the Carpool Across America and learn more about how you, your school, or your company can get on the social ride-sharing bandwagon at Zimride.com.