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1.34 miles ; also known as North Bay Drive, I-375 is not shown on many maps at all. On others, it gets a 375 marker off to the side with a little arrow; otherwise the marker would completely obscure the road. It goes east from I-275 in St. Petersburg and ends at 9th Street -- just short of a first down. To the south about 1/4 mile is I-175.
I-175 and I-375 were planned as "downtown feeders", intended to stem further decline of downtown St. Petersburg by providing quick access to the interstate system. (The city has since rebounded, mainly from tourism.)
Both were conceived as state routes originally. However, when a funding issue led the state to cancel part of I-75 in Hillsborough County, five miles of eligible interstate were released. This opportunity led to the designation of I-175 and I-375. 
I-375 opened between I-275 and 9th St. on Jan. 17, 1978, and was completed in January of 1979. I-175 opened a year later. 
1.06 miles ; spur in downtown Detroit, from I-75 to the Civic Center. Freeway dates back to 1965, but number dates back to 1970 or earlier.
Interstate 375 is the shortest signed interstate in the United States. A few interstate routes are officially shorter, but their interstate identity exists only on route logs, not on highway signs. (See more short 3di's.)
Thomas V. Ketchum writes: "I-375 came into being due to a relocation of I-75 from Toledo into Detroit. The original plan was to hug the riverfront, but that was rejected by the city. When I-75 was built to the north of downtown, the connection to the original I-75 was made through interchange ramps (excessively sharp to avoid the Stroh brewery). The orphaned section south to downtown then became 375. The interchange from 75 to 75 (how many other times does a through interstate turn through exit ramps?) is noted for trucks and cars flipping over guardrails when exceeding the 20 mph ramp."
In July 2000, the Michigan DOT proposed a three-block extension of I-375, moving its terminus further south. This would better serve the waterfront area, including the Renaissance Center (GM headquarters) and a relocated casino area. Construction is planned for 2002. 
The end of I-375 will be moved from Jefferson (where traffic continues west) to Atwater. Dyche Anderson explains the unconventional interchange design: "Heading southbound from Jefferson, there will be an exit for Franklin St Westbound, and an entrance - heading south - for Franklin Eastbound. There will be an exit for Atwater, but the freeway will do a U-turn and continue northbound. Heading north, there will be an entrance from Atwater, an exit to Franklin St Eastbound, and an entrance from Franklin St Westbound."
Yes, a U-turn at the end of the freeway!
Before construction on the $60 million project can start, the federal government must approve the plans and the state must acquire some land; but the only land involved is some parking spaces for a nearby church. No homes or buildings will be affected.