Why the Interstate 875 designation?
Look no further than Baltimore, Maryland as an example!
The Baltimore area has two tunnel crossings that go under the Patapsco River: The Fort McHenry Tunnel which carries Interstate 95 and the Baltimore Harbor Tunnel that carries Interstate 895. Before the Fort McHenry Tunnel opened the Baltimore Harbor Tunnel was the link between the northeast metropolis and Florida. By comparison the Tampa Bay region has three east-west bridge crossings that go over Tampa Bay: The Courtney Campbell Causeway (FL 60), the Howard Frankland Bridge (Interstate 275) and the Gandy Bridge (US 92 and the suggested Interstate 875).
After having taken so many trips to the Baltimore-Washington area and exploring Interstates 95 and 895 among other things, I think an Interstate 875 designation would be appropriate if Gandy Blvd. is upgraded to interstate standards including the makeover at Interstate 275's Exit 28. However, any interstate number designation is up to the folks at AASHTO (in particular, AASHTO's Special Committee on US Route Numbering within the Standing Committee on Highways) and concurrence with the Federal Highway Administration (FHWA).
I would imagine Francis Scott Key - while watching the rockets' red glare and seeing the bombs bursting in air on that night in 1814 and penning the words that would become The Star-Spangled Banner, our American national anthem adopted many years later on 3 March 1931 (which, ironically, is the same date that our State of Florida became the 27th State of the Union in 1845) - would one day envision two tunnels and the one bridge on Interstate 695 in Baltimore Harbor that carries his name.
