It was during this period San Francisco built some of its most important infrastructure. The following are the most densely populated cities with populations exceeding 50,000: The following table lists population densities for U.S. cities with populations of at least 50,000 as of 2016: #tablecaption  However, by 2016, San Francisco was rated low by small businesses in a Business Friendliness Survey. San Francisco has several nicknames, including "The City by the Bay", "Golden Gate City", "Frisco", "SF", "San Fran", and "Fog City", as well as older ones like "The City that Knows How", "Baghdad by the Bay", "The Paris of the West", or, as locals call it, "The City".  USGS has released the California earthquake forecast which models earthquake occurrence in California.  The portions of the Golden Gate National Recreation Area located within the city, including the Presidio and Ocean Beach, are patrolled by the United States Park Police. Population Density is computed by dividing the total population by Land Area Per Square Mile. These two districts are each sometimes further divided into two regions: the Outer Richmond and Outer Sunset can refer to the more western portions of their respective district and the Inner Richmond and Inner Sunset can refer to the more eastern portions. In 2015, Walk Score ranked San Francisco the second-most walkable city in the United States. city.. , Since the 1990s, the demand for skilled information technology workers from local startups and nearby Silicon Valley has attracted white-collar workers from all over the world and created a high standard of living in San Francisco. There is also Lombard Street, known for its "crookedness" and extensive views. 52–53, Quicksilver Press, 1997. As a result, the year's warmest month, on average, is September, and on average, October is warmer than July, especially in daytime. Due to its unique geography, and the freeway revolts of the late 1950s,Interstate 80 begins at the approach to the Bay Bridge and is the only direct automobile link to the East Bay.  Historically, trains operated by Southern Pacific Lines ran from San Francisco to Los Angeles, via Palo Alto and San Jose. San Francisco's War Memorial and Performing Arts Center hosts some of the most enduring performing-arts companies in the country. , As of 2010[update], 55% (411,728) of San Francisco residents spoke only English at home, while 19% (140,302) spoke a variety of Chinese (mostly Taishanese and Cantonese), 12% (88,147) Spanish, 3% (25,767) Tagalog, and 2% (14,017) Russian. Amtrak California Thruway Motorcoach runs a shuttle bus from three locations in San Francisco to its station across the bay in Emeryville. The regional sports network, Comcast SportsNet Bay Area and its sister station Comcast SportsNet California, are both located in San Francisco. The city is believed to have the highest number of homeless inhabitants per capita of any major U.S.  The construction of the Twin Peaks Tunnel in 1918 connected southwest neighborhoods to downtown via streetcar, hastening the development of West Portal, and nearby affluent Forest Hill and St. Francis Wood. Thruway service also runs south to San Luis Obispo with connection to the Pacific Surfliner. Rejecting calls to completely remake the street grid, San Franciscans opted for speed. In an effort to buoy small privately owned businesses in San Francisco and preserve the unique retail personality of the city, the Small Business Commission started a publicity campaign in 2004 to keep a larger share of retail dollars in the local economy, and the Board of Supervisors has used the planning code to limit the neighborhoods where formula retail establishments can set up shop, an effort affirmed by San Francisco voters.