A decade ago, few people would have dreamed of handing over their car keys to complete strangers in exchange for quick cash. Individual car owners could hardly risk becoming their own Avis, Hertz or Zipcar--all companies that own or lease vehicles by the thousands and have the financial muscle to insure against fender benders, spills and theft.
But San Francisco--based start-up RelayRides, which launched in two cities in 2010, went nationwide in March and now claims 20,000 members, is betting that neighborly car sharing can take hold across the U.S. Backed by $13 million in venture capital from the likes of...