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I-395 Connecticut; Massachusetts
I-395 Washington, D. C.; Virginia
13.39 miles . Interstate 395 is the Henry G. Shirley Memorial Highway, extending north from I-95/I-495 near Springfield, Va. to New York Avenue (US 50) in DC. Shirley was the Virginia Highway Commissioner, who died on July 16, 1941, just a few weeks after approving work on the expressway. 
Originally VA 350, the Shirley Highway opened on Sept. 6, 1949 from a point south of the Pentagon highway network to Woodbridge, VA.  A section in Arlington, including the "Mixing Bowl" interchange near the Pentagon, opened in 1942.  Around 1965, it was renumbered I-95, which originally was planned to penetrate the I-495 beltway and downtown DC. In 1977, when I-95 north of New York Ave. was cancelled, the Shirley was renumbered I-395. 
I-395 and US 1 cross the Potomac on a 3-span bridge, known for the Air Florida plane that hit one of its spans during an evening rush hour snowstorm in 1982. The oldest span, formerly the Rochambeau, is now named after Arland D. Williams, Jr., a passenger who perished while saving others from the icy waters.  
Some maps have confusingly shown sections of I-395 as I-195.
The interchange at I-95, I-495 and VA 644 (aka the "Springfield Interchange") has also been referred to as the Mixing Bowl, making the nickname a bit ambiguous. The local media appear to be at blame here: the state DOT calls it the Springfield Interchange. 
1.29 miles ; spur from I-95 in Miami to the MacArthur Causeway (US 41) into Miami Beach. I-395 is elevated for most of its length. It was added to the Interstate system on Oct. 15, 1964,  and opened to traffic in 1970.
Mr. Gorbachev, tear down this wall
The 1960s construction of the elevated six-lane I-395 is said to have devastated the Overtown section of Miami, once a thriving community. Today, the old design of the freeway causes traffic problems. The city and state have since the mid-1990s been looking at alternatives to modify or replace it.
In November 2002, plans were unveiled to replace I-395 with a below-ground freeway, creating an "open-cut" road from I-95 to MacArthur Causeway. Appreciation of land value along the corridor, including that freed by lowering the freeway, would help finance the project.  By some estimates, six to eight blocks of land would be opened up, paying for up to 80% of the estimated $500 million construction cost. 
Another alternative, proposed by the Florida Department of Transportation (FDOT), would build I-395 twice as high as it is today: a "sleek, graceful elevated span." . This design would cost less, allow the sun to shine beneath the freeway, and remove the plethora of support columns that block streets today, FDOT said. The Department contended that their alternative would be less of a barrier than a depressed, open-cut freeway
In July 2003, the I-395 Committee of the Metropolitan Planning Organization, a city and county planning group, approved the below-ground open-cut concept, freeing the way for FDOT to continue study. 
Design and construction could take about 10 years.
Thanks to Jason Learned and Mike Karels for sending Miami Herald articles.
1.98 miles ; a six-lane spur north from I-95 ending at Russell St. near Camden Yards in Baltimore. Interstate 395 opened in 1983.
Its interchange with I-95 is built entirely on an elevated bridge structure over the Middle Branch of Baltimore Harbor. I-395 itself is almost entirely an elevated twin-span viaduct. It ends in a twin branch, one branch to Howard St./Conway St. near Oriole Park at Camden Yards, and the other branch, multi-lane directional elevated ramps that connect to Harbor City Blvd.
Planning for I-395 started in the 1960's as a "pipeline" project; this appears to mean that planning activities not dependent on ingredients that were not ready (environmental policy, other regulations, funding, etc.) was still started so that when the other pieces of the puzzle came into place, the project was already well along .
So how long is it, anyway?
According to the 1998 federal interstate highway log, I-395 was the shortest signed interstate in the U.S., at 0.72 miles.  But in the 1999 highway log (updated June 30, 1999), I-395 is listed as 1.98 miles. No construction was done at the time; so how did I-395 more than double its length? The probable answer is that the interchange ramps at I-95, or the approaches at its north end, are now considered to be part of I-395 where before they were not.
I-395's end was left open for an eastern extension along ME 9 all the way to Calais: part of the planned Maine East-West Highway that would connect to New Brunswick route 1. 
A green I-395 entrance sign (looked fake) was featured in "Creepshow 2," which is still not worth seeing. However, the real I-395 is on the way to Stephen King's house: get off on US 1A/202, into Bangor, left at Cedar. At the end of Cedar, you can't miss the wrought iron two-headed bats.  (And for heaven's sake, watch out for pedestrians!)
MDOT studying I-395/ME 9 link
Maine DOT is mulling over improving access between I-395 and ME 9 easterly toward Calais. As of June 2003, there wasn't a list of alternatives suggesting or ruling out an extension of I-395. 
See also: I-395/Rt. 9 Connector Study (Maine DOT)
I-395 (numbered as another interstate) Pennsylvania
In 1958, according to a New York Times article*, Interstate 395 was to be the numbering for the Schuylkill Expressway (now I-76) from the Vine Street Expressway (now I-676) across the Walt Whitman Bridge. 
However, the highway was officially signed in 1959 as I-680. In 1964 and 1973, the number changed again, but from then on it has been part of I-76.
* "New Roads with New Numbers Will Parallel Old U.S. Routes". New York Times, September 19, 1958. Steve Anderson saw the article; I haven't seen it yet.