Milwaukee's regular public transit routes are limited, but effective. Check this site:

However, many special bus routes and charters that will take you directly to the Summefest grounds are added during Summerfest.

For Summerfest travel: If you are staying at a downtown hotel take the Summerfest shuttle bus that runs up Wisconsin Av.  If you are staying outside the downtown area many special Summerfest route busses leave from park and ride locations.  Several area bars also have free charter busses.

Nearly all of the construction on I-94 in the Marquette Interchange has been completed as of the beginning of 2009 (check before you drive, just in case).  If in need of alternate routes, some suggested ones are the following: One good alternate route is the Park Freeway (designated I-794) which can be reached east of I-94 from Layton AV, Howard or Oklahoma Av.  Howell Av is another  option to get to Summerfest from the south.  Or, you can park at one of the park and ride lots, such as College Av near the airport and take a bus to Summerfest.

 The advantage of taking a bus is you get dropped off right across the street from the main gate.  You will most likely have a long walk from any parking spot you find unless you are driving a motorcycle.  There is a motorcycle parking lot right at the main gate.