How many other states do this? So I-80 is the major 2di in Nebraska. I noticed that where it interchanges with I-76 (exit 102), I-180 (exit 401), I-680 (exit 446) and I-480 (exit 454), the corresponding interstate uses the I-80 exit number for its junction with I-80. This could be just because they all end at I-80. The only other place I have seen this before was in New Jersey, where I-195 meets I-295 at exit 60 on I-295. (Recently, I-195 had its exit numbers changed at I-295 from exit 60 A-B to exit 1 A-B.)
Utah has one instance of this, but I think it was an accident: it's on the southbound US 89 exit to I-84 westbound. (https://goo.gl/maps/W8rKUhaY5ck) 87 is the I-84 milepost for this junction; a proper US 89 mile-based exit would be 406 or 406B. It's not out of the question either, since 89 has exits 404 and 405 just to the south.
The reason I think it was an accident is that none of the other 89-to-84 exit ramps are numbered as anything, and no other signage for this ramp shows an exit number.
At I-24's Northwestern Terminus @ I-57 in Southern IL, the exit is I-57 Exit 44 and I-24 Exit 44 A/B
Its I-24 Mile Marker Zero, so it is certainly using the I-57 Exit numbering on the I-24 approach
I-90's exit to I-405 in Washington is 10, but I-405's exit is 11, so they missed it by one! This is impressive, because they both start at or near junctions with I-5. 90 goes almost straight east, while 405 goes east for three miles, then curves north for about eight miles. I-90's mileage actually starts with an at one time proposed interchange with the Alaska Way Viaduct which never happened, but you wouldn't think it would make that much difference.
The 80/180/34 cloverleaf near Lincoln has the two 80 to 180/34 movements signed as 401A and 401B, while the two 180/34 to 80 movements are signed as 401C and 401D.
As a child I went through this interchange regularly without thinking it was unusual... only now so many years later has it actually occurred to me.