Ms. Sandoval sees a cab ride as a chance to make a phone call or send a message or read a report. But if she is taking Uber, she makes sure her silence won’t be misinterpreted.

“I say, ‘I’m sorry, I have some work to do. Please excuse me,’ ” she said. “If the driver is a tour guide or a philosopher, you don’t want to alienate them.”

EBay pulled back on allowing sellers to review customers in 2008. Buying on eBay is a straightforward transaction with little personal interaction between seller and buyer, and so the reviews got in the way. Now eBay allows sellers to make only positive comments about buyers.

But the new platforms let reviews go both ways, and vary in their transparency about the process. Yelp is straightforward: Businesses can post replies to critical customers. On Lyft, the second-biggest of the new cab companies, passengers are vaguely warned that “a low star rating” means requests for rides may not be accepted. Uber does not mention passenger ratings at all in its user agreement but noted in a blog post that “an Uber trip should be a good experience for drivers too.”

It does not seem to take much to annoy some Uber drivers. On one online forum, an anonymous driver said he gave poor reviews to “people who are generally negative and would tend to bring down my mood (or anyone around them).” Another was cavalier about the process: “1 star for passengers does not do them any harm. Sensible drivers won’t pick them up, but so what?”

Even those who know Uber best appear surprised by how easy it is to fall from grace. “I was at a 5 for a long time, then I had a string of 4 stars,” Travis Kalanick, the company’s chief executive, recently told San Francisco magazine. “I don’t know what happened. I think what happened was I was a little stressed at work. I was not as courteous as I should have been.”

Part of the confusion stems from the fact that the rental economy — taking its cue from the Internet in general — sees everything as either horrible or great, with little room for nuance. Lyft nods to this when it tells passengers reviewing drivers that “anything lower than 5 indicates that you were somehow unhappy with the ride.” Drivers can be dropped from their services when they fall below 4.5, but it is unclear what it takes to get banned as a passenger.