All of this comes to mind because San Francisco is offering tax breaks to tech companies that relocate their offices to the city’s blighted neighborhoods. Twitter will be the first recipient of this largess when it moves into new offices in the Furniture Mart on a particularly desolate section of Market Street next year.

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Twitter, which is competing for talent with Google and Facebook, gives its employees free food. The question is whether those urban employees will leave the building often enough to dramatically improve the neighborhood.

Gabriel Metcalf, the executive director of the San Francisco Planning and Urban Research Association, or SPUR, an urban policy research group, says the impact of corporate cafeterias in an urban setting has not been rigorously studied. But, Mr. Metcalf says, “You certainly get more life on the street if everyone is going out to lunch.”

Twitter may be a good test case. Market Street, a wide avenue that bisects downtown San Francisco, begins at the Ferry Building. Smart designers transformed that structure from a seedy and little-used transportation hub; it now teems with shops and restaurants that attract tourists and office workers alike. During the twice-a-week farmers’ market, people stand in line for a half-hour to buy ramen, exotic sausages or rotisserie pork sandwiches from food carts.

But a mile and a half up Market, in the shadow of the golden dome of City Hall, it’s a different picture. Boarded-up buildings. Empty, littered lots. Even in boom times, the area never improved.

This is where the city has encouraged Twitter to set up shop. To transform the area, though, people will have to get out on the street. When the people come, shops open. When the shops open, more people come. A virtuous cycle begins.

But what happens when people don’t leave the buildings — when the culture of the suburban campus drops into an urban center? Two places in the city offer laboratories of sorts for a possible answer.

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In a former industrial neighborhood called Mission Bay, the University of California, San Francisco, built its medical center. The university stashed its cafeterias up off street level. As a result, the area has remained sterile and empty. Employees and students drive in, then drive out. The public stays away.

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That is expected to change when Salesforce.com, a company that has thrived providing Web services to corporate sales representatives, finishes building a new headquarters nearby. It will consist of eight buildings over 14 acres — a suburban campus in the city. The architects, Legorreta & Legorreta, of Mexico City, have incorporated street-level retail space into the project that is open to the sidewalk. They say they want to invite the public in. Salesforce has never had a corporate cafeteria.

Then there is a no-name neighborhood, south of San Francisco’s financial district, that is home to a satellite office of Google. While the fare in that office’s corporate cafeteria isn’t as extensive as it is in the company’s home office in Mountain View, 40 miles to the south, it is still copious and free for employees.

The results are not surprising. In the time that Google has been in San Francisco, few establishments that draw people to the sidewalk have opened in its neighborhood. There is a new expense-account-set restaurant across the street, and a taco truck popular with the employees at the nearby Gap headquarters, which also has a corporate cafeteria, though meals there are unsubsidized.

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EVEN a city like San Francisco, which in some ways likes to tell people how to live their lives — trying to ban McDonald’s Happy Meals, second-hand smoke outdoors and plastic shopping bags — is unlikely to start telling companies how to feed their employees.

“It’s not the kind of thing you can micromanage,” says Mr. Metcalf of SPUR. “It’s nice to have something that they leave alone.”

But he hopes that the executives who move into blighted city neighborhoods see a reason to encourage employees to get out.

“The city benefits,” he says, “when a company decides to integrate more with the public life of the street.”