Improving traffic flow throughout San Francisco's busiest streets.
I suppose you will find this crazy, but changing many or most of the busiest streets to one way streets would improve traffic flow and have a side benefit.
Make the downtown streets alternating one ways, both north/south and east/west. Same for Soma.
Make Mission Street one way Eastward towards the Embarcadero from Cesar Chavez.
The reason is that buses, delivery vehicles, double parkers (I know this is illegal, but it happens none the less) and commuters have less space with it being two ways. When streets are two way, illegal U-Turns are bound to happen, but not with one ways.
Although this may create another block for some to walk to reach a newly established direction (example: going east or west), the ease of use for motorized vehicles increases in some cases by two lanes. If a police car, ambulence or fire truck is needed on a particular street, then there is more space for traffic to clear or go around, meaning less blocked streets and grid locks.