The Downtown Austin Alliance has posted a street-closure map for SXSW, and it's quite detailed. As in previous years, large swaths of downtown will be, um, challenging to navigate for at least some hours every day between March 11 and March 20.
The areas around the Convention Center, Sixth Street and immediate cross streets from I-35 to Sabine Street, and most of Red River from 10th Street to River Street will be closed most nights and some afternoons.
Several on-ramps to I-35 near downtown will be closed 6am-3am daily during the Music festival (March 16-19).
Starting March 17, several South Austin streets that feed into Auditorium Shores will allow only local access, and the part of Riverside Drive adjacent to the Shores will be closed entirely from March 15-20.
East Austin residents will no doubt be delighted to see that there is only one SXSW-related restriction noted on the map for the area east of I-35 downtown: there will be no north curbside parking allowed on one block of East Fifth Street between San Marcos and Sabine streets March 14-20. However, a few surrounding streetlets can be closed at police discretion.
The DAA site has more details, as does the map, which you might want to pore over if you plan to attend the festival or live near downtown.