City officials acknowledge the potential problem. They are urging drivers to pull over before they pull up the city’s iPhone app, or to do so before they leave home. But the spots can disappear quickly, as any circling driver knows, and for plugged-in motorists in the habit of texting or glancing at the GPS, the urge to use the parking app is certain to mount as the frustration does.

Nathaniel Ford, executive director of the San Francisco Municipal Transportation Agency, said safety could actually improve if drivers quickly found a spot instead of circling and getting frustrated. “I get you off the streets as quickly as possible,” he said.

City drivers can testify to the frustration. Ms. Soltani drives into the city several times a week, she said, and spends 20 to 30 minutes searching for a spot each time. “That’s at least an hour I lose every week just looking for parking. It’s very frustrating,” she said.

She said she had heard about the new app, but had not yet downloaded it.

The $20 million parking project here, called SFpark, is backed by the Transportation Department and the Federal Highway Administration, which are looking into how to ease congestion and driver angst by making the most of limited parking.

San Francisco has put sensors into 7,000 metered parking spots and 12,250 spots in city garages. If spaces in an area open up, the sensors communicate wirelessly with computers that in turn make the information available to app users within a minute, said Mr. Ford, of the transportation agency. On the app, a map shows which blocks have lots of places (blue) and which are full (red).

San Francisco’s is by far the most widespread approach that several cities, universities and private parking garages are experimenting with.

Last December, Los Angeles worked with a company called Streetline to introduce a system covering spaces in West Hollywood, and it is expanding the program elsewhere. Streetline has since set up smaller projects on Roosevelt Island in New York City’s East River, as well as at the University of Maryland and in Forth Worth, Tex.