Presently, the Interstate Highway System includes four suffixed routes - the Interstate 35 split into Interstate 35E and Interstate 35W at Dallas-Fort Worth, Texas and a similar split into Interstate 35E and Interstate 35W at Minneapolis-St. Paul, Minnesota. However, there were once many more, as the three-digit Interstates were not designated until after all major routes were assigned numbers, including some short connections and spurs. (A few of the shortest, including I-190 and I-195, were assigned three-digit numbers almost immediately.) Most were not equal splits like on I-35, but had the main route continue through, and often the suffixed route never returned to its parent. In 1980, AASHTO abolished the majority of suffixes due to confusion, renumbering them as three-digit Interstates, but several that return to their parents were kept. Interstate 15E has since become Interstate 215, but both I-35E/I-35W splits still exist.

List of suffixed Interstates

Current routes

Interstate	Extent
I-35W I-35E	Hillsboro, Texas (I-35) to Denton, Texas (I-35)
I-35W I-35E	Burnsville, Minnesota (I-35) to Forest Lake, Minnesota (I-35)

Former routes

Interstate	Extent	Notes
I-5W I-5E	Tracy, California (I-5) to Dunnigan, California (I-5)	In the 1958 plan; I-5E changed to I-5 and I-5W to I-580, I-80 and I-505 in 1964
I-15E	Temecula, California (I-15) to Devore, California (I-15)	Renumbered from I-215 in 1973 and back to I-215 in 1982
I-15W	Pocatello, Idaho (I-15) to Rupert, Idaho (I-80N)	In the 1958 plan; became I-86
I-24W	Jackson, Tennessee (I-40) to Hayti, Missouri (I-55)	Did not connect to I-24; renumbered I-155 in 1964
I-35W	Wichita, Kansas (I-35) to Salina, Kansas (I-70)	In the 1958 plan; renumbered I-135 in 1980
I-59B	Bypass for I-59 around Birmingham, Alabama	Renumbered I-459
I-70S	Washington, Pennsylvania (I-70) to New Stanton, Pennsylvania (I-70/I-80S)	In the 1958 plan; became part of I-70 and former I-70 became parts of I-79 and I-76 in 1964
I-70S I-70N	Frederick, Maryland (I-70) to Washington, DC (I-66/I-95) and Baltimore, Maryland (I-83/I-95)	In the 1958 plan; became I-270 and I-70 in 1973
I-75E	Bypass for I-75 around Tampa-St. Petersburg, Florida	Renumbered I-275 in 1973, and swapped with I-75 in 1980
I-80N	Echo, Utah (I-80) to Portland, Oregon (I-5)	In the 1958 plan; became I-84 in 1980
I-180N	Spur from I-80N to Boise, Idaho	This was the only suffixed three-digit Interstate; all other spurs of suffixed routes had no suffix; became I-184 in 1980
I-80S	Big Springs, Nebraska (I-80) to Denver, Colorado (I-25/I-70)	In the 1958 plan; became I-76
I-80N	Neola, Iowa (I-80) to Loveland, Iowa (I-29)	Became part of I-680 in 1973
I-80N	Norwalk, Ohio (I-80/I-90) to Edinburg, Ohio (I-80)	In the 1958 plan; became part of I-80 by 1962
I-80S	Youngstown, Ohio (I-80) to Camden, New Jersey (I-295)	Extended west to Lodi, Ohio by 1962 over former I-80; east end truncated to Monroeville, Pennsylvania and the part east of Monroeville renumbered I-76 in 1964; the rest became part of I-76 in 1970
I-81E	Scranton, Pennsylvania (I-81) to Scotrun, Pennsylvania (I-80)	In the 1958 plan; became I-380 ca. 1973

References

• misc.transport.road Frequently Asked Questions - What are/were all the suffixed interstates?