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19.66 miles ; Long Beach Freeway; from Long Beach to I-10 in Monterey Park. Caltrans' intention to extend this north to I-210 in Pasadena has met with strong local opposition, especially from South Pasadena, a town of 24,000 that would be split by the freeway. The fight has been going on since 1965. 
The first section of the Long Beach Freeway opened in 1952; the last in 1970.  Originally numbered CA 15, it was renumbered CA 7 in 1964 when work on I-15 began. In September 1983 it was approved as a non-chargeable interstate, and on May 30, 1984, AASHTO approved the I-710 numbering. In October 1984, the FHWA approved another 1.6 miles from CA 1 to Ocean Blvd. in Long Beach. 
A Controversial Extension
The Federal Highway Administration signed a Record of Decision (ROD) in 1998 agreeing to extend Interstate 710.  Caltrans wants to start the 6.2 mile extension around 2005. Their web site projects the cost at $670 million, while Caltrans chief environmental planner Ron Kosinski estimates the cost at $1.4 billion. 
There would be six general-purpose lanes and two HOV lanes, with room for light rail in the median. Interchanges are planned at Hellman Ave., Valley Boulevard/Alhambra Ave., Huntington Dr., California Blvd., Del Mar Blvd., and Green St. To decrease the impact on South Pasadena, the proposed interchange with I-110 has been removed.
There are also two tunnels planned: a 1200-foot "cut and cover" near South Pasadena High School, and a 100-foot tunnel near the Buena Vista district.  In mid-2000, the Southern California Association of Governments (SCAG) unveiled another tunnel proposal, with a network of four "natural" tunnels (not cut and cover) under South Pasadena. 
South Pasadena, which opposes the freeway, has proposed what it calls a "low-build alternative", including synchronizing traffic signals, making some streets one-way, and relocating parking. 
"It's obvious why the City of South Pasadena doesn't want it. As for the City of La Canada, they feel that traffic will worsen because people will actually start using the northbound part of the 210. Up until now, it's been their own private little freeway, and they don't want that to change." -- Paul L. Talbot, Alhambra City Councilmember, discussing the two cities opposed to I-710, in May 2001 interview 
New tunnel plan surfaces in 2003
In January 2003, state Assemblywoman Carol Liu, D-Pasadena, and representatives from the Metropolitan Transportation Authority (MTA) and the Southern California Association of Governments (SCAG) unveiled a plan to build a 4.5-mile, 150-foot-deep tunnel carrying six lanes, from Alhambra to Pasadena. The tunnel would not be a "cut and cover" operation, meaning trees and properties on the surface would not be demolished or relocated. It's not certain how many interchanges the tunnel portion might have. The cost would be about $3 billion, and might be financed with tolls.  
Expansion plans for already-constructed portion
Meanwhile, the built part of I-710 is suffering from heavy traffic, and not just from commuters. The highway connects shipping ports at Long Beach with distribution centers in east Los Angeles, and the proportion of truck traffic, already well exceeding what was planned for when the freeway was designed, is only expected to increase as Pacific Rim trade does. 
In May 2002, a group of engineers and government officials unveiled a list of 12 alternatives (including no-build) for improving I-710 between the ports and the Pomona Freeway. The list will be narrowed down to five while working with local officials, and down to one by spring 2003.
In brief, the alternatives are: 
I-710 (cancelled) Arizona
Tucson's proposed Interstate 710 would have followed Kino Parkway, from I-10 near Ajo Way to Broadway Boulevard at Campbell Avenue.  It was added to Arizona's interstate system by AASHTO on Nov. 10, 1958. 
I-710 shared the fate of most proposed Tucson expressways: cancellation. This happened by 1976.