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There is a school district in Iowa called Interstate 35. (from Ryan Tucker)
95.74 miles ; from I-35 in Wichita to I-70 in Salina. Co-numbered with US 81 for the entire route (US 81 freeway continues another 15 miles north of I-70).
Before 1980, this road was marked I-35W. Construction started in the early 1960s, and Interstate 135 finished about 10 years later.
13.78 miles ; loop off I-35 and I-80 in Des Moines, Interstate 235 goes downtown while 35 and 80 bypass it.
An early (1957-58) Iowa numbering plan had the numbers I-235 and I-380 overlapping along the entire route, perhaps to be fair to both I-35 and I-80. On Nov. 10, 1958, AASHO directed Iowa to remove the I-380 overlap, while approving most of the remaining system. 
The idea for this freeway seems to have arisen in the 1950s. In 1957, the Des Moines City Council unanimously approved the freeway plan, and construction began shortly. The first segment of I-235 opened Dec. 14, 1961; the last segment opened Oct. 30, 1968, four years ahead of the original schedule.  
16.52 miles ; western loop off I-135 in Wichita. Was complete by 1965.
5.36 miles ; south from I-44 to I-35/40 in Oklahoma City. Opened in 1989, Interstate 235 is named the Centennial Expressway for the 100th anniversary of the opening of the Unassigned Lands, better known as the Land Rush. It was at the time called "The Last Interstate In The Original Plan To Be Built" (I don't have information on that plan, however). I-235, was shown as "proposed" since the 1970s, but never appeared on state maps until the late 80s. 
50.13 miles ; northeast from I-35 in Emporia to I-470 in Topeka.
On Oct. 23, 1987, I-335 was designated here.  Why? At the time, only signed interstate highways were allowed to go to 65 mph, while the Turnpike was limited to 55 mph. . Not good if you're counting on toll revenue.
I-335 (cancelled) Minnesota
2.74 miles ; proposed but unbuilt. Interstate 335 would have skirted the north side of downtown Minneapolis, connecting I-94 to I-35W near Broadway/Johnson Street. An 0.9-mile, $25M section was proposed as part of the 1970 highway bill.
Proposed in 1964, I-335 was killed around 1975 due to neighborhood resistance, as well as MNDot predictions that the route would actually increase congestion in the I-94 Lowry Hill tunnel.
A political action committee in the St Anthony East neighborhood, hearing of the planned elevated freeway, first requested that it be placed underground. When the highway department declined this proposal, the PAC moved on to opposing the freeway outright. 
It got as far as right-of-way acquisition, and some Evel Knievel ramps are visible on I-35W between Hennepin Avenue and Johnston St. After the highway was cancelled, the land was repopulated, with a swath of new homes going through an otherwise much older neighborhood. I-335 was withdrawn in early 1978, and funding was transferred to I-394 and other local projects. 
I-435 Kansas; Missouri
80.81 miles ; Interstate 435 is a full beltway around Kansas City. The newest section, west of I-29 and I-35, was proposed as late as 1970, at $155M for 47.4 miles. That section, however, finished around 1986. The eastern half got started in the late 1960s.
I-535 Minnesota; Wisconsin
2.78 miles ; from Duluth, Minn. into Superior, Wis. One of the few 3-digit interstates which goes into a state (Wisconsin) where the parent highway (I-35) does not.
Interstate 535 crosses over the 7.975-foot John A. Blatnik Bridge, which was dedicated on December 2, 1961, and got its current name on September 24, 1971. 
I-635 Kansas; Missouri
12.67 miles ; from I-35 across the river to I-29 in Kansas City. Interstate 635 finished around 1980.
Currently (2005) Interstate 635 is also noted for the Dallas High-Five interchange, under construction at US 75. The underpowered modified cloverleaf is being replaced with a four-level stack. The frontage roads, formerly severed by the interchange, will be connected through, creating the fifth level. It should be complete in 2006.
On Oct. 1, 1959, the Bureau of Public Roads (BPR) approved I-635 at a location leading from I-35W south of Dallas looping eastward to end at I-35 north of Dallas. 
In December 1971, Interstate 20 was relocated to the south in Forth Worth and Dallas (it used to follow I-30 here). The southernmost 13 miles of I-635 were transferred to I-20. 
In the summer of 1974, I-635 was legislatively extended approximately 9.6 miles westward from I-35W to TX 121. 
To increase capacity on I-635 between US 75 and I-35E, planners in 1999 were looking at a novel solution: six additional HOV lanes placed in underground tunnels. Erik Slotboom wrote:
"At a meeting Tuesday night, the status of the I635 study was presented. The locally preferred alternative for the segment between I35E north and US75 is the addition of 6 HOT lanes to the existing 8 lane freeway. Local opposition made it necessary to add the lanes without procuring any right-of-way. In the center section around the North Dallas Tollway, the two options under consideration are cut-and-cover tunnels beneath the feeder roads and twin bored tunnels, 2.5 miles long.
"The twin bored tunnels would be 17 meters below ground and would have 3 lanes with 10 ft and 4 ft shoulders. Overall width at the base is about 60 feet. Estimated price is $200 million - about $140 million more than the cut-and-cover option. Still, for 5 miles of a big tunnel, this is not too bad. Building the HOT lanes underneath the feeders would be a nightmare, especially considering the heavy commercialization along the corridor.
As far as I know, this would be the first bored tunnels in the US built for traffic diversion only (not for mountains or waterways). Don't look for this to get built anytime soon. In my opinion, it will be at least 10 years before construction starts, whatever the selected option is."