IN San Francisco recently, I was running late for a meeting, desperate for a ride. But no buses or cabs were in sight.

Then I remembered a service called Lyft. I pulled out my smartphone and quickly downloaded the application, which lets regular people act as chauffeurs for a fee. As instructed, I entered my location — then crossed my fingers and waited.

Five anxious minutes later, a clean black Audi pulled up, with a chatty young man at the wheel. I jumped in and we made friendly small talk. I gave him advice for his future trip to New York, and he invited me to a party that night in the city. We reached my destination, and I slid out of my seat, transferred $10 to the service through the Lyft app, and walked into my meeting with a smile.