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I-480 (cancelled) California
The widely unpopular Embarcadero Freeway, which would have provided a freeway connection between the Golden Gate and Bay bridges. Its downside was the double-decked roadway obscuring the view of Angel Island, Alcatraz, etc. The 1989 Loma Prieta quake condemned it, and it was later demolished; most people seemed happy to see it go. It weighed 150,000 tons. 
Part of the 1951 Trafficways plan, a short section of I-480 north of the Bay Bridge approach was designed in 1953 and opened in 1959. The intent was to connect it with the Golden Gate Bridge approaches, but this Golden Gate Freeway, following Lombard and 101, was rejected in 1966. (I-480 had been withdrawn from the Interstate highway system in 1965; the existing road was simply CA 480.)  In 1968, the S. F. Board of Supervisors voted 6-5 to stop building the freeway, resisting pressure from Sacramento.
In 1973, the Board declined to take part in another Golden Gate Freeway study, calling the proposal too similar to the 1966 plan. At that point, the city was already talking about tearing it down. In June, Sen. Alan Cranston talked to FHWA, who said: 1) If SF tore down 480, it would not have to pay back any federal funds used in building it; 2) SF could use federal funds to build a replacement on the ground or below; and 3) SF could not use federal money to simply tear it down without replacing it with another road. In September, FHWA clarified number 3, saying that "a sound transportation system," without a specific replacement, could suffice. The 1974 report proposed bringing the 480 ramps down to earth at Embarcadero and Howard. 
On Nov. 5, 1985, the Board voted to tear down what was there. The Loma Prieta quake damaged the road enough to close it. A rainy February 27, 1991, saw the "groundbreaking" of the Embarcadero demolition. Mayor Art Agnos remarked, "This is a good day for a rainfall... and it's a great day to take down a freeway." 
The 'Aquatic Freeway' plan
Among the proposals in the February 1966 plan by the state department of Highways and Public Works was Alternative FW, the "Aquatic Freeway" plan. I-480 would run slightly offshore in a double-decked tube, sometimes partly above water, and sometimes submerged. The 3.4 mile tunnel would have interchanges only at I-280 and near Broadway before connecting to the Golden Gate approach.
The advantages to the plan: minimal right-of-way would be taken, and most of the freeway would remain out of sight.
The disadvantages: the tube, acting as a sea wall when breaking the surface, would have to be reinforced against tidal pressures; the often shallow depth (15 feet) underwater would prove a hazard to ships; emergency and police access would be difficult, there would be a "severe psychological barrier [to enter a 3-mile long underwater tunnel] to many persons who might otherwise use this route," and finally, because of the minimal access to local streets, the road would be of benefit primarily to Marin residents only. 
I-480 Iowa; Nebraska
4.90 miles ; from I-80 in Omaha to I-29 in Council Bluffs, Iowa. In Omaha, it's signed as the Gerald R. Ford Freeway, an honor also given to I-196. Ford was born in Omaha under the name Leslie Lynch King, Jr. 
Extending I-480 southward contemplated, declined
There were plans afoot to extend I-480 southward from I-80 through Bellevue, Neb., and across a new bridge to meet I-29 in Iowa near Pacific Junction. Nebraska Congressman Doug Bereuter introduced Bill HR 2869 into the 106th Congress on Sept. 15, 1999 to "authorize the Secretary of Transportation to carry out highway and bridge projects to improve the flow of traffic between the States of Nebraska and Iowa and to direct the Secretary to designate certain highways in those States as an Interstate System route."
The extended Interstate 480 would incorporate the US 75 expressway leading south from Omaha, as well as an upgraded US 34 with a new bridge slightly south of the existing US 34 toll bridge over the Missouri River.
However, the Bereuter bill never made it out of committee, and in 2003 the plan for the new interstate-grade bridge was dropped. 
In the 1960s, this was proposed as part of I-80N (1962), then part of I-80 (1965). The section of Ohio Turnpike between the I-480 junctions would have been unnumbered;  the freeway from Youngstown to Akron would have been I-80, then I-80S (it's now I-76).
Nick DeCenzo writes that state troopers consider I-480 easy pickin's: "..it's the fastest highway in the state. The average ticket written on I-480 is for 78 mph, 18 above the 60 mph speed limit. This is probably due to the fact that the 8-lane I-480 goes around, rather that into, downtown (offical county maps call it the "Outerbelt South." Most people just call it 480), and is rarely congested, even in rush hour."
Just east of its interchange with I-77, Interstate 480 crosses the Cuyahoga River on twin spans 212 feet high and 4150 feet long. The original guardrails were only about 2.5 feet high, making some acrophobic drivers reluctant to take that route. Higher fencing was installed, but a few cars have nevertheless unfortunately gone over the side. 
Trivia: the Ohio DOT considers a short spur of I-480 to be I-480N: a separate route for administrative purposes. Official I-480N is the segment between the Warrensville Center Road and Northfield Road interchanges.  However, it's only signed for its destinations on either end: I-271/US 422, and I-480. 
See also: Ohio in 1962 (Cleveland area reroutings; Marc Fannin)
I-480 (numbered as another interstate) Pennsylvania
Old number for I-476, back before 1964 when I-76 was known as I-80S.