My family and I live on the corner of Mariposa and Pennsylvania. Because of the lack of stop signs, cars speed through this intersection without thinking much about pedestrians. Now that I have a baby, walking the dog with the baby stroller has become literally dangerous to cross the streets. When big trucks roll by at high speeds to zoom onto or off the freeway our whole building shakes.
Volunteers in the neighborhood have done an awesome job beautifying the vacant lots by transforming them into community gardens, however walking across the street to access the gardens are dangerous due to oncoming traffic that doesn't stop for the crosswalk.
As a driver, it's almost impossible for a car to exit in any direction from Pennsylvania Ave there, due to the relentless stream of cars coming off/getting on the freeway. Since I have lived in the neighborhood I've personally witnessed 4 accidents due to the lack of proper stop signs on this corner.
Even Google Maps street view caught a photo of what appears to be a near collision between vehicles and a van crossing the intersection while pedestrians (including a mother with stroller) are walking across crosswalks: http://bit.ly/Zfbz4N
The city of San Francisco needs to install stop signs on this dangerous corner immediately.