Several streets in and around downtown will be closed for the Ironman Triathlon on Sunday, September 13th.  Streets will be effected from 11:00 a.m. on Sunday to 1:00 a.m. on Monday, September 14th.  Traffic will not be allowed to cross the route at any time.     
 
Full list of street closures:
http://www.ironman.com/~/media/17a54a98086e4a409a1ba14363dd3aec/im%20wi%20spectator%20guide%208%2013%20online.pdf
 
Course maps:
http://www.ironman.com/triathlon/events/americas/ironman/wisconsin/athletes/course.aspx#axzz3lA7ZxrM2
 
Metro Transit detours:
http://www.cityofmadison.com/metro/detours/
 
See the attachment for a map of downtown traffic routes for Sunday during Ironman.