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57.22 miles ; Junipero Serra Freeway. From a huge interchange at US 101 and I-680 in San Jose to an abortive end in San Francisco just south of the I-80 approach to the Bay Bridge. The prom queen to US 101's ugly bucktoothed stepsister.
Interstate 280 was approved for California's Interstate system on Sept. 15, 1955. In January 1981, the 2-mile unbuilt segment between 6th Street and the Bay Bridge in San Francisco was withdrawn. 
Originally, I-280 was to connect to US 101 at the Golden Gate Bridge approach. The Junipero Serra and Park Presidio freeways would roughly parallel 19th Avenue to the east. This was part of a 1951 Trafficways Plan that was decimated by the 1959 Freeway Revolt, when the Board of Supervisors declared their opposition to most planned freeways in San Francisco.  Part of the Serra freeway was built, but is signed as CA 1 just north of I-280. 
The Southern Freeway, where I-280 runs now, was spared, and in 1961 the Board endorsed the current I-280 route to meet I-80 at the Bay Bridge. In 1965, this route became part of California's interstate system, and the Park Presidio route was withdrawn. Work was underway in the late 1960s. In October 1969, the city asked the state to stop work on the I-280/I-480 connection, over concerns that this would perpetuate the existence of a freeway the city wanted torn down. Work on the connection to I-80, however, was allowed to continue. 
I-280 never did make it to I-80. In 1973, it was completed to 3rd street, where it ended in midair [it has since been revised to outlet traffic onto King Street]. The 1989 Loma Prieta quake closed the double-decked section north of US 101 for six years. Compare this to I-10, which after Northridge was repaired in 90 days.
In 1990 CalTrans formally abandoned the 280/80 connection.* However, recent work was done at the terminus: a new onramp at 4th and Townsend, and new ramps to King Street and the Embarcadero, which opened November 1997. The old ramps were left standing, but disconnected from I-280 for a while;  but were torn down in early 2000.
* However, the state Streets and Highways code apparently leaves I-280 a way back in: "Notwithstanding the provisions of Section 89 of Chapter 1062 of the Statutes of 1959, construction of all or any portion of Route 280 from Route 101 near Alemany Boulevard to Route 480 near Harrison Street in San Francisco may be commenced at any time, if the City and County of San Francisco has conveyed or does convey to the State of California, without charge, all real property presently acquired by it for the construction of that portion of this route or any portion thereof."  
The Junipero Serra Freeway, which opened in the mid-1970s, is called by some "the most beautiful freeway in the world." Each direction is on a separate grade, and some bridges were designed with the intent to fit in with the surrounding terrain. I-280 officially becomes ugly as you approach CA 85.
The section between CA 85 and CA 17 was built around 1964; 85 north of that and 280 east made a continuous roadway. A few months before the 1964 election, Lyndon Baines Johnson released federal money to build 280 to US 101 as a favor to California senate nominee Pierre Salinger.  Salinger didn't win, but the eight-lane 280 did get built, and opened in December 1972.  Before this, 280 was routed north along CA 17 (now Interstate 880) to meet I-680 at US 101 in San Jose.
Eight lanes isn't nearly enough. The Caltrans mid-1980s "Route Concept Reports" projected a 2005 need for 14-16 lanes for I-280 between Route 85 and I-880; and for 14 lanes for I-880 from US 101 to Route 237.
I-280 (numbered as another interstate) Nebraska
Proposed/working number for what is now I-680 around Omaha. The 1962 and 1965 Rand McNally maps had two I-280s, the other one in Davenport/Quad Cities, proposed to go into Iowa. That was a problem, since two same-numbered 3di's in a state are not allowed. Davenport's 280 got to keep its number.
I-280 Illinois; Iowa
26.98 miles ; bypass of the Davenport "Quad Cities" to the south. Shares 9.4 miles with I-74, which actually sets foot in Iowa before ending at I-80. The Iowa segment, which completed the highway, opened on Oct. 25, 1973.  
I-280 border skirmish, early 1990s
Around 1990, Interstate 280 was involved in a controversy between Illinois and Iowa. Some parties in Illinois wanted to renumber I-280 as part of I-80, and number the "other side" of the loop it forms as 280. This would result in additional mileage in Illinois for I-80, a primary cross-country interstate.
Illinois pursued this through both the AASHTO Route Numbering committee and ISTEA hearings in Congress.
On June 10, 1991, the Illinois proposal for renumbering Quad Cities interstates was disapproved by AASHTO. The proposal would have relocated both interstates 80 and 74, eliminated I-280, and created a new Interstate 174 for the central route formerly part of I-74. AASHTO's reason for turning it down: the finding that "[Iowa] would not be submitting companion applications. Therefore, the Committee is unable to consider the applications from Illinois at the meeting." 
On June 13, 1991, Sen. Patrick Moynihan moved to table the Dixon-Simon amendment to ISTEA 1991. This amendment "requires the Secretary of Transportation to assign a number to a highway on the Interstate System whenever two or more bordering States that are connected by the highway on the Interstate System cannot agree to the number designation of the highway." I quote from the Congressional Record (the argument for tabling):
"This amendment is nothing but an attempt by the State of Illinois to steal commerce from the State of Iowa. For 34 years the policy of the Interstate Highway System has been to require that States that share highways in common agree to their numbering, and that they agree before changing any highway numbers. Interstate 80 is a major, national, east-west route. Its path was agreed to many years ago by all the States involved. Part of this Interstate goes through Iowa. Now Illinois wants to change this agreement.
The Dixon-Simon amendment was tabled, 72-26. To date, I-280 has not been renumbered.
Jason Hancock adds:
"Illinois wanted to do this in order to reduce accidents at the "Big X" interchange in Colona, a simple cloverleaf where you have to make a turn to "stay" on I-80 or I-74. It makes sense from Illinois' point of view but Iowa balked at the proposal, citing that businesses along I-80 would suffer due to relocation. On July 1, 1993, FHWA rejected the Illinois renumbering plan. Now Illinois is working on reconstructing the "Big X" by building new high-speed and flyover ramps so you wouldn't have to slow down if you want to "stay" on I-80." 
I-280 New Jersey
17.85 miles ; from I-80 southeast into Newark, ending at I-95. By the mid-1970s, this was mostly done, except for a small section between the Passaic River and I-95, which took an extra decade or so to complete. Has a drawbridge over the Passaic.
Reportedly the original 1954 plan for the highway called for it to connect I-80 with the Lincoln Tunnel -- a role now served by US 46 and NJ 3. "Essex County politicians, with the second strongest delegation in Trenton, successfully realigned the highway to fit almost entirely inside Essex County. The existing Stickel bridge was included in the design, and an enormous tunnel from the railway in Orange westerly through the mountain, onto a vast landfill, and over the second mountain was planned. That idea failed, and the current cut, deep dip, and second ascent was the replacement." 
12.41 miles ; serves Toledo. Has a drawbridge. Dates back to at least 1965. For about 40 years, the southernmost few miles were simply an undivided four-lane road, with traffic lights and a drawbridge. 
Interstate 280 opened in 1956 as part of the Toledo-Detroit Expressway, which also included what is now I-75 from the I-280 terminus to US 24 in Flat Rock, Michigan; and a segment south of the Ohio Turnpike, ending at US 20/23. The segment from Summit Street northward was marked US 24A, with the remainder marked OH 120.
The expressway segment south of East Toledo was not built to freeway standards: there were seven grade-level intersections at Walbridge Road, Lemoyne Road, Ayers Road, OH 795, Latcha Road, Bahnsen Road, & Hanley Road. ODOT had to build overpasses for these roads before the highway was awarded Interstate certification. I-280 ended at the Ohio Turnpike, while the 2-mile segment between the Turnpike and the Fremont Pike (US 23) was redesignated OH 420 in 1970.
For a brief time in 1958, the working definition for Interstate 77 included what is today's I-280. I-77 would have approached Cleveland from Canton and Akron, as it does today, but it would not have ended there. Instead, I-77 would have overlapped with I-90 nearing Toledo, used I-280 to connect to I-75, and then overlapped with I-75 to Detroit. Finally, the crooked I-77 would have followed today's I-94 to Port Huron, Mich.
In August 1958, Ohio requested that I-77 be truncated to Cleveland, I-280 be applied to the Toledo loop, and Michigan's portion of former I-77 be called I-75E. In November of that year, AASHO approved the changes.
The Craig Memorial Bridge was built specifically to carry the new highway over the Maumee River in 1956. After more than a generation, local transportation officials will see their wishes answered: ODOT has approved a new high-level bridge to carry I-280 across the Maumee. Along with this, I-280 will be widened to 6 lanes from OH 2 northward; access from Front Street will be redesigned; and the badly designed Summit Street interchange will be removed altogether.  
I-280 (numbered as another interstate) Pennsylvania
I-280 was an old number for the Schuylkill Expressway (today's I-76) through 1958,  and the Pennsylvania Turnpike eastern extension (today's I-276) from 1958 through 1963. Both routes are near Philadelphia.