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3.75 miles ; from I-43/94 east to the lake shore, then south to near South Shore park. Interstate 794 is completely elevated, including a two-mile bridge over the mouth of the harbor.  It was approved for the Interstate system on July 24, 1964.  It opened to traffic in 1977.  A severe structural failure of the Hoan Bridge in December 2000 has closed that part of the freeway (see below).
There were plans to connect a freeway from I-794's "elbow" north and west to the WI 145 freeway. The "Evel Knievel" ramps there were eventually connected to the Lincoln Memorial Drive. You've probably seen those ramps -- Jake and Elwood's car flew off one in "The Blues Brothers," at the end of the long Chicago chase scene. 
The Lake Parkway (WI 794)
In 1998 a parkway was constructed starting at the south end of I-794. Michael G. Koerner writes:
"This is the 'Lake Parkway' (formerly the 'Lake South Alternate Highway/Freeway'). It is one of the numerous Milwaukeeland freeways that was never built after the early 1970s. The only difference is that this project [will be] built, but not as an 'I-compatible' freeway, only as a 'parkway' (access will only be at interchanges, though, but this highway will be substandard). This proposed freeway was always popular with the local South Shore neighborhoods and suburbs, as it will take a LOT of commuter traffic off of local city streets and make it much easier to get to and from downtown, but the delays were money related. It runs southward from the 'Hoan Bridge' along the Union Pacific Railroad's 'Air Line' subdivison to Layton Av in Cudahy, ending with a (temporary) direct feed into Pennsylvania Av just south of Layton (further southward extensions, possibly as far as to WI 31 at the Racine/Kenosha county line are proposed, but are being held up by money and NIMBYs). The original proposal for this route was to connect with proposed never-built freeways in Illinois, this to take traffic pressure off of I-94."
Some like I-794, some hate it
How is I-794 regarded now? Let's look at an exchange between freeway advocate (and shopowner) George Watts and mayoral aide James Rowen, debating tearing down the WI 145 spur in northeast Milwaukee:
"Comparing the area around the Park East Freeway to the southern downtown, Watts said the Third Ward has become a better place since the construction of I-794 because the freeway takes trucks off local streets.
'That's one of the stupidest things I've ever heard about the transportation system in downtown Milwaukee,' Rowen said. 'George Watts knows a lot about marketing wedding gifts, but he knows nothing about transportation.'
The destruction wreaked on the Third Ward to build I-794 is 'legendary,' Rowen said." 
Hoan Bridge fails, Dec. 13, 2000
A failure on the northbound span of the Hoan Bridge caused the roadway to sink about 10 feet. I-794 was immediately closed east and south of the Lincoln Memorial Drive interchange, and will remain closed likely until at least spring 2001. See a Milwaukee Journal-Sentinel photo (thanks to Steve Anderson for the link, and Michael G. Koerner for posting to MTR).
On Dec. 28, 2000, a 130-foot section of the damaged northbound span was demolished, in a precision blast that kept the falling slab from damaging bridge supports or historic building nearby. Even the southbound lanes, left standing, will remain closed to traffic for at least 3 months. 
On Feb. 17, 2001 at 10 a.m., the Hoan Bridge was partially reopened to traffic: both directions are temporarily using the southbound lanes. The Wisconsin DOT will determine whether the bridge should be retrofitted (6 months, $19M) or rebuilt (more than 1 year, $41M). 
As the planned reconstruction of the Marquette Interchange (interstates 43, 94, and 794) is studied, officials are deciding if and how to retain the offramp to Plankinton Avenue. The ramp creates safety problems with weaving traffic. A proposal to close the ramp was unpopular with local businesses, but a design to retain the ramp would bring the freeway as close as 12 feet from Wisconsin Energy Corp. offices. An alternative design: double-deck the freeway. 
Mayor Norquist proposes renumbering
On January 21, 2003, Milwaukee mayor John Norquist included an interesting proposal in his "State of the City" address: eliminate the I-894 designation, reroute Interstate 94 in its place, and extend I-794 westward about 5 miles to incorporate former I-94 between exits 305 and 310.
The intent: to encourage thru traffic to bypass the city center and the Marquette interchange, lessening the need for double-decking or extensive interchange work.  It is debatable how much effect the renumbering would have.