Sharrows, or 'shared lane' signage, are applied on traffic lanes to clearly mark these lanes to cyclists and motorists that both have access to them. Generally they are used on narrow curb lanes where vehicle parking exists to prevent cyclists from biking to closely to the parked cars so as to prevent being 'doored'. However, the far-right, bus-only lanes on Post and Sutter made that impossible, so the San Francisco Municipal Transportation Agency (SFMTA) chose to sharrow the center lane as cyclists are not legally allowed to use lanes exclusive to buses.
"On two main thoroughfares in and out of downtown - Post and Sutter Streets between Van Ness Avenue and Union Square - the transportation agency has placed (sharrows) in the middle of busy, one-way streets. This puts cyclists in the center lane of three, surrounded by fast cars on all sides.
Joshua Citrak, an avid cyclist with 10 years' experience on San Francisco streets, pedaled the route at the request of The Bay Citizen." See his excellent video of biking the hills in urban San Francisco.
From Streetsblog SF (6/25/10): New Sharrows on Sutter and Post Streets Not Popular with Cyclists
New sharrows have been placed on the center lanes of Post and Sutter, but "drivers often zoom by at alarming speeds, breaking the 25 mile an hour speed limit, narrowly avoiding crashes, and treating the three-lane arterial like a highway...
The SFMTA recently installed the sharrows on both Sutter and Post Streets (Bicycle Route 16) as part of its Bike Plan directive to add 75 miles of new sharrows on bike routes across the city."
Thanks to SF Streetsblog