According to the SF MTA, due to President Obama’s visit, the following street closures will be required from 11 AM tomorrow (Tuesday) to noon, Wednesday:
4th Street from Market Street to Mission Street
Mission Street from 3rd Street to 4th Street
These street closures will also impact these bus routes:
8X /AX/BX Bayshore Express
14 L Mission Limited
14 X Mission Express
Presumably, Muni will be rerouting around the closed areas, though they don’t specify exactly how. A quick call to 311, however, got us information on where to catch and get off the 14 if I needed to go somewhere in the closed area, so that seems to be your best bet if you have bus questions.