Google Bus

Private Bus (Zynga) - San Francisco  2013

Actually, in this case, a Zynga Bus (parked in front of Zynga HQ in San Francisco's SOMA district).

From a recent George Packer article in The New Yorker:

A lot of younger employees of Silicon Valley companies live in the city and commute to work in white, Wi-Fi-equipped company buses, which collect passengers at fifteen or so stops around San Francisco. The buses - whose schedules are withheld from the public - have become a vivid emblem of the tech boom's stratifying effect in the Bay Area.

And just one more obstacle for motorists trying to navigate into San Francisco using Lombard and Van Ness.

Google Buses, Lombard Street - San Francisco  2013