What happened to letter extensions on Interstate highways?
AASHTO eliminated them in 1980 and told state DOTs to think of other numbers. Two still remain on I-35. I-15E in California stayed until 1983 (see 6.2.1.3). Jan 18, 2002 6.2.1.2 Q: Why does I-35 split in Dallas and Minneapolis?A: As stated in the previous question, some Interstates once carried letter suffixes. All but three were changed in 1980, and one of those (I-15E in CA) was gone by 1983. The remaining two are the I-35 splits in Dallas and Minneapolis, which remain because there was no consensus on what the new numbers would be. The smaller cities, St. Paul and Fort Worth, did not want their Interstate highways “demoted” to 3di’s. Jul 9, 2003 6.2.1.3 Q: What are/were all the suffixed interstates? A: Actually existed I-5W (CA) — I-505, I-80, I-580, I-205, CA-120 (see http://www.cahighways.org/001-008.html#005 for details) I-15E (CA) — I-215 (see http://www.cahighways.org/009-016.html#015 , section 2, for details) I-15W (ID) — western I-86 I-24W (MO/TN) — I-155 and US 412 I-35W
